SURFACE ELECTROMYOGRAPHY

SURFACE ELECTROMYOGRAPHY

INTRODUCTION
There are numerous studies that support the physiological basis for using quantitative electromyography in the diagnosis of temporomandibular disorders (Moyers, 1949; Perry, 1954; Jarabak, 1956; Perry, 1957; Porritt, 1960; Grossman, 1961; Moller, 1966; Yemm, 1976; Bakke et al., 1980; Riise et al., 1982; Sheikholeslam et al., 1983; Riise et al., 1984; Kydd et al. 1986).

There is evidence, based on controlled studies that used extensive statistical tests, that surface electromyography is reliable and reproducible (Goldensohn, 1966; Lloyd, 1971; Mitani and Yamashita, 1978; Riise, 1983; Hermens et al., 1986; Burdette and Gale, 1987).

Controlled studies that used extensive statistical tests show that there is a strong relationship between EMG and muscular force (Lippold, 1952; Bigland et al., 1954; Molin, 1972; Milner-Brown, 1975; Pruim, 1978).

Several studies have quantitatively investigated the EMG during postural activity of the mandible and during maximal bite in the intercuspal position. The EMG values for the temporal and masseteric muscles have been quantitatively investigated in these studies for control subjects without functional disorders and for patients with functional disorders. (Lous et al., 1970; Moller et al., 1971; Sheikholeslam et al., 1980; Sheikholeslam et al., 1982; Moller et al., 1982; Cram and Engstrom, 1986). These studies replicated the results that quantified statistically significant differences between the normal population and the patient population. The slight variability among these studies was due to the type of EMG instrumentation used in each study (i.e. range of filter frequency).

There is evidence based on controlled studies that used extensive statistical tests that maximal bite force and the electrical muscle activity during maximal bite in the intercuspal position are significantly weaker in patients with functional disorders of the masticatory system than controls without such disorders (Molin, 1972; Helkimo et al., 1975; Randow et al., 1976; Sheikholeslam et al., 1980; Moller et al., 1982; Sheikholeslam et al., 1982; Kydd et al., 1986).

Controlled studies that used extensive statistical tests conclude that postural activity of temporalis and masseter muscles are significantly higher in patients with functional disorders of the masticatory system than controls without such disorders (Moller, 1966; Lous et al., 1970; Moller et al., 1971; Sheikholeslam et al., 1982; Pantaleo et al., 1983; Geraris et al., 1989).

Clinical studies investigating Electromyography of temporal and masseteric muscles concluded that EMG was effective in the diagnosis of Myofascial Pain Disorders (Sheikholeslam et al., 1986; Pantaleo et al., 1983; Cooper et al, 1986; Moller, 1969; Helkimo et al., 1975; Mylinski et al., 1985; Riise et al., 1982; Sheikholeslam et al, 1983; Riise et al., 1984.) These studies further validate the basis for the use of EMG in clinical dentistry. The patients examined in the above studies exhibited high levels of EMG postural activity and weak EMG activity during maximal bite in the intercuspal position. Occlusal therapy resulted in significant improvement in symptoms and pain, and the successfully treated patients had significantly lower postural activity and significantly improved and symmetrical maximal bite activity. The rationale for utilizing EMG to monitor response to occlusal therapy was further substantiated when occlusal splints were removed causing onset of pain and elevated postural activity.

In summary, based on well controlled empirical and clinical studies that have been conducted in several universities over the past three decades throughout the world, there is unequivocal evidence to strongly support the use of EMG for the evaluation and diagnosis of temporomandibular disorders.

ARTICLES REVIEWED IN THIS PUBLICATION

SECTION 1: STUDIES THAT DOCUMENT THE PHYSIOLOGICAL BASIS FOR THE EFFICACY OF ELECTROMYOGRAPHY IN DENTISTRY

1. Vig, P. Electromyography in dental science: A review. Australian Dental Journal, pp. 315-322, August, 1963.

2. Jarabak, J.R. An electromyographic analysis of muscular and temporomandibular joint disturbances due to imbalances in occlusion. Angle Orthodont., (26)3: 170-190, 1956.

3. Perry, H.T. and S.C. Harris. Role of the neuromuscular system in functional activity of the mandible. Journal of Am. Dent. Assoc., 48: 665-673, 1954.

4. Perry, H. T. Muscular changes associated with temporomandibular joint dysfunction. Journal of Am. Dent. Res., 54: 644-653, 1957.

5. Yemm, R. Neurophysiologic studies of temporomandibular joint dysfunction. Oral Sci. Rev., 7: 31-53, 1976.

6. Bakke, M. & Moller, E. Distortion of maximal elevator activity by unilateral premature tooth contact. Scand. J. Dent. Res., 80: 67-75, 1980.

7. Riise, C. and Sheikholeslam, A. The influence of experimental interfering occlusal contacts on the postural activity of the anterior temporal and masseter muscles in young adults. J. Oral Rehabilitation, 9: 419-425, 1982.

8. Sheikholeslam, A., and Riise, C. Influence of experimental interfering occlusal contacts on the activity of the anterior temporal and masseter muscles during submaximal and maximal bite in the intercuspal position. J. Oral Rehabilitation, 10: 207-214, 1983.

9. Riise, C. and Sheikholeslam, A. The influence of experimental interfering occlusal contacts on the activity of the anterior temporal and masseter muscles during mastication. J. Oral Rehabilitation, 11: 325-333, 1984. See Abstract on page 25.

10. Kydd, W. L., Choy, E. & Daly, C. Progressive jaw muscle fatigue and electromyogram activity produced by isometric unilateral biting. The Journal of Craniomandibular Practice, (4)1: 18-21, 1986.

SECTION 2: STUDIES THAT DOCUMENT THE RELATIONSHIP BETWEEN ELECTROMYOGRAPHY AND MUSCULAR FORCE

1. Lippold, O.C.J. The relation between integrated action potentials in a human muscle and its isometric tension. J. Physiol., 117: 492-499, 1952.

2. Bigland, B., and Lippold, O.C.J. The relation between force, velocity and integrated electrical activity in human muscles. J. Physiol., 123: 214-224, 1954.

3. Molin, C. Vertical isometric muscle forces of the mandible: A comparative study of subjects with and without manifest mandibular pain dysfunction syndrome. Acta Odont. Scand., 30: 485-499, 1972.

4. Milner-Brown, H.S. The relation between the surface electromyogram and muscular force. J. Physiol. 246: 549-569, 1975.

5. Pruim, G.J., Ten Bosch, J. J., and De Jongh, H. J. Jaw muscle EMG-activity and static loading of the mandible. J. Biomechanics, 11: 389-395, 1978.

SECTION 3: STUDIES THAT DOCUMENT THE REPRODUCIBILITY AND RELIABILITY OF SURFACE ELECTROMYOGRAPHY

1. Hermens, H.J., Boon, K.L., and Zilvold, G. The clinical use of surface EMG. Medica Physica, 9: 119-130, 1986.

2. Riise, C. Clinical and electromyographic studies on occlusion. From the Department of Stomatognathic Physiology, Karolinska Institutet, Stockholm, Sweden, pp. 20-21, 1983.

3. Goldensohn, E. Electromyography. From Disorders of the Temporomandibular Joint. Lazlo Schwartz, ed. W.B. Saunders Co., Philadelphia/London. pp. 163-176, 1966. See

4. Lloyd, A.J. Surface electromyography during sustained isometric contractions, J. Applied Physiology, 30: 713-719, 1971.

5. Mitani, H., and Yamashita, A. On the power-spectra of the surface electromyograms of masticatory muscles. J. of Osaka Dental Univ., 6: 1-12, 1978.

6. Burdette, B.H., and Gale, E.N. Intersession reliability of surface electromyography. Journal of Dental Research, Abstract No. 1370, Volume 66, 1987.

SECTION 4: STUDIES THAT DOCUMENT THE STATISTICAL COMPARISON OF QUANTITATIVE ELECTROMYOGRAPHY FROM THE MASTICATORY MUSCLES OF THE “PATIENT” POPULATION TO THAT OF THE “NORMAL” POPULATION

1. Moller, E. The chewing apparatus: An electromyographic study of the action of the muscles of mastication and its correlation to facial morphology. Acta Physiologica Scandinavica, Vol. 69, Supp. 280, 73-74, 1966.

2. Lous, I., Sheikholeslam, A., and Moller, E. Postural activity in subjects with functional disorders of the chewing apparatus. Scand. J. Dent. Res., 78: 404- 410, 1970.

3. Moller, E. Sheikholeslam, A., and Lous, I. Deliberate relaxation of the temporal and masseter muscles in subjects with functional disorders of the chewing apparatus. Scand. J. Dent. Res., 79: 478-482, 1971.

4. Sheikholeslam, A., Moller, E., and Lous, I. Pain, tenderness and strength of human mandibular elevators. Scand. J. Dent. Res. 88: 60-66, 1980.

5. Sheikholeslam, A., Moller, E., and Lous, I. Postural and maximal activity in elevators of mandible before and after treatment of functional disorders. Scand. J. Dent. Res., 90: 37-46, 1982.

6. Moller, E., Sheikholeslam, A., and Lous, I. Response of elevator activity during mastication to treatment of functional disorders. Scand. J. Dent. Res., 90: 37-46, 1984.

7. Cram, Jeffrey R, and Engstrom, David. Patterns of neuromuscular activity in pain and nonpain patients. Clinical Biofeedback and Health, Vol. 9, No.2, pp. 106-115, 1986.

8. Michler, L., Moller, E., & Bakke, M. On-line analysis of natural activity in muscles of mastication. Journal of Craniomandibular Disorders: Facial & Oral Pain, Vol. 2, No. 2: 65-82, 1988.

9. Gervais, R.O., Fitzsimmons, G.W., and Thomas, N.R. Masseter and temporalis electromyographic activity in asymptomatic, subclinical, and temporomandibular joint dysfunction patients. The Journal of Craniomandibular Practice, Vol. 7, No. 1, pp. 52 – 57, 1989.

SECTION 5: STUDIES THAT SUPPORT THE CLINICAL EFFICACY OF ELECTROMYOGRAPHY IN DENTISTRY.

1. Moller, E. Clinical electromyography in dentistry. mt. Dent. J., 19: 250- 266, 1969.

2. Helkimo, E., Carlsson, G.E., and Carmeli, Y. Bite force in patients with functional disturbances of the masticatory system. Journal of Oral Rehabilitation, 2: 397-406, 1975.

3. Myslinski, N.R., Buxbaum, J.D., and Parente, F.J. The use of electromyography to quantify muscle pain. Meth. and Find. Exptl. Clin. Pharmacol., 7(10): 551-556, 1985.

4. Riise, C., and Sheikholeslam, A. The influence of experimental interfering occlusal contacts on the postural activity of the anterior temporal and masseter muscles in young adults. J. Oral Rehabilitation, 9: 419-425, 1982.

5. Sheikholeslam, A., and Riise, C. Influence of experimental interfering occlusal contacts on the activity of the anterior temporal and masseter muscles during submaximal and maximal bite in the intercuspal position. J. Oral Rehabilitation, 10: 207-214, 1983.

6. Riise, C., and Sheikholeslam, A. The influence of experimental interfering occlusal contacts on the activity of the anterior temporal and masseter muscles during mastication. J. Oral Rehabilitation, 11: 325-333, 1984.

7. Pantaleo, T., Prayer-Galletti, F., Pini-Prato, G., and Prayer-Galletti, S. An electromyographic study in patients with myofacial pain-dysfunction syndrome, Bulletin Group. Int. Rech. sc. Stomat. et Odont., 26: 167-179, 1983.

8. Cooper, B. C., Alleva, M., Cooper, D., & Lucente, F. E. Myofacial pain dysfunction: Analysis of 476 patients. Laryngoscope, 96: 1099-1106, 1986.

9. Sheikholeslam, A., Holmgren, K. & Riise, C. A clinical and electromyographic study of the long-term effects of an occlusal splint on the temporal and masseter muscles in patients with functional disorders and nocturnal bruxism. Journal of Oral Rehabilitation, 13: 137-145, 1986.

STUDIES THAT DOCUMENT THE PHYSIOLOGICAL BASIS FOR THE EFFICACY OF ELECTROMYOGRAPHY IN DENTISTRY

In a thorough review of the EMG literature in 1963, Vig concluded that “orthodontic appliance technology is well in advance of our understanding of the physiological basis of occlusal dysfunction. By methods such as electromyography, a sounder approach, with the elimination of some futile treatment procedures leading to disappointing results and relapses, may be expected.” Vig, P. Electromyography in dental science: A review. Australian Dental Journal, pp. 315-322, August, 1963.

INTRODUCTION
Electromyography is not a new field of study – – references in scientific literature date back to 1908 (Buchanan). However, it was not until 1949 when Moyers (1949, 1950) initially applied this technique to a study of the musculature related to the temporomandibular joint, that dental research discovered this versatile tool.

PHYSIOLOGICAL BASIS
Skeletal muscle activity is controlled by efferent nervous impulses, mediated either at a segmental reflex level or modified by cortical control, as is the case in the more complexly coordinated movements required to maintain posture and function of the masticatory system. The nature of cortical activity is modified by both conscious control and the afferent sensory and proprioceptive messages relayed from the periphery.

Generally, larger motor units are found in the muscles which perform slow, sustained contractions, while the smaller motor units are found where rapid, non- sustained tension is required for finely coordinated movements. The innervation ratio, i.e., the proportion of muscle fibres to the motor neurones in the central nervous system, in the motor neurone pool varies over a wide range, from the order of 300:1 as found in the gluteus maximus, to as low as 3:1 in the extrinsic musculature of the eye. The innervation ratios of the muscles of mastication have not been accurately determined, although electromyographic results suggest values of 110:1 for the masseter and 200:1 for the temporalis muscles (Wyke, 1949). Knowledge of this property is important as theoretically the smallest increment of contraction is due to the additional recruitment of a single motor unit.

When the nervous impulse reaches the resting mass of protoplasm, ions, and water, it adopts the functional posture which we usually associate with muscle (Perry, 1955). Concomitant with this, there are several changes such as development of tension, a shift in mass, production of metabolites and heat, as well as minute electrical changes of the order of microvolts.

THE METHOD
Electromyography has evolved from the now well-established techniques of electrocardiography and electro encephalography. The essentials of the equipment required are:

  1. The electrodes- -which pick up the minute electrical activity.
  2. An amplifying system to magnify these signals.
  3. A means of recording the amplified signals.

THE ELECTRODES
There are basically two types of electrode in common use:

  • The surface electrode consisting of a small metal disc attached to the skin over the muscle by a suitable conducting material such as celloidin.
  • The needle electrode which consists of a small bore hypodermic needle, containing an insulated wire.

Surface electrodes are therefore suitable for the study of integrated activity of the muscle mass immediately beneath them. By suitable, symmetrical placement of these electrodes over paired muscles, their relative activity may be studied at rest and in various movements

What then are the uses of electromyography?

Electromyography has contributed by affording a method of studying the physiological basis of function of such vital activities as mastication, deglutition, and speech. Furthermore, electromyography and cineradiography are the only methods of observing dynamically these activities. By understanding the normal we can base treatment on a rational level and eliminate empiricism from our clinical assessments

Perry (1949, 1950) used cephalometric monitoring of the electromyographs coupled with sound units. Identical tracings were repeatedly obtained from roentgenograms taken at the silent audio positions in nearly all cases. In his two cases where audio silence could not be obtained, he found a level of activity persisting in the temporalis muscle. On clinical examination, these patients were found to suffer from occlusal disturbances and temporomandibular joint symptoms. Following correction of these conditions, audio silence and cephalometrically reproducible rest position were obtained.

Schpuntoff and Schpuntoff found that general posture, pain, fatigue, and central nervous system excitation were major factors affecting the establishment of a physiological rest position (1956)

Jarabak (1957) recorded “electrical silence” from the elevators (masseter, temporalis) and a depressor (digastric) at rest position. In edentulous patients, with dentures operating through an excessive interocclusal clearance (i.e., between 8-12 mm.), he found spontaneous activity at rest, both in the temporalis and digastric muscle.
Recordings from the perioral and suprahyoid musculature by Schlossberg also showed minimal activity at rest (Franks, 1957).

RELATION OF ELECTROMYOGRAPHY TO CLINICAL PROBLEMS OCCLUSAL REHABILITATION:
Minor discrepancies in a single occlusal restoration can produce alteration from a bilaterally balanced to an asymmetrical muscular contraction pattern (Porritt, 1960). This can inhibit normal function and result in altered occlusal patterns.

In denture prosthesis the assessment of the rest position is of prime importance. The numerous clinical methods used to record it, attest to the empirical nature of this procedure.

Electromyography has been used to obtain reproducible records of the rest position (Schpuntoff & Schpuntoff, 1956) and it has been suggested that electromyographic comparison of the various maneuvers should be undertaken to assess their reliability and accuracy

TEMPOROMANDIBULAR JOINT DYSFUNCTION
Cases suffering from temporomandibular joint dysfunction with atypical pain patterns are frequently a source of frustration to both dentist and patient. Many of these apparently atypical neuralgias have previously been wrongly ascribed to psychogenic causes.

Perry (1955) has shown that many of these pains are due to muscle spasms in the masseters, pterygoids, and temporalis muscles. With the establishment of more physiological jaw relations, the electromyograms show a reduction of these muscle spasms with relief of pain.

Grossman (1961) reported on joint disorders where clinical deviations occasioned by premature cuspal contacts in malocclusions, produced characteristic myograms.
Correction of the malocclusion resulted in symmetrical function and resolution of the joint symptoms…

This early EMG study by Jarabak (1956) suggests many clinical possibilities for evaluating the relationship between occlusion, the temporomandibular joint and the muscles of the mandible. The findings of Jarabak, as will be demonstrated in this report, have been confirmed by numerous studies in the past 30 years.

Jarabak, J.R. An electromyographic analysis of muscular and temporomandibular joint disturbances due to imbalances in occlusion. Angle Orthodont., (26)3: 170-190, 1956.

Functional disorders of the temporomandibular joint characterized by clicking, crepitus, trismus and pain are sufficiently prevalent to warrant physiological analysis. Muscles surrounding a joint, and responsible for its movements, frequently go into a state of hyperactivity or spasmodic contraction when this joint becomes painful.

This study was designed to serve a threefold purpose, the first of which was to determine whether or not occlusal interferences cause functional imbalances in the temporal muscles. Second, if so, are these imbalances a contributing factor to clicking, trismus and pain in the temporomandibular joint? Third, is it possible to eliminate abnormal muscular activity by re-establishing a correct functional occlusion?

Records of neuromuscular activity from the temporal muscles of biting, rest after biting, and rest after speaking, were taken from three groups of patients. In the first group were subjects with good occlusions who were free from temporomandibular joint symptoms. In the second were orthodontic patients who had a clicking or who had developed a clicking in the joint during reduction of a distal occlusion with intermaxillary elastics. The third group were patients with functionally broken down occlusions accompanied by clicking, pain and trismus in the temporomandibular joint. Many of these subjects had, in addition to pain in the joint, deep pain in some of the muscles used in mandibular posture.

METHODOLOGY

MATERIALS
From three groups of subjects, neuromuscular activity from the temporal muscles was electromyographically studied. In the first group were four patients, in the second, seven, and in the third, eleven.

The subjects with normal occlusion had fourteen teeth in each arch and there had been no previous extraction. None had signs of clicking, crepitus or pain in either temporomandibular joint.

Three of the seven orthodontic patients in the second group had clicking in the point before orthodontic treatment was started; the remaining four developed a resounding click shortly after intermaxillary elastics were placed to reduce a distal occlusion.

The third group were adult subjects. All had at some time or another lost posterior teeth.

FINDINGS
I. Neuromuscular activity from a subject with normal occlusion free from joint disturbance. A typical rest record (Figure 3), taken from a control subject . . . shows no appreciable neuromuscular activity.

II. Neuromuscular activity from a subject in orthodontic treatment. At a point in the reduction of a distocclusion with intermaxillary elastics some orthodontic patients develop a resounding clicking in . . . temporomandibular joints. Electromyograms shown in Figure 4 indicate that a spontaneous hyperactivity of a short duration occurs in the posterior fibers of the temporal muscle. This spontaneous hyperactivity is seen during rest after biting and in rest after speaking.

Ill. Neuromuscular activity from adult non-orthodontic subjects who have gross occlusal interferences. In rest after speaking, as in rest after biting, spontaneous hyperactivity continues (Figure 6). At times it is greater than at other times. Generally, when it is greater these patients complain of a deeply seated radiating pain over the area of the temporal muscle. Seemingly, these muscles do not come to rest for long periods of time during these spasm seizures.

IV. Neuromuscular activity after placement of interocclusal splint. Spasms in the temporal muscle have already been observed when pain is present in the temporomandibular joint. Thus, electromyographic data of neuromuscular activity seem to substantiate the premise that functional occlusal interferences, coupled with temporomandibular joint clicking and pain, occur concurrently with muscle spasms. When the occlusal interferences were removed in this group with interocclusal splints, spasms disappeared.

V. Neuromuscular activity after interocclusal splint was removed. To further illustrate that a correct occlusion or one free from occlusal interferences is an important link in the chain of events leading to proper neuromuscular balance in the temporal muscles, let us consider what occurs when the interocclusal splint is removed. Its removal permits the teeth to return to their original occlusion beset with functional occlusal interferences. Electromyograms shown in Figure 9 were taken approximately five minutes after the splint was removed. Rest activity after biting (Figure 9) clearly illustrates that spontaneous hyperactivity returns as an immediate after-discharge following the voluntary motor activity. If this hyperactivity – – the spasm -• is compared with that from this subject, for the same exercise, before the interocclusal splint was inserted, it appears nearly identical. Upon removal of the splint the temporal muscles were thrown once again into a functional imbalance; spasms occur in the muscle and clicking returns to the temporomandibular joint.

DISCUSSION
The temporal muscle, by virtue of its origin from the side of the cranium and its insertion to the coronoid process of the mandible, has the function of elevating the mandible. (Although) its prime activity is to elevate the mandible, it also has several ancillary functions.

In order to have the myriad of complex movements associated with chewing and swallowing rhythmic and smooth, it is necessary to have muscle activity precisely timed and synchronously carried out by some muscles which are contracting, by others which are relaxing, and by still others which are holding, guiding or poising the mandible. It is the integrated action of several muscles or muscle groups acting in unison through a precise timing of nerve impulses from the central nervous system which makes coordinated muscle activity possible. Thus the mandible is a human lever whose movements in speaking, and at rest, and in chewing, and in swallowing, are neuromuscularly controlled.

If any member in this community of muscles falters or becomes sick, it is logical to assume that its activity may alter the activity of the remaining members of the group and may in this manner affect the joint whose movements they control. Since a precise starting and stopping equilibrium (reciprocal innervation) exists between antagonistically acting muscles, it is possible to visualize how a muscle spasm in the temporal muscle may disrupt or change the timing sufficiently to throw their antagonists, the lateral pterygoid and the digastric muscles, out of a contraction and relaxation sequence.

The spontaneous hyperactivity in the temporal muscle occurring as a result of orthodontic treatment was quite revealing because it pointed out that stimuli originating in the teeth can make a muscle spastic where spasticity did not previously exist. It also brought out the significant fact that these tooth stimuli manifest their effects on patterns of muscular activity only when the teeth in the upper arch are in a certain position to the teeth in the lower arch. Spontaneous hyperactivity occurred when the teeth formerly in distocclusion were placed into a cusp-to-cusp occlusion. As a result of this position of the teeth in their respective arches, it was possible for the mandible to slide either forward or backward. It was during this period of backward and forward sliding that a resounding clicking developed in the temporomandibular joint. It was also at this time that the spontaneous hyperactivity occurred in the posterior fibers of the temporal muscles.

As soon as the teeth began occluding in a normal occlusal relation the clicking diminished, but did not disappear completely until the occlusion had functionally equilibrated itself, or until it was equilibrated. Thus the mere reduction of a distocclusion is not tantamount to a balanced muscular activity. This comes only when the denture has functionally equilibrated itself or when it is equilibrated

The method (interocclusal splint) used to control muscle spasms in the temporal muscle leaves very little doubt that occlusion and the maintenance of correct vertical dimension between the maxilla and the mandible are important factors in the physiology of the temporomandibular joint. Let us consider some of the sources of sensory stimuli which may cause muscular imbalance, an imbalance which may in turn cause joint pain, clicking and trismus.

If an adverse stimulus is short in duration, that is, if the patient can remove the occlusal interference in night grinding, or if it is removed by the dentist by equilibrating the occlusion, the musculature returns to its normal activity. If, on the other hand, a cuspal interference persists or if multiple interferences develop and adverse stimuli continue to be imposed, reflexly, upon the muscles, it is possible to visualize the development of muscular confusion muscle spasms.

Duchene (1867) observed that muscles surrounding a joint would go into a state of hypertonus when the joint was inflamed, injured or dislocated. In severe dislocations the muscles might even go into a “state of spasmodic contraction.” Though muscle spasms can be initiated reflexly in the periodontium of the teeth, they can also come from the sensory stimuli having their origin in the capsule of the joint.

The electromyograms in Figure 7 show that, when occlusal interferences were removed and correct vertical dimension was restored with an interocclusal splint, muscle spasms disappeared. This immediate change from muscle spasm to one of normal muscle activity indicates, in the light of our present evidence, the basic etiology for these spasms to lie in cuspal interferences of the teeth. Because these spasms disappeared almost immediately with the placement of the splint, it is difficult to visualize, if the cause for the spasms was inflammation of the capsule, how this inflammation could be spontaneously eliminated. For the present, experimental evidence leans in the direction of placing the cause for muscle spasms in the temporal muscle to occlusal interferences rather than on inflammation in the temporomandibular joint

SUMMARY
Electromyographically it is possible to show that muscle spasms occur in the two fractions of the temporal muscles. These spasms were found in orthodontically treated and in non-orthodontic subjects. In the orthodontically treated as well as in the non-orthodontic patients, spasms were present when clicking, trismus and pain occurred in the temporomandibular joint. In the orthodontic subjects who had developed a clicking during treatment, the hyperactivity was not as great and as frequent as it was in the non-orthodontic patients who had clicking, trismus and pain for a long period. The severity and duration of spasms depend on the length of time and degree of impact arising from interocclusal interferences. In orthodontic subjects adverse proprioceptive stimuli arising from an end to end occlusion of cusps were of short duration, provoking only mild spontaneous hyperactivity. Where the adverse stimuli arising from multiple cuspal interferences were of long duration, muscular hyperactivity was greater and lasted for longer periods. In the latter, muscle spasms were distinctly concurrent with clicking, trismus and pain in the temporomandibular joint.

Because spasms in the temporal muscles were eliminated with an interocclusal splint in subjects who had gross occlusal interferences and an excessive interocclusal space, and because these spasms re-occurred almost immediately on removal of the splint, the etiology for muscle spasms in the temporal muscles is attributed to occlusal interferences and an excessive interocclusal space. This does not mean that other factors cannot cause temporal muscle spasms.

CONCLUSIONS

  1. The behavior of skeletal muscle is a faithful index of the state of the motor nerve center (lower motor neurone).
  2. Temporal muscle spasms occur simultaneously with functional disturbances in the temporomandibular joint.
  3. Functional disturbances in the temporomandibular joint may have their etiology in occlusal interferences of the teeth.
  4. Muscle spasms disappear in the temporal muscles when occlusal interferences were removed (in our subjects with an interocclusal splint).
  5. Upon removal of the splint, muscle spasms re-occurred in the right temporal muscle.

This 1954 study by Perry & Harris at Northwestern University Dental School documented the significance of EMG in providing a physiologically valid occlusion. According to Perry the placement of dental restorations which are not perfectly adapted to the existing patterns of parts and their functional movements will cause pounding of the tooth and its restoration through the action of resisting oral musculature during occlusion.

Perry, H.T. and Harris, S.C. Role of the neuromuscular system in functional activity of the mandible. Journal of Am. Dent. Assoc., 48: 665-673, 1954.

Most anatomy books assign a particular function to each muscle, but they do not point out that each muscle may have many other functions as well. For instance, all muscles attached to the mandible have some active part in mastication, either in contracting or in relaxing. The contribution of muscle to oral activity is governed by the architecture of the jaw and muscle attachment, by occlusal relations of the teeth, by the demands of various types of food and by the innate or acquired functional pattern of the individual.
The placement of dental restorations which are not perfectly adapted to the existing pattern of parts and their functional movements will cause pounding of the tooth and its restoration through the action of the resisting oral musculature during occlusion. This pounding will continue until the units are thrust aside, the functional pattern is changed, or the parts are lost. In the words of Todd, “Mechanically, physiologically, and psychologically, the human body is compelled to struggle for a state of equilibrium.” (1937).
Application of these principles and equipment has demonstrated that structural differences, such as malocclusions (Moyers, 1950), pathologic conditions of nervous or muscular origin (Pruzansky, 1952) and the alteration of individual tooth position (Perry, 1954), produce distinctive results on myograms.

The electromyograms of Figures 1, 2, and 3 show the mastication patterns of a patient with normal occlusion; they were taken in immediate sequence and reveal the following features: (1) the relation of the pairs of masseter and temporal muscles to each other in extent and time of activity, (2) the occurrence of greater activity on the working side in mastication, (3) the qualitative uniformity of activity when a response is repeated and (4) the gradation and extent of activity with increasing load

INTERPRETATION OF MYOGRAMS

This instrument and technique were used to obtain myograms of the action of the masseter and temporal muscles of ten patients with cephalometrically and anatomically normal occlusion. The myograms, of which Figures 6 and 7 are typical, revealed the following characteristics of muscular action during the chewing of gum:

  1. As they reached maximal activity, the temporal and masseter muscles on both sides were synchronized.
  2. The temporal muscles always displayed electrical activity before the masseter muscles went into action.
  3. There was more harmony and smoothness of action-potential discharge on the side preferred for mastication.
  4. The amplitude of the myogram was in direct proportion to the resistance of the bolus being chewed.

Myograms may prove of clinical value. The following typical characteristics were observed in Class II division 1 malocclusions (Fig. 8 and 9).

  1. The temporal and masseter muscles on both sides reached maximal activity asynchronously.
  2. The masseter muscles frequently were first to manifest electrical activity.
  3. There was much less harmony and virtually no smoothness of action- potential discharge on both sides, although there was less disharmony on the preferred side.

It may be concluded that muscle has the dominating role in mastication, supplying function and resisting any degree of mechanical disharmony. The evidence, already abundant, is corroborated by electromyograms.

Figure 10, for example, is an electromyogram of the right side of the same subject shown in Figure 6. The occlusal condition of this patient was declared normal by six orthodontists who did not use electromyography in their diagnosis. The electromyograph, however, revealed that something was wrong; by further examination of the study models and of the mouth, it was found that the upper right third molar was in premature contact.

CONCLUSION
Many neuromuscular phenomena are not yet understood; future research in neuromuscular physiology will unquestionably be of great assistance in the solution of clinical problems. In the meantime, the dentist must recognize that all force in the oral cavity is produced by muscle. The number, origin and insertion of fibers and their innervation are established genetically. These are the structures most resistant to change in the entire body. Therefore, the best means of helping the patient is to restore or maintain the normal physiology of the mouth, seeking to cooperate with these structures rather than causing them to resist and to produce pathologic conditions.

More than 30 years ago, Perry at Northwestern University monitored the postural EMG activity of patients with temporomandibular disorders before and after treatment. Electromyographic evidence of spasm at rest was observed before treatment, he concluded that patients who were treated by occlusal splinting demonstrated substantially lower or non-existent postural activity after treatment.

Perry, H. T. Muscular changes associated with temporomandibular joint dysfunction. Journal of Am. Dent. Res., 54: 644-653, 1957.

Too often in their endeavors to restore oral health, dentists are oblivious of the whole patient and direct their efforts to special dental problems, the placement of a single inlay or bridge, or the orthodontic correction of the teeth into an acceptable anatomic arrangement.

In the dentist?s efforts to establish anatomic normalcy, he often forgets the principles of physiology – – “how” and “why” all of these various parts are anatomically interrelated to achieve function. Too often knowledge of basic science is eclipsed by the pragmatism of office efficiency. This shortsightedness results in a neglected service to the total needs of the patient.

Dentistry has made great strides in the fields of dental materials, improved cutting methods, local anesthetics, control of dental decay and oral rehabilitation; in short, the dentist can well manage the structures he sees. A certain percentage of patients, however, present themselves with problems of pain, malaise and poorly defined complaints of discomfort. The suffering some of these patients have endured and the future pain they may suffer when they leave the dentist?s office without an attempt by the dentist to help them may not be realized. Often the patients are conveniently classified as neurotics.

The orthodontic department of Northwestern University Dental School has long been concerned with functional problems of the occlusion and temporomandibular joints. Research studies by graduate students have been designed to enlarge the scope of understanding of the anatomy, physiology and pathology of this joint.

The electromyograph has supplied another useful adjunct in studying functional problems. When used in conjunction with the cephalometric roentgenogram and a careful clinical examination, many interesting features of oral function have been more thoroughly studied than was previously possible. A great deal of the material in this report is based on electromyographic findings of the past four years.

The material presented in this paper is derived from 42 patients treated at the Northwestern University orthodontic department?s temporomandibular joint clinic. The number of patients treated and observed in this special clinic exceeds this number many times; however, each one of these 42 patients had one or more electromyographic examinations. Another 84 instances were taken from the private practice of J. R. Thompson, because of the completeness of his records. A few of the latter patients had also been investigated with the electromyograph. The total number of patients studied was 126.

Models, cephalometric and intraoral roentgenograms, functional wax records (Thompson, 1954), pain distribution charts and complete functional examinations were obtained in each instance. Electromyographic records were taken in many instances. Tape recordings of the patient describing the subjective nature and distribution of the pain were taken of the clinic patients. In certain instances it was also desirable to examine the head of the condyloid process with temporomandibular joint roentgenograms by using the Donovan modification of the Lindblom apparatus.

PATTERN OF PAIN
Utilization of electromyographic examination in diagnosis permits an evaluation of possible involvement of muscle as related to pain. All of the patients studied in the clinic were examined electromyographically during treatment and also after the corrective treatment was administered.

When a healthy, normally innervated, voluntary muscle is at rest, there are no signs of activity recorded by surface electrodes (Hoefer, 1949). Should the slightest movement occur or the least bit of contraction take place, however, electrical discharges are “picked up” by the surface electrodes.

THE RESTING MUSCLE
When a muscle is at rest a small amount of energy is expended in maintaining a slight tension, referred to as “tonus.” Tonus is like a “continual alertness” which keeps the muscle constantly prepared to act. The activity of tonus is maintained by a few widely separated and deeply located motor units. With surface electrodes this electrical activity is not recorded, because only a few motor units are randomly active in dispersed regions within the entire muscle.

Therefore, a healthy resting muscle displays no evidence of innervation recordable by surface electrode and an electromyographic record from one of these resting muscles shows only a straight base line when the amplification level is set at a gain of 4 as was used in these studies.

In patients with occlusal disharmonies and temporomandibular joint dysfunctions, the absence of a true electromyographic resting pattern from the muscles of mastication has been noted. The temporal and masseter muscles, when more severe dysfunction is present, constantly exhibit a low grade electrical discharge even when the mandible is supposedly in the physiologic rest position. Certain findings with needle electrodes indicate that this is also true for the external and internal pterygoid muscles. This sustained electrical discharge and absence of rest is characteristic of muscle spasm which is due to an overactive neural discharge.

In all of the patients studied electromyographically, the pattern of muscle spasm closely followed the topographic distribution of subjectively recognized pain. This finding presented a definite basis for the dull, aching type of pain which was so characteristic in these patients.

In many of the earlier patients in the clinic it was noted that with the correction of the occlusal problems and the cessation of the temporomandibular symptoms of clicking, crepitus and pain, the electromyogram of the resting muscle no longer showed the electrical evidence of spasm. In other words, the “resting” muscles which had previously showed a spasm activity of electrical discharge, after treatment caused no base-line deflection of the pens and resembled a healthy electromyographic pattern of resting muscle. This seemed, in itself, some basis for assuming that the pain was related directly to the dysfunction. The cause and the effect seemed present but between there must be a connecting link. This was to be found in the realms of anatomy and physiology.

NEUROANATOMIC BASIS OF MANDIBULAR MOVEMENT
In those patients possessing only minor premature contacts or few occlusal irregularities, adjustment occurs as a normal process when occlusal conditions present many severe functional interferences in mandibular movements; however, the inconstancy and irregularity of the stimuli accompanying function produce a continual bombardment within the mesencephalic nucleus. This constant discharge of sensory impulses is relayed to synapses in the motor nucleus with the motor cells of the trigeminal nerve. The impulses then travel peripherally in the motor nerves to the muscles responsible for mandibular guidance and posture. The various components of each muscle attempt to guide the mandible into occlusal positions of least interference. In time a position of occlusion may be found at which the protective stimuli are minimal. Then the musculature strives to relocate this position with each occluding movement. This position of least occlusal interference is found and maintained at the expense of normal muscle function.

The musculature as a result of the continual overwork and guidance becomes fatigued and metabolic products accumulate. With these metabolic products the sensory endings for pain in these muscles are finally stimulated and a conscious recognition of localized pain results.

In the clinical treatment of these patients by occlusal splinting or occlusal equilibration in which interfering restoration or tooth structure is removed, the musculature is again permitted to carry out its action unhindered by excessive protective impulses. This is readily proved by the electromyographic recordings on occlusal and temporomandibular joints after correction. A few days after occlusal relief, the spasm in rest position is diminished, and in a week, if all premature contacts are relieved, the spasm and pain are gone and the rest position becomes constant.

In these instances wherein the proprioceptive stimuli of a poor occlusion constantly signal the mandibular musculature to find and maintain a position of least interference there is little opportunity for division of labor. With reflex guidance of the mandible fatigue in one segment of a muscle or muscle group is soon apparent.
This resulting fatigue eventually sets up the afore-mentioned spasm activity electromyographically noted at rest. With this slight degree of muscle contraction at rest, the posture of the mandible is upset. A true physiological rest position of the mandible no longer exists. The body, however, attempts to compensate for this imbalance by calling into action its second line of muscular reserves. Just as increased load values on one side of the body results in contraction of other distant balancing muscles, so do the postural and functional disturbances of the mandibular musculature. In this action the postcervical muscles are called into play in a distant attempt to establish balance in the function of the masticatory muscles.

This assisting role by certain of the postcervical muscles may result in temporary relief to the mandibular musculature. If, however, the offending parts of the occlusion are not corrected, the mandibular musculature and postcervical musculature (or parts of each) will soon present signs of over work, fatigue, spasm and pain. .

RESULTS OF TREATMENT
In 118 of the 126 patients in this study the pain was relieved by either occlusal equilibration or occlusal splinting. The remaining eight patients were relieved of some of the pain but have not, as yet, been completely relieved. In every instance wherein electromyographic evidence of spasm at rest was noted before treatment, the final electromyogram indicated a near normal or normal picture of rest that is, an absence of electrical activity at rest.

Yemm at the University of Bristol in England presents a neurophysiologic overview of temporomandibular joint dysfunction that supports many of the clinical neuromuscular protocols and their underlying neuromuscular basis. After an extensive review of the literature, and studying the EMG of a group of normal subjects and a group of dysfunctional patients, Yemm concludes, “There is an increasing weight of evidence that hyperactivity of jaw closing muscles may originate in the central nervous system. It is concluded that such centrally induced activity may be sufficient to cause muscle damage, which leads to disturbed function, local pain and tenderness and to pain referred to adjacent structures. The results demonstrate a clear difference between [the EMG of] the normal group and the patient group.”
Yemm, R. Neurophysiologic studies of temporomandibular joint dysfunction. Oral Sci. Rev., 7: 31-53, 1976.

INTRODUCTION
The clinical condition now commonly known as temporomandibular joint dysfunction was described, and interest aroused in it as a clinical entity, by the work of Costen, whose first report was published in 1934. It was considered at this time that the cause was pressure of the mandibular condyle upon related structures such as nerves, permitted by overclosure of the mouth. The overclosure hypothesis has since given way to the concept that other defects in the dental occlusion are the primary cause. More recently still, the emphasis has been changing once again, this time to the consideration of abnormalities of muscle and muscle activity as being responsible.
The purpose of the present paper is to review the neurophysiological evidence in favor of these hypotheses.

EVIDENCE THAT MUSCLE HYPERACTIVITY CAN CAUSE DYSFUNCTION
As Costen?s ideas have been questioned, and generally found unacceptable, alternative hypotheses have been advanced. Two fundamentally different suggestions have been made. The first is that defects in the dental occlusion initiate or necessitate sufficiently abnormal muscle activity to cause dysfunction. The second, and more recent, is that muscle hyperactivity, not necessarily in the presence of occlusal abnormality, is the primary factor.

Common to both hypotheses is the concept that the dysfunction condition arises from hyperactivity of one or more of the jaw muscles. It is reasonable to examine the validity of this argument, without for the moment considering the cause of the hyperactivity.

A frequent clinical observation has been that many patients with temporomandibular joint dysfunction exhibit a tendency to clench or grind their teeth (e.g. Franks, 1965, Weinberg, 1974). Such observations are of necessity subjective, depending upon both the patient?s awareness and upon the clinician?s definition of what constitutes abnormality. Nevertheless it is proposed that this hyperactivity initiates and maintains the clinical condition. The validity of the clinical impression has been tested by Vestergaard Christensen (1967, 1971).

In two studies, normal adult subjects were asked to grind their teeth for a period of 30 minutes. Obviously, such voluntary hyperactivity has a different origin from that of the nocturnal, unconscious tooth grinding of bruxism, but it is reasonable to assume that its effects upon the system might be similar. All but one of Vestergaard Christensen?s subjects in the later experiments experienced pain or discomfort in the period of time following the voluntary hyperactivity. The site, nature and duration of the symptoms described varied from subject to subject. His results are summarized in Fig. 1. The distribution of pain sites seems very similar to the varied reports of patients with temporomandibular dysfunction. The relatively short duration of the experimentally induced signs and symptoms is obviously different from that seen in the majority of patients. However, the hypothesis demands that the dysfunction patient be a habitual tooth clencher or grinder, and a limited duration of pain would be expected following a single relatively brief period of hyperactivity.

This evidence constitutes the main experimental support for the clinical contention that dysfunction is caused by muscle hyperactivity. Studies of the muscle activity of dysfunction patients, the majority employing electromyographic techniques, are less conclusive, since in many it is not possible to determine whether any demonstrated hyperactivity is causative or a consequence of the dysfunction. Reports of this nature will be considered in a later section.

The suggestion that muscle hyperactivity can lead to muscle pain is not limited to jaw muscles. Swanson (1971) considers that muscle pain with this origin is common. Examples quoted are occupational muscle pain in typists and writers. In addition Wolf (1972) has discussed the involvement of muscle hyperactivity and muscle pain in some types of headache.

EVIDENCE OF CENTRALLY INITIATED HYPERACTIVITY
The absence of scientific support for the theory that temporomandibular joint dysfunction is due to hyperactivity initiated by occlusal defects has led, together with observations of the type of patient most often subject to dysfunction, to the proposal by numerous authors that hyperactivity of the jaw muscle originates centrally in the nervous system (e.g. Copland, 1954, Kydd, 1959, Franks, 1965, Berry, 1967, Newton, 1969, Weinberg, 1974). It is suggested that psychological or physical stress experienced by the individual leads to an increase in muscle activity of the jaw muscles, and that this contributes to, if not actually causes, the onset and maintenance of dysfunction.

The phenomenon of increased muscle tension in the human under stress is well documented. Increases in activity of a number of muscles have been shown to occur during both mental and physical stress. For instance Malmo and Shagass (1949) showed increases in activity of some neck muscles when radiant heat was applied to the forehead. Goldstein and co-workers (Goldstein et al, 1964, Shipman et al, 1964, Goldstein, 1965) have shown that activity of several muscles, including biceps, frontalis, trapezius and some neck muscles, can increase during interviews by a psychiatrist, the response being dependent upon the character of the interview.

Using the method of inducing stress already described, which permits repeated exposure within one experimental session, three groups of subjects were examined. The first group was composed of ten normal subjects, the second of ten dysfunction patients, and the third of ten subjects for whom treatment of dysfunction had been completed. The three groups, who were matched as far as possible for age and sex distribution, provided a means of testing whether any difference detected between normal subjects and patients was also present when the clinical condition had been resolved. As in the previously described studies of stress induced activity in normal subjects, electromyography was used to determine muscle activity changes. The signals were tape recorded for subsequent analysis using integration.

In each of the 30 subjects tested, recording of muscle activity was carried out for one masseter muscle for four attempts at the task. A brief rest was permitted between each two minute exposure to the task. At the time of testing, several of the dysfunction group exhibited tenderness of one masseter, in which case the recordings were taken from the clinically normal contralateral muscle. Each subject was given a short practice period, and no mention was made of the fact that the surface electrodes were sampling muscle activity.

The results demonstrate a clear difference between the normal group and the patient group. The latter consistently failed to demonstrate the progressive relaxation shown by the normal subjects. Fig. 7 is a typical individual response from the patient group for comparison with that of a normal subject (Fig. 6). Of the subjects with a previous history of dysfunction, only three showed any evidence of a progressive reduction in response. The responses of the three groups are shown in Fig. 8.

A number of reports have been made to the effect that the jaw muscles of temporomandibular joint dysfunction patients are hyperactive. In some instances it is reported that this activity is continuous, and considered to be a feature of the clinical condition (Jarabak, 1956, Perry, 1957, Griffin and Munro, 1971, Chaco, 1973). Others indicate the hyperactivity diminishes at times when, in the light of the observations of stress induced muscle activity, the patient is encouraged or permitted to relax (Kydd, 1959, Franks, 1965, Moller et al, 1971). Some of these studies are without control samples of normal subjects; the implication of some that muscle hyperactivity is characteristic of the jaw closing muscles in dysfunction patients will be examined in a subsequent section.

THE ACQUIRED PATHOLOGY
Recognizing the frequent muscle involvement in the generation of the signs and symptoms observed, it has been suggested that the problem is one of muscle spasm. Spasm of skeletal muscle is usually defined as uncontrolled muscle contraction of a brief or maintained character (Rushworth, 1961). The existence of muscle spasm in the jaw muscles of patients with dysfunction is suggested by a number of authors.

Travell (1960) considers that the spasm arises from trigger areas within the affected muscle, stimulation of which results in maintained activity. Several studies have advanced evidence that continuous activity, claimed to be spasm, is a feature of the muscles of dysfunction patients (Jarabak, 1956, Perry, 1957, Griffin and Munro, 1971, Chaco, 1973). However, others report that although hyperactivity was observed, this reduces if the patient is permitted or encouraged to relax (Kydd, 1959, Franks, 1965, Moller et al, 1971, Yemm, unpublished observations). The presence of continuous, spastic activity in the jaw muscles must be considered doubtful, therefore.

Evidence suggesting that continuous spasm is unlikely to exist is provided by studies of jaw movements. A frequent feature of dysfunction is the existence of a restriction of mouth opening or trismus.

SUMMARY
There is an increasing weight of evidence that hyperactivity of jaw closing muscles may originate in the central nervous system. It is concluded that such centrally induced activity may be sufficient to cause muscle damage, which leads to disturbed function, local pain and tenderness and to pain referred to adjacent structures.

The influence of afferent proprioceptive input from occlusion causes an asymmetry of muscle activity according to Bakke and Moller. Bilateral bipolar surface electrodes were the mode of choice. The authors, in a study at Royal Dental College in Copenhagen, demonstrated that “biting in the intercuspal position with unilateral premature tooth contact causes asymmetry of the elevators, with strongest activity on the side of interference” and “a reduction of mean voltage on both sides with increasing height of the overlay (prematurity)”.

Bakke, M., & Moller, E. Distortion of maximal elevator activity by unilateral premature tooth contact. Scand. J. Dent. Res., 80: 67-75, 1980.

In four subjects the electrical activity in the anterior and posterior temporal masseter muscles during maximal bite was recorded bilaterally with and without premature unilateral contact. Muscle activity was measured as the average level and the peak of the mean voltage with layers of strips of 0.05. 0.10, 0.15 and 0.20 mm. placed between first molars either on the left or the right side, and compared with the level activity with undisturbed occlusion. Unilateral premature contact caused a significant asymmetry of action in all muscles under study with stronger activity ipsilaterally. With increasing thickness of the overlay, the mean voltage decreased in parallel on both sides. We suggest that the asymmetry was caused by a larger spindle afferent activity on the ipsilateral as compared to the contralateral side, and that the all-over decrease of muscle activity was due to gradual reduction of activity from periodontal pressoreceptors.

The present study deals with the influence of a unilateral premature contact on muscle activity during maximal bite in the intercuspal position. It was the aim to explore the immediate response of the temporal and masseter muscles to such contacts located on first molars.

SUBJECTS
The experiments included one female and three males, 20-35 years of age, with complete support in the lateral sections of the dentition, and without neuromuscular disorders of functional disorders of the temporomandibular joint.

RESULTS
Unilateral premature contact caused a marked asymmetry between the action of ipsilateral and contralateral muscles during maximal bite. Compared to control values (undisturbed occlusion) the general tendency of elevator activity with strips inserted was an increase on the side of the premature contact and decrease on the opposite side. However, with increasing thickness of the overlay the ipsilateral activity returned to the level of control, while the mean voltage in the contralateral muscle continued to decline.
Anterior temporal muscles – In terms both of the average level and of the peak of the mean voltage, differences between the activity in the ipsilateral and the contralateral muscles were highly significant (P<0.01), irrespective of the thickness of the overlays. The increment of muscle action on the side of the premature contact was marked in terms of peak values with one layer (P<0.001), two layers (P<0.01), and three layers (P<0.05), but not with four layers while the average level did not change significantly. Contralateral, reduction during the experimental conditions was more pronounced in terms of the average level (P<0.01 and P<0.001) than of the peak.

Masseter muscles – In spite of larger numerical differences, premature contact was reflected less in the activity in the masseter muscles as compared to the other muscles under study. Premature contact caused a marked difference between the overlay-side and contralateral muscles (P<0.01 to P<0.05), but the average level of ipsilateral activity did not differ significantly from the control level at any number of layers, and peak values were only enhanced with 0.05 and 0.10 mm (P<0.05). Contralaterally, the average level of the mean voltage decreased with one, three and four strips P<0.05), while peak values did not differ significantly from control, irrespective of the number of strips inserted.

DISCUSSION
From electromyographic studies of patients with functional disorders of the chewing apparatus (7, 9, 11, 12, 20, 21) and subjects in whom function was impaired by artificial premature contact (22),it has been inferred that occlusal interferences disturb muscle function. However, the initial influence of such interferences on muscle coordination has not been specified. The present study has demonstrated that biting in the intercuspal position with unilateral premature tooth contact causes asymmetric action of the elevators, with strongest activity on the side of the interference.

The significant findings of the present study were 1) asymmetry of muscle activity on the side of premature contact and, with this asymmetry maintained, 2) a reduction of mean voltage on both sides with increasing height of the overlay. A comparison with data obtained during unilateral contact at larger jaw separation emphasizes these two features.

In a controlled longitudinal study at the Department of Stomatognathic Physiology at Karolinska Institute in Stockholm, Riise and Sheikholeslam analyzed the influence of an intercuspal occlusal interference that was experimentally introduced in 11 normal subjects. All subjects had complete natural dentition without any symptoms of functional disorders.

Riise and Sheikholeslam?s thorough study of quantitative EMG on the temporalis and masseter muscles of the subjects resulted in three important papers that were published in the Journal of Oral Rehabilitation in 1982, 1983 and 1984. Using extensive statistical tests, these studies concluded that:

1] “Experimental occlusal interferences similar to those often produced in the daily dental practice of occlusal rehabilitation, such as fillings, crowns, and bridges affect the neuromuscular pattern of postural activity in the mandibular elevators at rest. The pattern of postural activity is influenced sometimes as early as one hour after the insertion (of interference). After 48 hours, there was a significant increase of the activity in the anterior temporal muscles. This increased activity persisted (up to one week) after the interference was removed.”

2] “A single, small occlusal interference is able to disturb the almost symmetrical pattern of muscular activity. This was especially evident during submaximal bite. After the application of the interference, the maximal (bite) muscular activity was reduced significantly in all muscles under study.”

3] “Disturbance of the occlusion by a small, experimental interference in the intercuspal position significantly changes the timing and the level of muscular activity during mastication, at least over the experimental period (one week). Furthermore, there was also a change towards a unilateral chewing pattern, which, if persisting over the years, has been shown to evoke sequalae in the masticatory system. The importance of occlusal stability is further emphasized by the fact that four (of eleven) subjects broke off participation in the investigation because of pain in jaw muscles and/or temporomandibular joints caused by the interference.”

Riise, C., and Sheikholeslam, A. The influence of experimental interfering occlusal contacts on the postural activity of the anterior temporal and masseter muscles in young adults. J. Oral Rehabilitation, 9: 419-425, 1982.

SUMMARY
The effects of an intercuspal occlusal interference on the pattern of postural activity of the anterior temporal and masseter muscles were studied in eleven volunteers with complete, natural dentitions. The results indicate that, in man, there is postural activity in the anterior temporal and sometimes the masseter muscles. The pattern of postural activity is influenced by the occurrence of an experimental occlusal interference, sometimes as early as 1 hour after the insertion. After 48 hours there was a significant increase of the activity in the anterior temporal muscles. This increased activity persisted until the interference was removed 1 week later and had almost disappeared 1 week after the removal.

INTRODUCTION
In subjects with unilateral cross-bite, Troelstrup & Moller (1970) and Moller & Troelstrup (1975) demonstrated that the postural activity in the posterior temporal muscle tended to predominate on the side of the cross-bite, and this tendency was interpreted as an adaptation to the cross-bite. Ingervall & Thilander (1975) have confirmed these observations. In patients with functional disorders in the chewing muscles, the level of the postural activity was found to be increased especially in the anterior temporal muscle (Lous, Sheikholeslam & Moller, 1970). Deliberate relaxation in the sitting position with a headrest had little effect in such patients (Moller, Sheikholeslam & Lous, 1971) though in the supine position, the activity in the anterior temporal muscle was reduced virtually to the same level as that in relaxed normal subjects.
The purpose of the present study was to investigate whether there is any postural activity in the mandibular elevators at rest and, if there is such an activity, what effect an occlusal interference can have on the neuromuscular pattern.

MATERIALS AND METHODS
Eleven male dental students, aged 24-32 years, volunteered for this study. they were selected according to the following criteria: (1) with complete natural dentitions, (2) without any symptoms of functional disorders in the stomatognathic system The surface electrode was chosen in preference to the needle electrode to avoid pain and anxiety effects on muscular tension. Furthermore, the surface electrode is able to sample the activity from a greater number of units (Yemm, 1977). The postural activity was recorded in the anterior temporal and masseter muscles in alert subjects, sitting upright, with heads unsupported and with eyes open.

The artificial occlusal interference was inserted each time by the same operator. The interference was a small, rounded, and 0.5 mm high amalgam filling on the maxillary right first molar (the mesial facet of the disto-buccal cusp).

CLINICAL COMMENTS
hree hours after insertion of an occlusal interference, seven subjects complained of pain, tenderness and fatigue mostly in the elevator muscles. Psychologically, most of the subjects felt that the filling caused them to be under constant nervous tension and reported awareness of bruxism. Two knew that they tried to avoid the occlusal interference. In eight subjects, symptoms of functional disorders developed within less than 12 hours, and shiny facets appeared on the surface of the amalgam interference.

RESULTS
The alert pattern of postural activity before the insertion of an occlusal interference showed that there were some active tonic units in the anterior temporal muscle (Fig. 1). Some of these units were able to continue firing for more than 15 mm without any sign of fatigue. In the masseter muscle, postural activity was usually absent, but occasionally there was slight tonic activity (four subjects).

One hour after the insertion of an occlusal interference, there were no significant changes in the pattern of the postural activity except in two subjects, who showed increased activity in the anterior temporal muscle of one side. After 48 hours, the increase of the postural activity was significant (t:P<=O.O1) in the right and left anterior temporal muscles (Fig. 2, Table 1). In the masseter muscles there was no significant change except in four subjects with an increased postural activity.

One week after the insertion of the interference there was still a significant (Table 2) increase of postural activity in either the right or the left anterior temporal muscle while there was no significant change in the masseter muscles. Immediately after the removal of the remaining interference there was no significant response in the pattern of postural activity. One week later the postural activity had returned almost to its original pattern in all subjects.

DISCUSSION
After removal of the remaining occlusal interference, there was no immediate response in the pattern of the postural activity and it took about a week for adaptation to a new posture, thus, the fusimotor drive can be a possible cofactor for postural hyperactivity.

After about an hour, in two subjects the early response to occlusal interference appeared as asymmetrical hyperactivity in either the right or the left temporal muscle. This early. asymmetrical spontaneous postural hyperactivity probably depended on an attempt to shift the position of the mandible to avoid the interference. Within 48 hours, the postural hyperactivity increased in the anterior temporal muscles of eight subjects, and in four of these even in the masseter muscles. As the subjects were instructed not to keep their teeth in contact during the recordings, such contact could no initiate the hyperactivity. Moreover there was no significant change in the pattern of postural activity immediately after removal of the occlusal interference. It seems that the hyperactivity shown in the present investigation may have been the result of increased fusimotor drive caused by stimuli from the occlusal interferences. In the long run, the hyperactivity may be followed by structural adaptation such as teeth movements, muscular reactions and remodelling of the temporomandibular joints or may lead to pathological changes in the masticatory system.

CONCLUSION
The present results indicate that there is postural activity in the anterior temporal and sometimes in the masseter muscles, and that experimental occlusal interferences similar to those often produced in the daily dental practice of occlusal rehabilitation, such as fillings, crowns, and bridges effect the neuromuscular pattern of postural activity in the mandibular elevators at rest.

Sheikholeslam, A., and Riise, C. Influence of experimental interfering occlusal contacts on the activity of the anterior temporal and masseter muscles during submaximal and maximal bite in the intercuspal position. J. Oral Rehabilitation, 10: 207-214, 1983.

SUMMARY
The effects of an intercuspal occlusal interference on the pattern of activity of the anterior temporal and masseter muscles during submaximal and maximal bite, were studied in eleven volunteers with complete, natural dentitions.

The results show that, during maximal and submaximal bite the occlusal interference (about 0.5 mm) in the intercuspal position is able to disturb the almost symmetric pattern of muscular activity in the anterior temporal and masseter muscles. Further, the level of muscular activity during maximal bite decreased significantly in all muscles studied. In some subjects, the decrease of muscular activity could still be observed one week after insertion of the interfering contact. After eliminating the interference, the muscular coordination pattern improved and the level of muscular activity increased significantly.

INTRODUCTION
The static bite force of the masticatory muscles was investigated as early as the 17th century.

Mechanical pressure applied to the teeth is known to stimulate periodontal receptors (Pfaffman, 1939; Ness, 1954). These receptors have been considered by some authors to play a limiting role in maximal bite force (Black, 1895; Schroeder, 1927; Worner & Anderson, 1944; Steenberghe & De Vries, 1978), while others have reported a positive feedback on the degree of elevator muscle contraction (Czeche, 1968; Arnold, 1972; Lund & Lamarre, 1973)

There is evidence that the maximal bite force (Molin, 1972; Helkimo, Carlsson and Carmeli, 1975) and the electrical muscle activity during maximal bite in the intercuspal position (Sheikholeslam, Moller and Lous, 1980) are significantly weaker in patients with functional disorders of the masticatory system than in controls without such disorders. In a short-term clinical and electromyographic study of the effects of an intercuspal interference (gold-inlay) in one of the mandibular first molars, Randow et al. (1976) reported that the interfering contact immediately disturbed the coordinated pattern of muscular activity in the elevators during biting with maximal effort in the new intercuspal position. Furthermore, all subjects but one showed signs and symptoms of functional disorders of the masticatory muscles during the 2-week experimental period.

The purpose of the present study was to investigate quantitatively the short- term effects on the pattern of muscular activity of an experimental occlusal contact interfering in the intercuspal position during submaximal and maximal bite.

MATERIALS AND METHODS
Eleven males, aged 24-32 years, with complete natural dentitions, excellent periodontal status and without signs and symptoms of functional disorders, volunteered for the present investigation. The electromyographic methods have been described previously (Riise & Sheikholeslam, 1982). The electrical activity in the anterior temporal and masseter muscles was recorded bilaterally, using surface electrodes, during: (1) submaximal bite in the intercuspal position (i.e. a continuous EMG recording of the gradually increasing muscular activity from the mandibular rest activity to almost 20% of the full effort of the right anterior temporal muscle- -„reference muscle?); (2) maximal bite in the intercuspal position (i.e. hardest possible clenching for 1 second and relaxation for 3 mm between each clenching to avoid muscular fatigue).

The muscular coordination and correlation pattern during submaximal bite was evaluated semiquantitatively, while the full effort activity was calculated as the average of the electromyographic mean voltage of four recordings EMG recordings were performed before, 1 hour, 48 hours, and 1 week after the insertion of the interfering filling, as well as before and immediately after elimination of the interference. After 1 week or more, final control recordings were made. In the statistical evaluation the difference between means of distributions were matched using the Student?s t-test for paired comparisons.

CLINICAL FINDINGS
In less than 12 hours following the insertion of the interfering amalgam filling, signs and symptoms of functional disorders had developed in eight subjects. Within a week the occlusal interference was reduced in height and the symptoms gradually subsided. However, in four subjects, clinical examination revealed that moderate signs of functional disorders persisted which disappeared within a week after eliminating the interfering filling.

RESULTS
The immediate EMG recordings after the insertion of the interfering filling showed that in eight subjects, there were two or more different patterns of muscular activity during submaximal and maximal bite in the altered intercuspal position. During submaximal bite the occlusal interference disturbed the almost symmetrical and coordinated pattern of muscular activity in the anterior temporal and masseter muscles in nine subjects. The electrical activity during the maximal bite in the altered intercuspal position was significantly reduced (P<0.001) in all muscles studied.
Forty-eight hours after inserting the occlusal interference, in seven subjects, the EMG recording during submaximal bite still showed lack of coordination and symmetry in the activity pattern of the anterior temporal muscles and in three subjects also of the masseters. The electrical activity during maximal bite showed a significant decrease (P<0.01) in all the muscles studied when compared to the activity before the insertion of an occlusal interference.

After one week, in four subjects, the pattern of EMG during the submaximal bite showed that, still, there was some asymmetry and lack of coordination in the activity of the temporal muscles. The electrical activity during the maximal bite was significantly decreased in the right anterior temporal (P<0.001) and the right masseter muscles, (P<0.05).
Immediately after removal of the remaining occlusal interference the earlier asymmetric submaximal EMG pattern became more symmetric, and furthermore, the electrical activity of the maximal bite in the intercuspal position increased significantly. However, when the EMG at maximal bite was compared, directly after the removal, to that before insertion of the occlusal interference, the results still showed a significantly reduced electrical activity in the right anterior temporal and masseter muscles (size of occlusal interference). One to four weeks after the elimination of the interfering occlusal contact, in all but one subject, the EMG of maximal bite in the intercuspal position showed no statistical difference when compared to that before insertion of the experimental interference.

DISCUSSION
In the present investigation, the disturbance in coordination and correlation of elevator muscle activity during the maximal bite after insertion of an interfering amalgam filling almost disappeared after the elimination of the interference.

The most striking result of the present study was finding that a single, small occlusal interference is able to disturb the almost symmetrical pattern of muscular activity. This was especially evident in the anterior temporal muscles during submaximal bite.

After application of the interference, the maximal muscular activity was reduced significantly in all muscles under study.

There are several assumptions which could be advanced partly to explain the incoordinating effects of an occlusal interference on the muscular contraction pattern of maximal bite and the reduction of the level of activity:

(1) It may be the effect of an increased negative feedback from high threshold mechanoreceptors and nociceptors in or around the teeth (Anderson & Picton, 1958) with the occlusal interference.

(2) It may be due to a reduction of the number of teeth in contact in the altered intercuspal position, as Steenberghe & De Vries (1978) reported that there was a positive correlation between the maximal bite force which can be reached and the number of teeth in contact. Since the maximal bite force has to be distributed over a smaller number of teeth in contact, the high threshold mechanoreceptors and nociceptors in and around these teeth will be stimulated more easily, thus giving a negative feedback to the mandibular elevator motoneurons. On the other hand, assuming that the periodontal receptors have a facilitating effect on the motoneurons of the jaw closing muscles via cortical level (Lund & Lamarre, 1973), the degree of positive feedback would be reduced with a less number of teeth in contact.

(3) It may be due to the discrepancy between the direction of the resultant of the bite in the altered intercuspal position and the former forces acting on the teeth. As a result of altered contact positions, the teeth may be pushed in a different direction by forces different to those for which the actual periodontal receptors have been programmed and, consequently, their feedback may be changed. By recording the activity from the inferior alveolar nerve in man, Johansson & Olsson (1976) reported that a mechanical stimulus directed distally or lingually on a lower premolar evoked a vigorous response whereas forces applied in the mesial and buccal direction did not. Further, they found that lingually and axially directed stimuli were the most efficient to excite this tooth.

(4) Displacement of the condylar position may also have changed the feedback to the elevator muscles, as it was reported by Abe, Takata & Kawamura (1973) that the receptors in the TMJ are able to modify the stimulation threshold of the motoneurons of the masticatory muscles.

(5) Psychological factors, such as fear of pain and fracture of teeth may have played some minor role in the reduction of muscular activity. However, the subjects were instructed and encouraged to clench as hard they ever could.

The increase of muscular activity after removal of the experimental occlusal interference was significant. The explanation may be that the factors which had increased the inhibitory feedback during maximal bite, had been reduced. However, the level of muscular activity on the right side (interference side) did not return to the pre-experimental level immediately after removal of the interference. This reduction of muscular activity may be due to inhibition arising from pain in or around the teeth which interfered, and/or muscular fatigue, spasm caused by development of functional disorders and postural hyperactivity (Riise & Sheikholeslam, 1982).

The results of this study are in line with Bakke & Moller (1980), who demonstrated that unilateral premature occlusal contacts (by using layers of celluloid strips), as small as 0.05 mm, caused a significant asymmetry of action in the temporal and masseter muscles during the maximal bite. Furthermore, by increasing the thickness of the premature contact the muscular activity decreased in parallel on the ipsi- and contralateral sides.

CONCLUSION
The results of the present study suggest that, most probably, the periodontal receptors in the man are able to modify the pattern of muscular activity of the anterior temporal and masseter muscles during submaximal and maximal bite. Furthermore, disturbances of the input of the periodontal receptors caused by an occlusal interference in the intercuspal position may lead to functional disorders in the stomagnathic system.

Riise, C. and Sheikholeslam, A. The influence of experimental interfering occlusal contacts on the activity of the anterior temporal and masseter muscles during mastication. J. Oral Rehabilitation, 11: 325-333, 1984.

SUMMARY
Quantitative electromyography (EMG) was used to study, in eleven volunteers with complete, natural dentitions, the effects of an experimental intercuspal occlusal interference on the pattern of activity of the anterior temporal and masseter muscles during mastication.

The results show that a small occlusal interference (about 0.05) in the intercuspal position can change the coordination of muscular activity during mastication. In general, there was a prolonged contraction time as well as a reduction of the activity in all the investigated elevators, especially on the side of the interference. Furthermore, after 48 h several subjects preferred to chew unilaterally. After removal of the interference, the pattern of coordination of muscular activity returned almost to the pre-experimental pattern within 2 weeks.

INTRODUCTION
Human mastication is considered to be an event where the rhythm is generated by a brainstem „pattern generator? as described by Dellow & Lund (1971), Lund (1976), Nakamura et al. (1976) and Dubner, Sessle & Storey (1978). The generator can be modified by inputs from a variety of central as well as peripheral sites. This partly explains the difficulties in interpreting the results obtained by different observers using different methods concerning the effects of occlusal interferences on the mastication. Such effects on human mastication have been studied by observing changes in the masticatory system after either experimentally inducing occlusal interferences (Anderson & Picton, 1958; Schaerer, Stallard & Zander, 1967); De Boever, 1969; Hannam et al., 1977; Hannam & Lund, 1981), or removing natural interferences, in the latter case in healthy individuals as well as in patients (Ramfjord, 1961; Hannam et al., 1977; Ingervall & Carlsson, 1982 and Moller, Sheikholeslam & Lous, 1984).

One of the main goals in the field of stomatognathic physiology has been to understand the interaction between muscular activity and the interocclusal relationship during mastication. The muscular activity during natural chewing has been studied quantitatively in detail by Moller (1966) who found the principal feature of the chewing pattern to be: lead in time of the temporal muscle and predominance in strength of the masseter muscle on the side of the chewing.

The purpose of the present study was to investigate quantitatively the short- term effects of an experimental occlusal interference in the intercuspal position on the pattern of muscular activity during mastication.

MATERIALS AND METHODS
Eleven males, aged 24-32 years, with complete natural dentitions, excellent periodontal status and without signs and symptoms of functional disorders, *volunteered for the present investigation. The electrical activity in the anterior temporal and masseter muscles was recorded bilaterally, using surface electrodes. The electromyographic, methods have been described in detail previously (Moller, 1966, 1974; Riise & Sheikholeslam, 1982).

The patterns of elevator activity during natural chewing (apple) were measured quantitatively, with onset of the activity in the right anterior temporal muscle as reference. Onset and cessation of activity were measured directly on the electromyograms, while the tracing of the mean voltage was used to measure the time course to 50% maximal mean voltage ascending (MV1), to maximal mean voltage (MV2), and to 50% maximal mean voltage descending (MV3).

The experimental intercuspal interference was a rounded amalgam filling, 2 mm in diameter and about 0.05 mm in height, inserted on the maxillary right, first molar on a pre-existing amalgam filling.

CLINICAL COMMENTS
Within less than 12 hours following the insertion of the interfering amalgam filling, signs and symptoms of functional disorders had developed in eight subjects, and after 48 hours in nine subjects. Four of these withdrew from the investigation after 48 hours because of their symptoms, and the interferences were removed. In the remaining five subjects with signs and symptoms the amalgam interference was somewhat reduced in height and the symptoms gradually subsided. Clinical examination, however, revealed that moderate sign of functional disorders still persisted in four subjects at the end of the experimental period, but disappeared within a week of eliminating the interfering filling.

RESULTS
Before insertion of the occlusal interference the EMG-pattern of muscular activity showed that all individuals but one chewed on the right and left side as well as bilaterally.

DISCUSSION
The results of the present study demonstrated, in general, that an occlusal interference as small as about 0.05 mm (inserted in the intercuspal position in a pre-existing amalgam filling) can influence the pattern of muscular coordination during natural mastication, and can reduce the intensity and prolong the duration of muscular activity.

CONCLUSION
Disturbance of the occlusion by a small, experimental interference in the intercuspal position significantly changed the timing and the level of muscular activity during mastication, at least over the experimental period (1 week). Furthermore, there was also a change towards a unilateral chewing pattern, which, if, persisting over the years, has been shown to evoke sequalae in the masticatory system. The importance of occlusion in the investigation of pain in jaw muscles and/or temporomandibular joints caused by the interference is apparent.

Kydd et al., in a controlled 1986 study induced subjective pain and fatigue in 30 healthy patients as a result of unilateral isometric biting with a uniform force of 55 N, a level of force that caused discomfort and pain. The authors concluded that “In the subjects with contralateral pain discomfort, EMG evaluation demonstrated that integrated EMG activity on the non-stressed
contralateral side was twice that of the ipsilateral side where the force was applied”.

Kydd, W.L., Choy, E., & Daly, C. Progressive jaw muscle fatigue and electromyogram activity produced by isometric uniItera1 biting. The Journal of Craniomandibular Practice, (4)1: 18-21, 1986.

The purpose of our study was to quantify the duration of force required during unilateral biting to produce the onset of subjective fatigue and pain in the masseter and anterior temporalis muscles of healthy adult female subjects. We defined pain-fatigue as the appearance of an initial, intense discomfort in one or more jaw muscles after the onset of unilateral biting or clenching.

MATERIALS AND METHODS
The 30 human subjects were healthy female volunteers. The mean age of the subjects was 28 years. with a range from 19 to 43 years. All the subjects were clinically examined by careful palpation of the jaw muscles and the TMJs and all were symptomatic.

RESULTS
In 10 of the test subjects who had complained of pain on the contralateral side and on whom EMG recordings were made, we found the mean integrated EMG in the masseter to be 90% greater on the contralateral side than on the ipsilateral side in all 10 subjects. This increase occurred on the contralateral side whether biting first took place on the right or left side.

SUMMARY
As a result of unilateral isometric biting with a uniform force of 55 N, discomfort and pain occurred. The localization of discomfort and pain occurred primarily on the side contralateral to the force application and near the origin of the masseter, slightly inferior in the zygomatic arch. In the subjects with contralateral pain discomfort, EMG evaluation demonstrated that integrated EMG activity on the nonstressed contralateral side was twice that of the ipsilateral side where the force was applied.

STUDIES THAT DOCUMENT THE RELATIONSHIP BETWEEN ELECTROMYOGRAPHY AND MUSCULAR FORCE
This early classic work of Lippold, at the Department of Physiology of University College in London, established the linear relation between integrated electromyograms and the tension produced by a voluntary isometric contraction in a human muscle. The author always found a direct linear relationship between integrated EMG output and muscle activity using thirty experiments on different subjects.

Lippold, O.C.J. The relation between integrated action potentials in a human muscle and its isometric tension. J. Physiol., 117: 492-499, 1952.

The results show that under the limited conditions of the experiment there is a linear relation between the integrated electromyogram and the tension produced by a voluntary isometric contraction in a human muscle.

Although there is no proportionality between the mechanical and the electrical responses of a single motor unit, when the summated effect of a large number of units is recorded by surface electrodes from the whole muscle, the variation is statistically cancelled out. It has been shown (Adrian & Broak, 1929; Eccles & Sherrington, 1930; Gilson & Mills, 1941) that changes in the strength of contraction of a muscle are brought about in two ways: as the strength of contraction increases the number of motor units active becomes greater and there is a rise in the frequency at which these units repetitively contract.

Both these factors must increase the integrated electrical output of the muscle, so that the existence of this linear relationship indicates that the recruitment of motor units, bringing about increased strength of contraction, is spatially random. Similarly, there are either random increments of discharge frequencies of the active units, or once a particular unit has become active its rate of contraction smoothly increases.

SUMMARY
[1] The relation between the isometric tension of a voluntarily-contracting human muscle and its integrated electromyogram has been investigated.
[2] In thirty experiments on different subjects the relationship is always directly linear.
[3] The coefficient of correlation in these experiments varies between +0.93 and -0.99

Bigland and Lippold, in a controlled study at the Department of Physiology of University College in London, showed a linear relation between electrical activity and tension during constant or zero velocity of shortening. This classic study confirms the rationale for the clinical EMG protocol using maximal bite or clench. The function EMG test is designed to quantify the efficacy of muscle motor unit recruitment at a given mandibular position. This test shows that as the electrical activity increases, the proportion of overlap between potentials arising in different parts of the muscle remains constant. The excitation is related to the number and discharge frequency of active units.

Bigland, B. and Lippold, O.C.J. The relation between force, velocity and integrated electrical activity in human muscles. J. Physiol, 123: 214-224, 1954.

DISCUSSION
These results show a direct proportion between the integrated electrical activity in a muscle and the tension it is exerting, during constant (or zero) velocity of shortening (or lengthening). This indicates that, as the electrical activity increases, the proportion of overlap between potentials arising in different parts of the muscle, causing addition and subtraction in the final output, remains constant. Thus at any given velocity the area under the action potential curve is a measure of the „excitation? in the muscle.

The excitation is related to the number and discharge frequency of active units. Assuming that the fibres are randomly distributed within the muscle in terms of the size and hence of the force of contraction of each fibre, the tension developed in the muscle must be related directly to the number of Units which are active. It has been shown in nerve muscle preparations that the tension developed in response to maximal shocks is directly proportional to the frequency of stimulation, until a maximum tension is reached (Adrian & Bronk, 1929; Brown & Burns, 1949). This is also true in intact human muscles (Bigland & Lippold, unpublished). Thus, within certain limits on these grounds, any changes in either the number or frequency of active units would be expected to result in a linear relation between electrical activity and tension.

SUMMARY
Tension, velocity and electrical activity are thus interdependent, and integration of the electrical record provides a composite measure of the number of active fibres and their frequency of excitation.

In a carefully controlled study of voluntary isometric biting forces at maximal and submaximal levels, Molin (1972) demonstrated that there were progressively increasing force differences between the (control and patient) groups”. The joint study conducted at University of Karolinska and University of Gothenberg in Sweden concluded that “the patients generally produced only one half to two thirds of forces produced by the control subjects”.

Molin, C. Vertical isometric muscle forces of the mandible: A comparative study of subjects with and without manifest mandibular pain dysfunction syndrome. Acta Odont. Scand., 30: 485-499, 1972.

ABSTRACT
The aim of the investigation was to study isometric biting forces at „maximal? as well as at submaximal levels as defined in subjective terms. Special attention was paid to differences between subjects with and without manifest mandibular pain dysfunction syndrome (MPD). Thirty-one female subjects with manifest MPD constituted the patient group and 30 healthy females the control group. The reliability of the measurements was tested and the results obtained in both groups indicated that the discriminating capacity was not affected by the disorder. Except for the lowest force level (approximating the absolute threshold), substantial force differences between the groups were obtained. These differences increased with the force levels, and the patients generally produced only one-half to two-thirds of the forces produced by the control subjects.

SUBJECTS
The experimental group consisted of the same 31 female patients from the Department of Oral and Jaw Diseases, Karolinska Sjukhuset, Stockholm, who had take part in the study of the horizontal mandibular forces (Marklund & Molin, 1972). Their age range was 16-45 with a mean of 18.3 years. All of them had manifest mandibular pain dysfunction syndrome (MPD), (Molin, 1973a).

The control group consisted of the same 30 healthy females (dental nurses, student dental nurses and student physiotherapists) who also participated in the previous investigation. They were aged 18-28 with a mean age of 22.1 years. The control subjects were paid. None of the subjects had appreciable bite defects or periodontal disease.

APPARATUS
The biting force transducer was designed according to principles outlined by Linderholm and Wennstrom (1970). In principle the transducer consisted of two cantilever beams with electrical strain-gauges cemented close to the fixed ends.

PROCEDURE
The subject was instructed to perform close bites in accordance with the rating scale. The scale was placed in front of the subject, and the test leader pointed out the requested biting force category. The subject was asked to bite intermittently for periods of about 3 seconds with 10 seconds rest intervals. To keep the effects of experimental praxis, fatigue and memory errors under control, the order of the biting force levels was rotated at random.

RESULTS
When the patients were compared with the control subjects (Table IV), it can be seen that there were progressively increasing force differences between the groups with rising values on the rating scale. The degree of significance also increased with rising force levels. The „maximum forces? (initial „maximum? measurements and recordings at scale level 5) produced by the patients were all between two-thirds and half of those produced by the control subjects

In a detailed study of surface electromyography versus muscular force conducted at the University of Alberta?s Department of Physiology, Milner-Brown (1975) demonstrated that integrated “EMG is linearly related to the force produced by the muscle.

Milner-Brown, H. S. The relation between the surface electromyogram and muscular force. J. Physiol. 246: 549-569, 1975.

SUMMARY
1. Motor units in the first dorsal interosseus muscle of normal human subjects were recorded by needle electrodes, together with the surface electromyogram (e.m.g.). the wave form contributed by each motor unit to the surface e.m.g. was determined by signal averaging.
2. The peak-to-peak amplitude of the wave form contributed to the surface e.m.g. by a motor unit increased approximately as the square root of the threshold force at which the unit was recruited. The peak-to-peak duration of the wave form was independent of the threshold force.
3. Large and small motor units are uniformly distributed throughout this muscle, and the muscle fibres making up a motor unit may be widely dispersed.
4. The rectified surface e.m.g. was computed as a function of force, based on the sample of motor units recorded. The largest contribution of motor unit recruitment occurs at low force levels, while the contribution of increased firing rate becomes more important at higher force levels.
5. Possible bases for the common experimental observation that the mean rectified surface e.m.g. varies linearly with the force generated by a muscle are discussed. E.m.g. potentials and contractile responses may both sum nonlinearly at moderate to high force levels, but in such a way that the rectified surface e.m.g. is still approximately linearly related to the force produced by the muscle

In a 1978 study of EMG versus static loading of the mandible, Pruim (et al, at the Department of Orthodontics, University of Groningen, The Netherlands) further confirmed “the linear relationship between integrated EMG and the (mandibular) force exerted by individual muscles in isometric conditions.” The authors demonstrated that “at high bite force levels the activities in all muscle pairs are considerably increased.”

Pruim, G.J., Ten Bosch, J.J., and De Jongh, H.J. (1978) Jaw muscle EMG activity and static loading of the mandible. J. Biomechanics. 11, 389-395.

ABSTRACT
A method is described to relate jaw muscle EMG-activity to static bite forces. Bite forces are measured bilaterally in several reproducible positions on the human dentition by means of small wedge shaped transducers. Electromyographic methods are used to derive a relative measure of the activity in the opener and closer muscles. Visual feedback methods are used to obtain bite recordings at various levels of bite force and muscle activity.

There is no reason to doubt the linear relationship between integrated EMG activity and the force exerted by individual muscles in isometric conditions. The anterior and posterior parts of the temporal muscle show a different functional behavior. The role of the opener muscles as antagonists is of such importance, that it should not be neglected in muscle force analysis.

MATERIALS
Seven adult male students served as experimental subjects, some of them several times The subjects had no temporomandibular joint dysfunctions, as judged by subjective symptoms and by objective symptoms such as manual palpation during mandibular movements.

ELECTROMYOGRAPHY
To record muscle action potentials bipolar surface electrodes were applied.

RESULTS
It can be demonstrated that at high bite force levels the activities in all muscle pairs are considerably increased. To which extent the increase occurs depends on the muscle.

DISCUSSION
The question of linear or alinear relationships between the integrated EMG (EMGI) and isometric muscle force has been discussed by many authors (Lippold, 1952; Bigland and Lippold, 1954a,b; Ralston, 1961; Ahlgren, 1967; Ahlgren and Owall, 1970; Goubel, 1971; DeLuca and Forrest, 1973; Cnockaert et al., 1975; Yemm, 1977; Hof and Van den Berg, 1977; Maton and Bouisset, 1977). Common postulate was the existence of a linear relationship between integrated EMG and isometric muscle force, as far as contractions at submaximum level are concerned. As can be shown in Fig. 4, our experimental results parallel previous findings. However, it can also be noticed that antagonistic activity occurs as soon as agonistic muscle activity deviates from linear behavior.

In most investigations bearing on jaw mechanics, bite forces were measured during unilateral biting (e.g. Ahlgren and Owall, 1970; Carlsson, 1974; de Boever, 1975; Mansour and Reynik, 1975). In our opinion in this situation a pattern of forces could arise at the contralateral (balancing) side, which may result in a departure from static and isometric conditions. Therefore bilateral biting was assumed to be a better method for loading the mandible.

In those papers where other joints were investigated (e.g. Lippold, 1952; Bigland and Lippold, 1954; Goubel, 1971; De Luca and Forrest, 1973; Cnockaert et al., 1975; Hof and Van den Berg, 1977), forces were transferred to the force transducers through intervening soft tissues. In such cases at high force levels an inhibition initiated in the soft tissues can never be excluded. In this investigation the bite forces were transferred to the skull, with only the periodontium as underlying tissue. By means of the splint construction the bite forces were evenly distributed over the dentition. Thus an inhibition initiated by the pressoreceptors in the periodontium was prevented as far as possible and high bite forces could be expected. Moreover, the feedback methods stimulated the subjects to exert full effort.

Since the shape of the articular surfaces in the temporomandibular joints does not mechanically prevent displacement of the condyles, maintenance of the mandibular position must be provided by muscle action. In our experiments the acrylic splints and the flexible telescope construction served as an aid to coordinate this muscle action.

STUDIES THAT DOCUMENT THE REPRODUCIBILITY AND RELIABILITY OF SURFACE ELECTROMYOGRAPHY
At the University of Technology (The Netherlands) in 1986, Hermens et al. conducted a thorough study and literature review on the clinical applications and reproducibility of surface electromyography. Advantages of surface over needle electrodes in electromyography were cited.

Hermens, H. J., Boon, K. L., and Zilvold, G. The clinical use of surface EMG. Medica Physica, 9: 119-130, 1986.

ABSTRACT
The use of surface EMG as a tool for quantification is described. First the specific advantages of surface EMG are discussed. Time registrations are processed by means of a computer, From each registration an amplitude histogram and a power density spectrum is calculated. The parameters standard deviation, kurtosis and skewness are used in order to describe the histogram. The power density spectrum is characterized by a number of frequency parameters (first peak, maximum, median, – 6dB and – 10dB) and relative power parameters (relative power in % above – 6dB, – 10dB and 100 Hz). Furthermore the gradient EMG – activity (expressed in standard deviation) and force and the quotient between antagonist and agonist EMG activity are studied. By means of a pilot investigation a standard procedure is evaluated and “normal values” are found.

INTRODUCTION
It can be observed that with needle EMG the properties of the motor unit action potentials are primarily registered (firing frequency and shape of the potential), whereas with surface EMG more or less the activity of a large part of a muscle is registered. In this way a better idea of the functioning of a muscle is obtained.

This is one of the main reasons why surface EMG is often applied in kinesiological studies.

Another important advantage of the use of surface electrodes is its noninvasive character. In the clinical situation this point proves to be very important especially with follow- up investigations and with investigations with children.

In the literature many models can be found that relate physiological processes to characteristics of the surface EMG signal (Lindstrom, 1974; Guha and Anand, 1978)

THE PARAMETERS TO DESCRIBE SURFACE EMG
In most investigations the IEMG appears to increase linearly with the exerted isometric force in a rather large region (Rau, 1973; Hof van v. d. Berg, 1981). An additional increase is caused by fatigue (Petrofsky, 1979; Kramer, 1979)

DISCUSSION
In this study we tried to answer the question: can surface EMG be applied in a clinical situation, especially as a tool for quantification? In our opinion it is obvious that needle EMG is hardly applicable for quantification. Patterns obtained by means of needles are almost always only related to single motor units and the relative position of the leadoff area causes a large variability with respect to the registered potentials. With surface EMG, leadoff electrodes are always situated outside the motor unit area. The signal is dominated by an interference pattern caused by the summation of many motor unit action potentials. If disorders occur, changes can be detected in this signal in a more reliable way, at least from a statistical point of view. Our results show that surface EMG can certainly lead to a reproducible method of quantification.

It is possible to indicate a range of “normal” values to discern pathology. It is also possible to measure changes in parameters during a follow-up investigation.

The reproducibility of the parameters derived from the spectrum is somewhat better compared with the results described by Viitasalo and Komi (1975) although comparison is somewhat difficult due to the different presentation of the spectrum. The improved reproducibility will be obtained by the longer time registration (2 seconds) that is used to calculate the spectrum.

Riise (in the Department of Stomatognathic Physiology, Karolinska Institutet, Stockholm, Sweden) strongly prefers the use of bipolar surface electrodes to evaluate the general activity of the anterior temporal and masseter muscles.

Erickson (1982) also advocates the preferred use of surface electrodes due to the mixed, heterogeneous fibers of the mandibular elevator muscles.

Riise, C. Clinical and electromyographic studies on occlusion. From the Department of Stomatognathic
Physiology, Karolinska Institutet, Stockholm, Sweden, pp. 20-21, 1983.

The preference of bipolar surface electrodes (Riise & Sheikholeslam – 1982, 1983, 1983a) to needle electrodes was based on the character of the studies, where the general activity of the anterior temporal and masseter muscles was essential, not the activity of single or few motor units. The prevalence of mixed types of fibres in different parts of the elevator muscles (Eriksson, 1982) also motivates the choice of surface electrodes. As surface electrodes record the activity from a larger area than do needle electrodes, they pick up more potentials during weak effort (Moller, 1966). The amplitude of these potentials varies less than when using needle electrodes due to the amplitude-distance relationship (Buchtal, Guld & Rosenfalck, 1957). Furthermore, surface electrodes seem not to create pain and psychological effects in the subjects, interfere less with natural function, and the mean amplitude recorded varies almost linearly with the force generated at constant length, or during contractions with constant velocity (Milner-Brown & Stein, 1975).

The distance between the surface electrodes, their placement and size were kept constant since the mean voltage varies with these parameters (Moller, 1966). By palpitation of the muscles the electrodes were placed parallel to the main direction of the muscle fibres. When using bipolar surface electrodes with a small interelectrode distance, compared to the size of the muscle, the activity conducted from adjacent muscles is nearly identical on the two leads and therefore rejected (Moller, 1966). Provided a similar electrode position is used, a similar powerspectral distribution curve may be produced when recording one month later as reported by Naeije & Zorn (1981).
Goldensohn in Electromyography (Chapter 11 from the text Disorders of the Temporomandibular Joint) further confirms the preference for recording EMG activity with surface electrodes in cases of TMJ studies.

Goldensohn, M.D., also states, In the electromyographic studies conducted thus far, the majority of patients tested complaining of facial pain and mandibular dysfunction showed recordings different from that of asymptomatic individuals. This suggests a fruitful field for further investigation utilizing surface electrodes

Goldensohn, E. Electromyography. From Disorders of the Temporomandibular Joint Lazlo Schwartz, ed. W.B. Saunders Co., Philadelphia/London. pp. 163-176, 1966.

CHOICE OF ELECTRODES
The choice of electrodes for recording depends upon the specific problem at hand. For temporomandibular joint studies it is best, when possible, to use surface electrodes on the skin overlying the muscle because such electrodes do not cause pain or interfere significantly with joint function

DISCUSSION
Unlike other parts of the body where maximum muscle tension is usually achieved by exerting force against the external environment, the jaw muscles can build up considerable tension from within through the contact of the teeth during clenching or gnashing

In the electromyographic studies conducted thus far, the majority of patients tested complaining of facial pain and mandibular dysfunction showed recordings different from that of asymptomatic individuals. This suggests a fruitful field for further investigation utilizing surface electrodes and ink writing equipment.

Lloyd, in a study at the U.S. Army Medical Research Laboratory, emphasizes the value of using electromyography as a representational measure of neuromuscular activity. The use of surface electrodes for obtaining composite recordings of multiple motor units is described.

The preferability of bipolar surface electrodes to document motor unit recruitment (muscle contraction) is clearly delineated in the study. The use of EMG to evaluate maximal motor recruitment is strongly endorsed by the author?s findings. Special note should be made of the thesis proposed regarding reasons for loss of recruitment ability following sustained maximal bite. The multiple causalities further support clinical neuromuscular protocols and procedures.

Lloyd, A. J. Surface electromyography during sustained isometric contractions. J. Applied Physiology, 30: 713-719, 1971.

ABSTRACT
Ten volunteers were asked to maintain isometric contractions involving elbow flexor muscles as long as possible at levels equal to 30, 50 and 70% of their maximum voluntary strength and to report when they experienced five successive levels of pain resulting from the contraction. Surface electromyographic recordings were made on the biceps muscle as well as three peripheral muscles. The results indicated that the maximum duration of the contraction could be reliably predicted from the reports of mild and moderate pain intensities

Studies of human physical performance have shown it increasingly profitable to incorporate the electromyogram (EMG) as a representational measure of neuromuscular activity. Typically, surface electrodes are placed directly over the active muscle for recording EMGs during strenuous work. The resulting EMG is a composite recording of numerous motor Units within a relatively large muscle area. A number of studies have demonstrated that a consistent relationship is obtained between the level of recorded EMG activity and such measures as increased isometric contractions, increased tension levels, and maximum endurance times (Bigland & Lippold, 1954; Close, 1964; Eason, 1960; Inman et al., 1952; Wilcot & Beenken, 1957). When EMG activity has been compared with the levels of tension produced by increased isometric contractions, a positive relationship was obtained (Inman et al., 1952). This relationship has been confirmed by studies in which the EMG activity has been represented in digital form by an integration procedure (Wilcot & Beenken, 1957), and by a count of the EMG spikes (Close, 1964). Both procedures appeared to be highly related and demonstrated that as the force of a muscle contraction increased, the level of EMG activity concomitantly increased.

It has been determined that surface EMG activity is a composite of the activity of a relatively large number of motor units even with a moderate contraction. Therefore, an assessment of the frequency and amplitude change in individual motor units. The purpose of the present experiment was to investigate the basis for the increase in EMG activity during maximally sustained isometric contractions.

METHODS
Bipolar recordings of EMG activity were obtained with silver-silver chloride surface electrodes.

DISCUSSION
Surface EMG recordings during strenuous work endurance seemed to provide considerable information about muscle activity. Amplitude increase resulted from a shift in motor unit activity – – a shift from random firing of motor units in the unfatigued muscle to synchronized firing in the fatigued muscle. The process appeared to be the same regardless of the contraction force. Synchronization of motor units became the major contributing component for the increased amplitude. The surface electrodes provided a reflection of motor unit activity over a considerable area of muscle tissue and should have adequately represented the activity within the entire muscle (Close, 1964). Amplitude increase could have resulted from either of two previously proposed sources. In terms of the recruitment concept, Olson et al, (1968) determined that, in a passive reflex response, motor units were recruited in an orderly fashion with motor units of smaller amplitudes appearing early. As passive stretch was increased, motor units with larger amplitudes appeared. These results suggested that motor units have varying thresholds which are based on the amount of muscle tension present. Present experimental results suggested that, with the ensuing maximum endurance, there resulted an initial recruitment of higher threshold motor units characterized by a low firing frequency; a similar recruitment process seemed applicable to explain differences in EMG amplitudes at the 30, 50 and 70% contraction forces.

The results of the present experiment suggested that, with an increase in the duration of constant force, synchronization and recruitment produced an increase in the amplitude of the surface EMG. Perhaps, as a constant force was sustained, metabolic reserves were being depleted and the sub-maximum contraction force continued to increase in value until maximum endurance was reached. At this point the force would become the equivalent of 100% maximum and 70% contraction forces where motivation appeared to have less influence on endurance estimations. The frequency shift would accompany the arterial occlusion and potential metabolic depletion from muscle ischemia proposed by Humphreys and Lind (1963). Another alternative could be the resulting compression of the motoneurons, originally suggested by Reid (1928). Either hypothesis was considered feasible on the basis of the present results. Nerve compression, as a source for loss of high-frequency signals in the EMG, could account for the rapid recovery observed after a sustained submaximum contraction. Anabolic and catabolic processes would take considerably longer than recovery functions indicate.

This controlled study of the power-spectra of surface electromyograms of masticating muscles defines the technical and physical parameters of contemporary clinical electromyography. The work of Mitani and Yamashita, at Osaka Dental University, has been pivotal in defining electromyography requirements for a clinical EMG.

Mitani, H., and Yamashita, A. On the power-spectra of the surface electromyograms of masticatory muscles. J. of Osaka Dental Univ., 6: 1-12, 1978.

INTRODUCTION
For the observation of the coordinating style of muscular activity as a whole, from the standpoint of Kinesiology, the surface electromyogram (EMG) is considered particularly well fit for the purpose. In the dental field, also, many reports have so far been made on the participation of the masticatory muscles in holding the position of the jaw, fundamental movement and functional movement of the jaw since Moyers (1950) through an analysis of the surface EMO. In the early stages of research, a subjective analysis only of EMG bursts was performed, but in 1954, Perry and Harris succeeded in plotting the amplitude at 50 msec intervals from high-speed recordings of the EMG and observing the muscle activity quantitatively and qualitatively. Since 1965, Muguruma (1965), Hashimoto (1969) and Tawa (1971) have undertaken to transform the EMG simultaneously recorded from masticatory muscles into integrated curves, and, through the calculation of their areas, clarified the coordination pattern of those muscles at their functional movement.

In this way, since after Piper (1912), in the field of general medicine, first took not of the rhythm of the surface EMG during voluntary contraction, a possibility has been anticipated that the information would be made available concerning the activity of neuromuscular units (through) the frequencies of the EMG. Of late, electronic (instruments) have improved and consequently, an analysis of frequency and correlation has come to be made automatically, and it has become to be known by degrees that information contained in the surface EMG is closely related to the activity of a single NMU

EXPERIMENTAL METHODS

1. Recordings of EMG

The subjects and muscles tested:
Three male youths with normal tooth alignment and occlusion were selected. The muscles and regions tested were the lower middle part of the masseter (Mm), the anterior portion of the temporal (Ta) and the posterior portion of the temporal (Tp) on the left and right sides, totaling six regions. The bipolar disk surface electrodes were placed through the electrode jelly along the muscle fibers on the concerned muscle.

Amplifier and recorder:
A push-pull type EMG amplifier* of which time constant was 0.03 sec was used and the amplification degree was so adjusted that the input signal of 100 (microvolts) would show a 10 mm reflection of the pen. While monitoring by an ink writing oscillograph (at the feeding speed of the recording paper of 50 mm/sec), simultaneous EMG recordings were made at a tape speed of 30 inch/sec on a data recorder.

Contents of the record:
In centric occlusion, which was considered to be the fundamental and most stable position of the mandible, the subjects were instructed to sustain moderate biting for about 3 sec.

Record analyzing equipment and analyzing method:
By using a Real-Time Digital Correlator and Spectrum Analyzer, the EMG obtained under Item 1 above was put to analysis (Fig. 1).

The EMG recorded on the tape was reproduced at 1/10 speed and, through the Correlator and Spectrum analyzer, the power-spectrum was recorded with an Electronic Polyrecorder.

In Tp on both sides the greater part of the spectrum was occupied by a low frequency component (13-30 Hz), without forming a noticeable peak, thus indicating a muscle inactivity (Table 1). In Ta on both sides, a broad spectrum structure was seen, which was 133 Hz and 239 Hz on the right, while, on the left, it was 146 Hz and 239 Hz, respectively forming two perceptible peaks. Both on the left and right, the frequency range was indicated less than about 500 Hz (Table 3).

The power-spectrum of Tp in this case indicated a similarity to that of Figure 2. In respect to Ta, the main frequency component was less than 350 Hz on both sides, a peak being formed on the right, near 160 Hz, while on the left, a multi-peak spectrum structure was seen, a peak being formed at the frequency of 60, 120, 186, 253 Hz. Also, harmonics were presumed to exist, because the frequency at each peak was seen to have a relationship of integers (Table 2). With respect to Mm, a far broader spectrum structure than other muscles was shown both on the left and right, the frequency component being distributed between 13- 630 Hz with no outstanding peak found around 13-320 Hz, indicating a mono-peak spectrum structure with a peak formed at 133 Hz (Table 2). With respect to Mm, the frequency range was 13-400 Hz both on the left and right, indicating a multi- peak spectrum structure on the right, their positions being found approximately on the frequency ranges obtainable by multiplying by integers (e.g., 60, 120, 186 and 226 Hz) (Table 3).

Thus, the frequency components (observed) when a moderate biting was sustained in the centric occlusion were seen to be about 13-600 Hz in Mm and Ta, and somewhat less in Tp, being roughly 13-300 Hz, Tp being known to be more or less inactive than the other two muscles. A rather indistinct peak was seen adjacent to 50-60 Hz and when the power was somewhat increased, an (additional) indistinct peak formed near 146 Hz. On the other hand, in respect to Mm, the main peak was found between 120-150 Hz, the peaks were formed near 50 Hz, 200 Hz, 300 Hz and 400 Hz, indicating a multi-peak structure, which implied the greatest activity. Ta, as the fundamental form of a pattern of normal distribution centering around 133 Hz, showed 2 or 3 minor peaks before and after it. Thus, insofar as the power-spectrum pattern of Ta is concerned, it was seen to be positioned midway between Tp and Mm or rather nearer to Mm.

DISCUSSION
The method of studying the analysis of the frequency distribution of the surface EMG may be classified roughly into (the following) categories:
[1] Method of mathematic analysis
[2] Analysis employing the optical method
[3] Band width analysis
[4] Successive short range spectrum analysis
[5] Correlation analysis.

Any of these methods may be used in accordance with varying objectives. However, not many reports on the study regarding the frequency analysis of the surface EMG are mad in the dental field, quantitative determination having so far been made of the EMG burst in the light of the amplitude values of the original wave. The present study was made (in order) to explore the activity of muscles, quantitatively and qualitatively, in the light of the frequency distribution of the surface EMG of the masticatory muscles through a correlation analysis.

Regarding the relation of muscle contraction and the surface EMG, Inman (et al., 1952), Lippold (1952) and others have reported that a rectilinear relationship exists between the power of the muscle contraction and the integrated value of the EMG in the case of isometric contraction. While, in reference to the frequency spectrum, Sato & Tsuruma (1967) and Hayes (1960) have reported that the frequency spectrum should be constant in the case of contraction stronger than moderate.

As a result of moderate biting sustained in the centric occlusion the present authors were led to presume the following, in the light of the frequency distribution and the power values of EMG of temporal and masseter muscles:

[1] On the power values and distribution of frequency

As the power value of muscle grows larger, the frequency range of powerspectrum expands and the main frequency component of spectrum is known to shift to a higher range. The surface EMG is known to be a collection of NMU activities and when the muscle is in a state of normal contraction, the synchronous activity of NMU becomes low, each NMU apparently having a disorderly activity. The extent of contraction, strong or weak, depends on the number of NMU joined and the frequency of NMU spikes. In case the power value is large, the number of NMU participating in the activity and the frequency range of their spikes increase, and through temporal and spatial summation, the frequency range of spectrum expands and the high frequency component is assumed to be dominant.

[2] On the formation of higher harmonics

In the case of multi-peak spectrum pictures as seen in left Ta of Fig. 3, and right Mm of Fig. 4, the peaks were seen to be in approximately integer-multiplied frequency ranges. In voluntary biting as performed in the present experiment, the impulse from the pyrimidal tract participates in the firing of the motoneuron of the jaw elevators. By those impulses from the upper center of the brain, the excitatory postsynaptic potential caused in the alpha motoneuron reaches a critical level and discharges an efferent impulse towards the muscle, inviting contraction. It is known that EPSP causes a temporal and spatial summation, and there is a constant relationship of integer multiplication between the firing of the motoneuron and presynaptic impulse (Homma et al., 1970). While Fex and Krakau (1958) reported that the surface EMG is composed of the fundamental wave of NMU cycle of discharge and higher harmonics. If the peak at 60 Hz is considered to be the fundamental wave among the peaks at 60, 120, 186, and 226 Hz (Fig. 3,4), the idea expressed by them appears to be in perfect agreement with the results of the present study. Samejima (1971) likewise agrees that the high frequency component of auto-power-spectrum of the surface EMG has higher harmonics at a frequency corresponding to the cycle of discharge of NMU spikes of the motor unit system. He explains, however, that such correlation is of extremely low extent and from this, the power-spectrum of this sort is assumed not likely to appear frequently.

SUMMARY
The surface EMG of the bilateral middle part of the masseter muscle and of the anterior and posterior portion of the temporal muscle of three young male subjects with normal tooth alignment and occlusion was recorded on magnetic tapes. Then, the recording thus obtained was reproduced, and, through a digital correlator and spectrum analyzer, a power-spectrum was obtained.

In their recent, carefully controlled study at (State University of New York, Buffalo) Burdette and Gale confirmed the replicability and reliability of surface EMG.

Burdette, B.H., and Gale, E.N. Intersession reliability of surface electromyography. Journal of Dental Research, Abstract No. 1370, Volume 66, 1987.

The purpose of this experiment was to compare EMG data from trials both within the same session without moving the electrodes, and between sessions two weeks apart. A custom plastic template was used to reproduce electrode placement sites as closely as possible the second visit.

Seventeen TMD patients were instructed to swallow water and then sit quietly without swallowing for 30 seconds. During this time, EMG data was gathered from the masseter and temporalis muscles on their pain side using 4 bipolar surface electrodes. Three 30-second trials were run and the data analyzed from the last two. This protocol was repeated in two weeks. Head posture was also standardized between sessions.

Correlation coefficients were calculated for each muscle between trials both within sessions and between sessions. The r values ranged from .6584 to .9852 between trials within the same session for both muscles. The r values for the masseter muscle ranged for .6112 to .6671 between trials separated by the two-week interval. Intersession values for the temporalis muscle ranged from .2624 to .3261. Using the standardization technique described, replicability of surface EMG data between recording sessions was considered satisfactory for the masseter muscle.

ABSTRACT
In 39 patients with functional disorders of the temporomandibular joint and the muscles of mastication and in 45 dental students without such disorders, the activity in the temporal and masseter muscles was recorded with the subjects seated upright and the mandible at rest. On the average, the level of postural activity in the patients was significantly stronger than in the control group. The increase was not a general trait including all muscles under study, as its location differed among patients and was limited to single muscles in most cases. In the temporal muscles, increased activity was associated with tenderness by palpation. Strong postural activity in the masseter muscles was associated with pain in the cheek and tenderness of the deep part of the muscle, the individual variation of the electromyographic findings pointed to a differentiated etiology more than to a general increase of activity for emotional reasons.

This report gives the preliminary results of a quantitative electromyographic study of oral function in such patients and a statistical comparison with the activity in normal subjects. It deals with the postural activity in the temporal and masseter muscles and the aim was to investigate the concept of hyperactivity or muscle spasms and to relate the electromyographic and clinical findings.

SAMPLE OF PATIENTS
The patients were chosen at random among those referred to the Royal dental College in Copenhagen for the treatment of functional disorders. The sample included 30 females and 9 males, 14-70 years of age, and majority (22) were 14-24 years old.

SAMPLE OF CONTROLS
A group of dental students were selected: (1) without history of reduced mobility, clicking in the joints, or pain in the joints and the muscles of mastication, and (2) in whom no signs of functional disorders were revealed in the clinical examination. The sample consisted of 19 females and 26 males, 20-30 years of age.

ELECTROMYOGRAPHY
In the recordings with the mandible at rest the level of the mean voltage was measured at intervals of 1 sec; the postural activity in each muscle was characterized by the average of 40 single observations.

RESULTS
A comparison between average findings showed that the postural activity in patients with functional disorders was significantly stronger than in dental students without such disorders; this observation included all muscles under study except the right masseter (Table 2).

DISCUSSION
It emerged that the average level of postural activity in the temporal and masseter muscles was raised significantly in the patients as compared to the control sample
In our sample of patients electromyographic observations were only related to clinical findings in the muscles under study. Increased postural activity was associated with tenderness and in the case of the masseters, pain in the cheek as well.

In a carefully controlled 1971 study of quantitative EMG supported by a grant from the Danish Dental Association, Moller et al. replicated the results of Lous (1970) by studying the masseteric and temporal activities of 24 patients and 45 normal subjects. The findings indicated significant differences in the two groups. Such significant difference was evident even when relaxation was induced in patients and normal subjects. The study documented the variability of EMG resulting from various postural positions.

Moller, E., Sheikholeslam, A., and Lous, L. Deliberate relaxation of the temporal and masseter muscles in subjects with functional disorders of the chewing apparatus. Scand. J. Dent. Res., 79: 478-482, 1971.

ABSTRACT
The ability to relax the temporal and masseter muscles was tested in 24 patients with functional disorders of the muscles of mastication and the temporomandibular joints and in 45 dental students without such disorders. In spite of all efforts taken to induce relaxation in the patients, postural activity remained unchanged as long as they sat up, but a change to supine position caused significant reduction of the activity in the temporal muscle. On the return to an upright position these muscles attained their previous level of activity. In the students, deliberate relaxation was limited to the supine position and caused a reduction of the activity in the temporal muscle. Patients with hyperactivity in masseter muscles with the mandible at rest were too few to demonstrate any effect of relaxation, and the absence of effect in the control sample showed that these muscles do not contribute to mandibular posture.

The present study deals with the activity in the temporal and masseter muscles during deliberate relaxation in subjects with and without functional disorders.

MATERIAL AND METHODS
The sample of patients and controls, the clinical and electromyographic methods, and the activity with the mandible at rest have been described previously (Lous, Sheikholeslam & Moller, 1970). In 24 patients (19 females, 5 males, 14-15 years of age) of the total sample of 39 (30 females, 9 males 14-70 years of age), bilateral electromyographic recording from the temporal and masseter muscles were obtained during deliberate relaxation, first in the upright, sitting position with the head supported, then in the supine position.

The level of the mean voltage was measured at intervals of 1 sec and in each muscle the degree of 20 such measurements taken from recordings during two sessions. Data obtained during and after relaxation were compared to the average of the postural activity at the beginning and the end of a session (Lous et al., 1971).

In the control sample (45 dental students, 19 females and 26 males, 20-30 years of age), recordings during relaxation were limited to the supine position.

DISCUSSION
Electromyography permits direct observation of muscle relaxation. In the present study it showed that ability to relax depends on the position of the subject (up-right versus supine) more than on the condition of the muscles (patients versus normals).

It is concluded, therefore, that hyperactivity in the temporal muscle reflects a deviation in the posture of the mandible. In the etiology of abnormal posture, the importance of specific occlusal conditions appears from the influence of unilateral cross-bite on the postural activity in the posterior temporal muscle (Troelstrup & Moller, 1970). Since the masseter muscles are unaffected by posture, we assume that the cause of tenderness and hyperactivity (with the mandible at rest) in these muscles must be sought in functions involving stronger activity and tooth contact (chewing, swallowing, subconscious grinding and clenching). Such a differentiation between the temporal and masseter muscles could explain why tenderness was observed equally often in both muscles (Lous et al. 1970, Table 1), although increased postural activity was most pronounced in the temporal muscle.

In a controlled study of quantitative EMG at Karolinska Institutet in Sweden, Sheikholeslam compared the EMG resulting from the maximal bite of 39 patients with functional disorders against 45 normal subjects. The authors found significant statistical differences between the maximal bite activity of the patient group in comparison with the maximal bite activity of the control group.

The statistical difference was still significant (P<0.05) even when comparisons of female patients to female controls and male patients to male controls were made. Although the absolute values were smaller in the older patients, reduction by age was insignificant. The “normal” ranges of EMG during maximal bite were established for the temporalis and masseter muscles.

The study established a strong rationale for using EMG during maximal clench to distinguish normal from abnormal masseter and temporalis function. The findings of the controlled study further validates the rationale for EMG maximal motor unit recruitment testing to quantitate muscle status in the clinical environment.

Sheikholeslam, A., Moller, E., and Lous, L. Pain, tenderness and strength of human mandibular elevators. Scand. J. Dent. Res. 88: 60-66, 1980.

ABSTRACT
In terms of the mean voltage during maximal bite in the intercuspal position we compared the strength of the temporal and masseter muscles of 39 patients with functional disorders of the chewing apparatus and 45 controls. Maximal electrical activity was significantly stronger in the controls than in the patients. Differences between the two samples with respect to sex, age, number of opposing teeth in contact and the presence of pain and tenderness tended to favor stronger activity in the controls. However, the different levels of electrical activity, especially in the case of the masseter muscles, could only be accounted for on the basis of a difference in maximal strength. We suggest that the weaker elevator muscles of the patients was a predisposing factor making these muscles less fit to endure hyperactivity induced psychologically or as a reflex response to occlusal interferences and functional disorders of the temporomandibular joints or other elements of the oral neuromuscular system. The sample of controls had much stronger elevators, less susceptible to such hyperactivity.

Maximal strength in human muscles is positively correlated to the diameter (Ringquist, 1973, masseter muscles) and content of type II fibers (Tesch & Karlsson, 1978; Thorstensson et al., 1976; limb muscles). Exercise improves maximal strength (Saltin et al., 1976; Thorstensson, Hulten et al., 1976; limb muscles in man) and may cause an increase of the number of muscle fibers by splitting.

The present study deals with the strength of the temporal and masseter muscles in patients with pain and tenderness of these muscles as compared with a group of controls. Based on the linear relationship between electrical and mechanical muscle activity during isometric contraction (e.g. Lippold, 1952; Milner-Brown & Stein, 1975; Moller, 1966) muscle strength was assessed indirectly by electromyography. The aim was to explore the significance of muscle strength per se as a predisposing factor in the development of pain and tenderness in the muscles of mastication.

MATERIAL
The patients were questioned with respect to mobility of the mandible, clicking from the temporomandibular joints and the location of their pain. The clinical examination included measurement of the maximal opening movement, auscultation of the joints, and palpation of the joints and the muscles of mastication.

ELECTROMYOGRAPHY
Electrical muscle activity was picked up bilaterally with bipolar surface electrodes over the anterior and posterior temporal and masseter muscles, and the electromyograms and their mean voltages were recorded simultaneously (Troelstrup & Moller, 1970).

The activity during maximal bite in the intercuspal position was recorded during contractions of about 1 second. The degree of activity was assessed in terms of the average level and the peak of the mean voltage (Bakke and Moller, 1980).

RESULTS
The average level and the peak of the mean voltage during maximal bite in the intercuspal position was significantly stronger in the sample of controls than in the patients (Table 2).

Comparing female patients to female controls and male patients to male controls, maximal activity was still significantly stronger in the controls (Table 3).
Although absolute values were smaller in the older patients, reduction by age was insignificant (Table 4).

DISCUSSION
Therefore, the highly significant difference between patients and controls in our study of maximal electrical activity points to a difference with respect to maximal strength.

Since the maximal attainable strength depends on fiber-type characteristics (Ringquist, 1973; Thorstensson, Grimby & Karlsson, 1976; Thorstensson, Hulten, Dobeln & Karlsson, 1976; Tesch & Karlsson, 1978), which are partly determined genetically (Komi, 1977) the disposition for functional disorders of the muscles of mastication must vary individually. Therefore, measurements of the strength of these muscles in patients may be a guide to the level of accuracy to which a particular occlusion should be adjusted. The strength-pain relationship also points to muscle exercise as an alternative measure to eliminate muscle pain and tenderness.

In another carefully controlled study using 37 patients before and after treatment of functional disorders of the masticatory system and 43 control subjects the authors demonstrated reduction of postural activity and of symptoms and signs in all muscles under study. In both muscles, reduced activity was accompanied by less pain and tenderness – – most significantly in the masseter. The joint study at Karolinska Instituttet and Royal Dental College concluded that “maximal activity in the right and left anterior temporal and masseter muscles became more symmetrical due to treatment,” and that “increased postural activity and pain coincide for the muscles of mastication and that pain decreases with activity in response to treatment”.

This large-sample, controlled study makes evident the fact that the clinical application of EMG is a valuable quantitative modality for the clinician to measure progress and effectiveness of his therapy, by monitoring EMG postural activity of the anterior temporalis and masseter muscles.

Sheikholeslam, A., Moller, E., and Lous, L. Postural and maximal activity in elevators of mandible before and after treatment of functional disorders. Scand. J. Dent. Res., 90: 37-46, 1982.

ABSTRACT
In a longitudinal study we compared clinical data on pain and tenderness and electromyographic recordings of postural and maximal activity from 37 patients before and after treatment of functional disorders of the masticatory system. Forty-three dental students served as controls. Treatment was followed by a concomitant decrease of pain, tenderness and postural activity Following treatment, loading in patients decreased significantly and became more symmetrical in the anterior temporal muscles since treatment involved correction of the occlusion we conclude that the intermaxillary conditions of tooth contact during function play a role in the etiology of functional disorders and that the reduction of the relative postural loading of the elevators of the patients due to this treatment contributed to their relief. The relatively high percentage of full effort displayed as postural activity by the patients even after treatment may explain fluctuation of symptoms and signs of functional disorders of the masticatory system. It also emphasizes the need for improved methods of treatment to reduce the probability of recurrence.

If abnormal muscle activity is a source of pain in functional disorders of the masticatory system, treatment followed by the relief of pain must involve changes in the action of the muscles of mastication. Jarabak (1956), Perry (1957) and Ramfjord (1961) have repeated their electromyographic recordings after treatment and, based on observation of the electromyograms, they concluded that occlusal adjustment eliminated the spasms observed in the pre-treatment recordings.

MATERIAL AND METHODS
The present study was based on 37 patients, 28 females and nine males, aged from 14 to 62 (mean 29) years at the time of initial examinations

TREATMENT PROCEDURES
Different types of occlusal splints were used in 23 patients; occlusal adjustment was the sole treatment in 20 patients and was used after previous splint therapy in 14. Exercise, heat and infiltration with local anesthetics was administered as an adjunct to occlusal adjustment or splint-therapy in 17 patients. In one patient temporary correction of oral function was followed by 3-4 years of orthodontic treatment.

EFFECT OF TREATMENT
Treatment was evaluated with respect to improvement of symptoms and signs in the different regions of the masticatory system (Table 1). Pain was reduced by 60% (temple) to 90% (ear), mobility was improved in seven of 10 patients (70%), and joint sounds eliminated in 19 of 29 (66%). Tenderness was reduced by 70% in the temporal and masseter muscles and by 80-85% at the temporomandibular joints. Maximal opening was restored to normal in six of nine subjects (67%) after treatment. Finally, in 25 subjects with joint sounds assessed by stethoscopy before treatment, movements could by performed without such sounds after treatment in 14 patients (56%).

The statistical analysis included average differences between paired observations (pre- and post-treatment) of muscle activity and clinical findings evaluated by Student?s t-test.

RESULTS

POSTURAL ACTIVITY
On average occlusal adjustment resulted in a reduction of postural activity and of symptoms and signs in all muscles under study (Table 2). In the anterior temporal muscles, activity decreased by 1.5-2 (microvolts) (P<0.01). In both muscles the reduced level of activity was accompanied by less pain and tenderness, most significantly in the masseter muscles.

DISCUSSION

MAXIMAL ACTIVITY
During isometric contraction electrical and mechanical activity of muscle are linearly related (e.g. Milner-Brown & Stein, 1975; Moller, 1966). Furthermore, action of the elevators of the mandible during maximal bite in the intercuspal position represents the upper limit of the activity during chewing and biting (Moller, 1966) and the maximal force of clenching (Steenberghe & de Vries, 1978). It therefore seems justified to use the average of the mean voltage in the mandibular elevators during maximal bite as a measure of the strength of these muscles. Comparison showed that treatment did not affect muscle strength per se and consequently that subjects with weak muscles are more susceptible to pain and tenderness due to overload than are subjects with strong elevators (cf. Sheikholeslam, Moller & Lous, 1980). However, maximal activity depends on the number and evenness of the distribution of occlusal contacts (Bakke, Moller & Thorsen, 1980) this could be ascribed to the occlusal adjustment.

POSTURAL ACTIVITY RELATIVE TO FULL EFFORT
The present study has demonstrated that increased postural activity and pain coincide in the muscles of mastication and that pain decreases with activity in response to treatment.

It was justified to consider the postural activity in our patients and controls as a sign of continuous contraction. In the patients before treatment and associated with pain the level of activity in the temporal and masseter muscles was more than twice that of the controls (cf. Table 4). In response to treatment, postural activity of the patients on average decreased by 30% in the anterior temporal muscles (P<0.01) and 20-25% in the posterior temporal and masseter muscles (0.05<P<0.10). We suggest that the significantly lower incidence of pain after treatment was due to this reduction of postural activity.

Our study has demonstrated a significant reduction of pain and or the relative postural loading of the anterior temporal muscles (cf. Table 4). A tendency of a similar reduction in the posterior temporal and masseter muscles was observed. However, in all three pairs of muscles the proportion of full effort displayed as postural activity in the patients after treatment was still significantly above the controls. The smaller strength of the elevators in patients as compared to controls may in part explain why the patients had to activate a larger part of their muscles to support the mandible. This difference made the elevators of the patients more susceptible to overload and we hold it responsible for the long and short term fluctuation of symptoms and signs of functional disorders (Kopp, 1977). The relative excess of postural activity in patients after treatment also emphasizes the need for improving the methods of functional correction in order to diminish the probability of recurrence of pain and tenderness.

In a controlled electromyographic study of mastication in 37 patients and 43 control subjects, Moller, et al., demonstrated that by analyzing the EMG values from the muscles of mastication, parameters of muscle coordination are studied and a quantitative measurement of hyperactivity during mastication is determined.” Relative strength and relative contraction time differed between patients and controls. Differences between controls and patients with respect to relative strength and contraction times concerned all muscles under study, and improved significantly after successful treatment.

Moller, E., Sheikholeslam, A., and Lous, L. Response of elevator activity during mastication to treatment of functional disorders. Scand. J. Dent. Res., 90: 37-46, 1984.

ABSTRACT
The pattern of elevator activity during mastication in temporal and masseter muscles of 37 patients with functional disorders and pain in the masticatory system was studied before and after conventional treatment and compared with a control group of 43 subjects. As compared to controls, patients before treatment chewed with greater relative strength (percent of maximal elevator activity), longer relative contraction times (percent of total duration of chewing cycle) and stronger intermediary activity between strokes. These parameters of muscle coordination are proposed as quantitative estimates of “hyperactivity.” Conventional treatment abolished pain, tenderness and other symptoms and signs completely in 18 patients in whom the most significant findings in the muscles under study were reductions in absolute and relative contraction times ascribed to increased stability produced by occlusal adjustment. It is suggested that relatively shorter pauses before treatment impaired blood flow and that their prolongation following treatment improved circulation. During the strong, dynamic contractions of mastication, masseter muscles tended to be more susceptible to hyperactivity than the temporal muscles.

We have previously demonstrated coincidence of increased postural activity, pain and tenderness of elevators of the mandible and simultaneous decrease of this activity and of pain and tenderness after treatment (Sheikholeslam, Moller & Lous, 1982)

DISCUSSION

PAIN AND ITS RELIEF
The present study introduces two quantitative parameters of “hyperactivity” during mastication: relative strength and relative contraction time which both differed between patients and controls
Differences between controls and patients with respect to relative strength and contraction times as well as reduction of contraction times, absolute and relatively, after successful treatment concerned all muscles under study. However, the number of time parameters differed and the levels of significance were most pronounced for the masseter muscles. Hyperactivity in terms of relatively increased postural activity has also been shown and included all muscles under study (Sheikholeslam, Moller & Lous, 1982), but the most substantial differences concerned the temporal muscles. Hence, the temporal muscles tend to be susceptible to static overload while the masseter muscles are most likely to acquire their symptoms and signs of overload during the strong, dynamic contractions of mastication.

In a well controlled study of EMG activity in normal subjects and in patients with a history of chronic pain, Cram, et al. monitored the EMG levels of 10 muscle groups, first in the sitting and then in the standing posture.

The EMG study verified that in the sitting position, the patients exhibited significantly different postural activity than that of nonpain control subjects in the temporalis and the masseter muscles. The authors concluded that “the general findings of this descriptive study clearly suggest that the muscle activity of pain patients is different than that of nonpain subjects.” Because of their large number of nonpain subjects in this study (N=104), the investigators established a carefully collected normative data base for normal subjects.

Cram, J.K., and Engstrom, D. Patterns of neuromuscular activity in pain and nonpain patients. Clinical Biofeedback and Health Vol. 9, No.2, pp. 106-115, 1986.

The purpose of this study was to compare the patterns of surface EMG activity in normal subjects and in patients with a history of chronic pain. Surface EMG activity was sampled from the right and left homologous sites of ten muscle groups, first in the sitting and then in the standing posture. Statistical comparisons between the normal subjects (N=104) and chronic pain patients (N=200) were conducted on each of the 40 sources of surface EMG data. The results of the study indicate that chronic pain patients exhibit a higher level of surface EMG activity, with 38 percent of the sampled sites being significantly higher. An asymmetrical pattern of activation is clearly noted in the erector spinae muscles (while standing) for the pain population these findings support previous research indicating abnormal neuromuscular activity and posture in chronic pain patients.

SUBJECTS
The nonpain population consisted of 64 university students and 40 hospital outpatients screened in a general medical clinic for cardiovascular fitness. Prior to this screening, all 104 subjects were questioned about and denied ever being treated for chronic or recurrent pain during the past three years. . . . The mean age of the nonpain population was 29.6 years: 50% were female, 50% were male.

The population of pain patients consisted of 200 consecutive admissions to the chronic pain therapy program at the Swedish Hospital Pain Center.

RESULTS
Each muscle site for each posture was statistically compared using univariate F-tests. Because of the multiple comparisons, a probability value of .01 was used to decide as to the statistical separation of the two groups.

The mean and standard deviations for the two populations may be seen in Table 2. In all instances, a significant difference between the two populations represents a greater amount of activation of the muscles for the pain patients.

DISCUSSION
The general findings of this descriptive study clearly suggest that the muscle activity of pain patients is different than that of nonpain subjects.

This study identifies an objective diagnostic protocol by using EMG when comparing the postural activity and the maximal activity of the mandibular muscles against the normal ranges stored in a computer data base. Michler, et al. (Royal Dental College and University of Alborg, in Denmark) used EMG to differentiate between normal subjects and patients exhibiting functional disorder of masticatory system.

Michler, L., Moller, E., and Bakke, M. On-line analysis of natural activity in muscles of mastication. Journal of Craniomandibular Disorders: Facial & Oral Pain, Vol. 2, No. 2: 65-82, 1988.

ABSTRACT
A system for automatic on-line analysis of electromyographic recordings during natural activity in the muscles of mastication has been developed. Acquisition and digitizing of data, their filtering to mean voltage, gain read-off, statistical treatment, and table/graphic presentation of results are in control of a microcomputer. Amplitude and time measurements marked on direct tracings combined with tabulated statistics of the same data permit immediate check of their validity before further calculations. A screen menu directs standard examination reducing control of one pushbutton for start/compute and one for delete. With instant access to parameters of muscle action and appropriate statistics, electromyography is now available for direct functional control during clinical procedures and basic experiments.
1. Resting posture is recorded for 10 seconds with the subject sitting upright without a headrest and with eyes open.
2. Resting posture as in 1, but with eyes closed.

DISCUSSION
A standardized electromyographic examination of natural oral functions with manual, time-consuming acquisition of data and subsequent computer analysis has been employed for several years (Moller, 1966, 1970 and 1974).

Electromyography and Diagnosis: This article deals with the properties of an on-line measuring system opening for quantitative, statistical evaluation of findings in individual subjects versus normal materials available from control groups of previous studies (Moller, E. 1966; Moller, E., Sheikholeslam, A. & Lous, I. 1984; Lous, I., Sheikholeslam, A. & Moller, E. 1970; Sheikholeslam, A., Moller, E. & Lous, I. 1980).
Mean values with a standard deviation derived from that study are in the anterior temporal 3.5 mV, SD 1.7, in the posterior temporal 4.3 mV, SD 2.5, and in the masseters 2.4 mV, SD 0.8 (average of the right and the left side).

CONCLUSION
The present on-line analysis of natural activity in c1raniomandibular muscles builds on a quantitative method of assessing whole-muscle electromyograms used routinely in clinical and experimental work for more than 20 years. However, the advantages that are obtained by computerizing the procedure are epoch making:

It permits control of the data before they enter the final phase of calculations and offers instant guidance on treatment procedures or experimental set up. Any system with a delay exceeding the time the subject could reasonably be expected to remain available for a recording cannot fulfill this claim.

It limits duration for an extensive functional electromyographic examination to the time of recording. This represents saving 30 to 40 hours of measuring and a delay of about 2 weeks before having statistical data available.

In a controlled study of quantitative electromyography at University of Alberta, Gervais, et al, documented the comparison of the resting EMG levels from masseter and temporalis muscles of the “patient” subjects to that of the “normal” subjects. The 116 subjects in the study were classified into three experimental groups of asymptomatic (no signs or symptoms of TM dysfunction), subclinical (one or few signs or symptoms) and patient (significant signs or symptoms and pain).

The investigators concluded that “the patient group demonstrated significantly higher EMG activity than the asymptomatic or subclinical groups for all variables except the right masseter (F (3,220) = 6.65, p = <0.001). These findings strengthen diagnostic and assessment procedures and criteria as well as suggest alternate treatment.” The study?s extensive statistical tests further confirmed that “no sex differences were noted for the patient group, nor was patient age a clinically significant factor.”
Gervais, R.O., Fitzsimmons, G.W., and Thomas, N.R. Masseter and temporalis electromyographic activity in asymptomatic, subclinical, and temporomandibular joint dysfunction patients. The Journal of Craniomandibular Practice, Vol. 7, No. 1, pp. 52 – 57, 1989.

ABSTRACT
For the present study, resting EMG levels for each masseter and temporalis were obtained from three groups of subjects: asymptomatic (female, N = 24, mean age = 26.4); subclinical (female, N = 31, mean age = 28.6); and patient (N = 61, female 70%, mean age = 31.9). A Biocomp 2001 biofeedback system was used to gather the EMG data from each of the four sites during a six- to eight-minute resting baseline period. The patient group demonstrated significantly higher EMG activity than the asymptomatic or subclinical groups for all variables except the right masseter (F (8,220) = 6.65, p < 0.001), the temporalis was found to be the site of greatest EMG activity more frequently than the masseter. These findings strengthen diagnostic and assessment procedures and criteria, as well as suggest alternate treatment and research protocols.

METHOD
The 116 subjects for this study were classified into three experimental groups: asymptomatic, subclinical, and patient.
All respondents •were subsequently asked to complete a TMJ symptom questionnaire in order to determine group membership. The subclinical subjects were selected on the basis of a “yes” response to any of the symptoms in addition to facial or TMJ pain, or joint noises.

The selection of the asymptomatic subjects proceeded in a similar manner except that none could have positive signs or symptoms of TM dysfunction.
The patient group was comprised of 61 consecutive TMJ patient referrals directed to the author?s practice…. Forty-three (70%) were female (mean age 33.8), and 18 (30%) were male (mean age 31.2)….. Demographically, the patient group was consistent with the epidemiological literature which reports that 65% to 80% of clinical patients are female, and predominantly between 20 to 40 years of age.21.

PROCEDURE
The subjects were instructed to maintain a stable relaxed posture with eyes closed and to avoid extraneous movement during the recording periods, which were 6 to 8 minutes in length. No specific instructions to relax the jaw were given as the intent was to assess typical or habitual temporalis and masseter activity.

RESULTS
The null hypothesis of no difference in group means was tested and rejected using multivariate analysis of variance procedures (MANOVA). Significantly greater mean EMG levels were noted in the patient group as compared to the asymptomatic and subclinical groups, F (8,220) 6.65, p < 0.001.

Simultaneous confidence intervals for each of the four variables revealed that the patient group differed significantly, at the 95% confidence level, from the asymptomatic and subclinical groups on all variables except the right masseter.

A Hotelling?s T2 analysis of the patient group EMG levels was also conducted on the basis of sex. No significant differences between male and female subjects were noted, F (4, 56) 0.85, p 0.49.

DISCUSSION
In this study, three groups of subjects comprising asymptomatic, subclinical, and TM dysfunction patients were contrasted with respect to resting masseter and temporalis EMG levels. The patient group was found to have significantly greater resting muscle activity on three of the variables: left and right temporalis, and left masseter. No sex differences in muscle activity were noted for the patient group, nor were patient age a clinically significant factor.

The findings of this study suggest that for both female and male TMJ patients, resting EMG levels of 2 [microvolts] with a standard deviation of 1 [microvolt] indicate normal temporalis or masseter activity. Conversely, in assessing the appropriateness of biofeedback training, the presence of a neuromuscular component in the condition would be indicated by mean resting EMG of 5 [microvolts] with a standard deviation of 4 [microvolts] in one or more of the masseters or temporalis. A positive correlation would be expected between the magnitude of EMG activity and the severity of the reported clinical signs and symptoms.

In a treatment protocol based on the normative indications, the therapeutic goals would focus upon reducing the presenting neuromuscular hyperactivity for all four muscle sites to normal or subclinical levels, at which point the muscle-related symptoms should resolve.

STUDIES THAT DOCUMENT THE DIAGNOSTIC EFFICACY OF
ELECTROMYOGRAPHY IN DENTISTRY

In this well-known study, Moller (the head of the Department of Electromyography, Royal Dental College, Copenhagen), states that, “Electromyography of the activity during full effort is an important supplement to the clinical examination of the muscles of mastication.” This conclusion further verifies the rationale for the standard EMG function clinical protocol. Moller also concludes, “Electromyography provides an objective means of deciding the degree of predominance of one side during natural chewing.”

The author concludes: “The functional analysis includes the muscular activity at rest and during full effort, chewing and swallowing; it requires quantitation of the electromyograms and statistical evaluation of the findings. As a supplement to the clinical and radiological investigation, electromyography can contribute to a more precise diagnosis of functional disorders of the chewing apparatus and of the importance of function in malocclusion.”

Moller, E. Clinical electromyography in dentistry. Int. Dent. J., 19: 250-266, 1969.

INTRODUCTION
Electromyography is the recording of muscle action potentials. Therefore the electromyogram reflects the degree of motor innervation and gives an indirect measure of the force exerted by the individual muscle as well as the time of its activation.

The aim of the present report is to exemplify electromyography as a diagnostic method in dentistry.

Electromyography can be applied clinically in two ways:

1. Findings from intramuscular recordings during voluntary effort can contribute to the differentiation between myopathy and neuropathy; the electromyographic changes include duration, amplitude, and shape of single action potentials, the territory of individual motor units, and the pattern of activity during full effort (Buchthal, 1966).

2. During natural function simultaneous recordings from several muscles can be used to analyze their coordination in time and intensity. In the case of dentistry, functional analyses by electromyography can contribute to a more precise diagnosis of functional disorders of the chewing apparatus and of the importance of function in malocclusion.

TECHNIQUE
The electrical activity of muscles can be recorded intramuscularly with needle electrodes or over the muscles with surface electrodes. For the functional analysis, surface electrodes are applicable to study the action of the temporal, masseter, and orbicularis one muscles.

During natural function the activity in most instances appears as a pattern of interference. A measure of the amplitude, number and duration of the potential changes of the interference pattern can be obtained by recording its numerical mean voltage, the degree of activity at rest and during full effort (Fig. 1A) is characterized by the average level of the mean voltage.

ELECTROMYOGRAPHIC ANALYSIS OF ORAL FUNCTION
The different functions of the chewing apparatus demand a precise coordination in time and intensity of a large number of muscles. To demonstrate deviations in the patterns of coordination the electromyograms have been quantitated as described and compared with the findings obtained from 36 male dental students published earlier (Moller, 1966). The functional analysis includes the activity at rest and during full effort, chewing and swallowing

POSTURAL ACTIVITY
In subjects complaining of pain in the muscles of mastication or in the temporomandibular joints, the level of postural activity may be increased. A significant rise of the activity in a single muscle is demonstrated by recordings from a full denture subject with pain localized to the left side of the face (Fig. 2B) in the differentiation between causes of facial pain, electromyography can contribute by indicating if and where postural activity is increased.

FULL EFFORT
The clinical importance of recordings during full effort is demonstrated by three cases (Fig. 3, A-C).

In a 15 year old girl referred for electromyography with the diagnosis of hypertrophy of the masseters (A) a swelling over the right masseter had persisted after a parotitis at the age of twelve.
The activity recorded during maximal bite in the intercuspal position was 75 per cent less than normal on the right side (pseudohypertrophy) and twice the normal on the left side (hypertrophy). Duration of motor Unit potentials showed no significant difference from the normal. However, the average duration on the right side (8.9 msec) was significantly lower than on the left side (11.3 msec); this finding in connection with the difference between the mean voltages during full effort indicated a localized myogenic affection in the right masseter.

An electromyographic examination of a 47 year old woman with an apparent hypertrophy of the right masseter (B) showed normal level of activity in this muscle during full effort; recordings from the left masseter indicated a 75 per cent decrease. Average duration of action potentials in the right (10.4 msec) and left (9.4 msec) masseter showed no significant deviation from the normal (9.7 msec, standard deviation: 1.0 msec). Conclusion from the electromyographic examination: normal conditions on the right side, reduced number of motor units (atrophy) on the left side.

For a comparison, recordings are shown from a male dental student without clinical signs of hypertrophy of the masseter (C) but with 100 per cent increase of the level of activity on both sides.

Electromyography of the activity during full effort is an important supplement to the clinical examination of the muscles of mastication.

MASTICATION
Identical (symmetrical) innervation of the elevator muscles on the right and left side rarely occurs during natural chewing; in most instances there is a random shift of predominance and the extent and direction of mandibular movements. It is reasonable to assume that consistent predominance of one side can cause pain in muscles, joints and periodontal tissues. In fact, such a relation has been suggested from a clinical survey of 400 patients with disorders of the temporomandibular joints by Boering (1966).
Electromyography provides an objective means of deciding the degree of predominance of one side during natural chewing.

SWALLOWING
The action of the muscles of mastication during swallowing has mainly been considered in relation to malocclusion. Rix (1946), Tulley (1935) and Gwynne-Evans (1954) divided the oral phase during swallowing of saliva into a „normal? type performed with strong action of the masseter muscles, tooth contact and relaxed lips and an „abnormal? type without tooth contact and with intense action of the lips.

From electromyographic recordings during swallowing, it is possible to analyze the individual combination of muscle activity, and decide whether it has a compensatory or dysplastic effect on the malocclusion in question.

SUMMARY
The present report deals with the application of electromyographic analysis of oral function as a diagnostic method in dentistry. The functional analysis includes the muscular activity at rest and during full effort, chewing and swallowing; it requires quantitation of the electromyograms and statistical evaluation of the findings. As a supplement to the clinical and radiological investigation, electromyography can contribute to a more precise diagnosis of functional disorders of the chewing apparatus and of the importance of function in malocclusion.

In a controlled study of bite force in a group of patients and normal subjects, Helkimo et al. documented the statistically significant difference in bite force between the two experimental groups. The study at University of Gothenburg in Sweden found that “bite force in the patient group was lower than in the control group at the first registration but increased with palliation of the symptoms during treatment.” Since numerous studies have documented the strong positive correlation between electromyography and muscular force (Lippold, 1952; Bigland and Lippold, 1954; Molin, 1972; Milner-Brown, 1975; Prumi, 1978); the following study further supports the diagnostically valid EMG differences between patient groups and controls.

Helkimo, E., Carlsson, G.E., and Carmeli, Y. Bite force in patients with functional disturbances of the masticatory system. Journal of Oral Rehabilitation, 2: 397-406, 1975.

SUMMARY
In thirty patients (24 women and 6 men) treated because of dysfunction of the masticatory system at the department of stomatognathic Physiology, University of Gothenburg, bite force was registered before, during and after treatment had been completed. In the controls, thirty-six dental students and trainee dental nurses, with no dysfunction of the masticatory system, bite force was registered on two occasions.

Bite force was measured between the first molars on each side and between the central incisors. Also finger force was registered. The force measurements were made at five different levels, increasing from very weak to maximum force. Repeated tests of bite force in the control group, made at intervals of about 1 week, gave almost identical results. Bite force in the patient group was lower than in the control group at the first registration but increased with palliation of the symptoms during treatment.

In a 1985 study, Myslinksi et al., at the Department of Physiology of Baltimore College of Dental Surgery used EMG to quantify muscle pain in patients suffering from MPD before and after therapy. Using extensive statistical tests, the authors concluded that there was a parallel relationship between objective EMG and perceived pain. When pain attenuated and then returned, the EMG signals followed accordingly. The highest correlation between EMG readings and subjective pain ratings was demonstrated in the resting mode. The authors used multiple regression analysis to quantify the reduction in perceived pain from a proposed mathematical formula. Although the formula relates the change in pain levels using analgesics, the authors contend that the formula is also reliable for mechanical therapeutic techniques.

This study further emphasizes the importance of monitoring postural activity using quantitative electromyography for differential diagnosis of MPD and assessment of treatment progress.

Myslinski, N. R.., Buxbaum, J. D., and Parente, F. J. The use of electromyography to quantify muscle pain. Meth. and Find. Exptl. Clin. Pharmacol, 7(10): 551-556, 1985.

SUMMARY
This study was designed to determine the feasibility of using electromyography (EMG) to quantify muscle pain in patients suffering from chronic myofacial pain dysfunction (MPD). Ten patients were carefully selected to include those having mild to sever pain, but not any major psychological or other physiological dysfunction.

Measurements of perceived pain and EMG frequency and amplitude were recorded before and after standard analgesic therapy. EMG recordings were collected bilaterally from the masseter and anterior temporalis muscles during the resting, swallowing, clenching and chewing modes of activity. Multiple regression (R) analysis indicated that changes in perceived pain are correlated with changes in the EMG and can be determined by using the following formula: ?P = (?F) (0.405) + C where P = perceived pain level, F = EMG frequency, and C = 1.533. By computing the Phi coefficients, the highest correlation between EMO recordings and subjective pain ratings was demonstrated in the rating mode. In this mode, 64% (multiple R = 0.80) of the variance in perceived pain difference scores from pre- to post-therapy tests could be determined.

INTRODUCTION
EMG of the masticatory musculature has been employed as a method of diagnosis and pain assessment in both research and clinical situations (Sheikholeslam, A., Moller, E. and Lous, I. (1982). EMG has been of particular value in the diagnosis and treatment assessment of patients afflicted with MPD syndrome (Buxbaum, J.D., 1982; Mikhael, M., and Rosen, H., 1980). Munro has shown that EMG activity in the masseter and anterior temporalis muscles of patients with MPD is significantly greater than that of normal subjects (Munro, R. R., 1975). In laboratory tests on the patients in this study, we observed that levels of EMG activity appeared to have direct correlation with the subjective pain levels reported by the patients. This relationship is possibly due to a pain-spasm-pain cycle in the masticatory muscles. To evaluate the feasibility of using EMG to quantify muscle pain in these patients, the study was designed with the following questions in mind:
1. Is there a statistically significant correlation between changes in muscle pain and changes in EMG?
2. Which EMG parameter yields the strongest EMG/pain correlation?
3. Which mode of muscle activity yields the strongest EMG/pain correlation?

PATIENT SELECTION
Those that participated in the study were diagnosed as having MPD syndrome with muscle spasm as the major sign. They consist of six females and four males, aged 15 to 41 years, with mild to severe symptoms

EXPERIMENTAL PROCEDURE
Pain and electromyography readings were taken before and after therapy for each subject EMGs were obtained bilaterally with bipolar surface electrodes from both the masseter and anterior temporalis muscle during five modes of activity. Although other muscles may be involved in the pain, more than 90% of patients with MPD have involvement of the anterior temporalis or masseter muscles, or both (Mikhael, M. and Rosen, H. 1980). Subjects were their own controls. They had no knowledge of changes that occurred in their EMG. The investigators that administered the subjective visual scale for pain were different from those who analyzed the EMG.

STATISTICS
Difference scores were computed by subtracting each patients pre- and post- therapy pain, frequency, and amplitude scores.

RESULTS
There was a significant correlation between changes in pain and changes in EMG. They indicate that the most sensitive relationships between EMG measures and pain difference ratings were in the resting mode.

DISCUSSION
The present study attempted to correlate changes in muscle pain with changes in muscle spasm in a select group of patients chosen from MPD patients.

Spasm can be caused by muscle overextension, muscle over contraction, or muscle fatigue These habits can be triggered by such things as improperly occluding restorations, but are most often involuntary tension-relieving mechanisms (Laskin, D. 1969). The work of Christenson supports muscular fatigue as the direct cause of pain (Christenson, L. 1967). He found that prolonged voluntary loading can produce the signs and symptoms of MPD. In a study conducted by Yemm, subjects within a control group were able to relax their muscles between successive attempts at a task, but MPD patients could not relax their muscles between tasks (Yemm, R. 1969). The muscle spasm in MPD is self-perpetuating. The spasm causes pain and muscle tenderness, which in turn causes more spasm. The end result is a spasm-pain-spasm cycle. Bruno, in his work on premature occlusal contacts and masticatory muscle spasm, states that if muscle spasm is allowed to continue pain will result (Bruno, S. 1971). Travell states that when muscles are subject+ to pain they act only in one way: they develop spasm and shorten (Travell, J. 1960).

In the present study, there was a parallel relationship between objective EMG and perceived pain. It showed that the test for the multiple R was statistically reliable. As the pain attenuated after treatment and then returned, the EMG signals followed accordingly in a graded manner. The analgesic therapy breaks the pain-spasm-pain cycle. Eliminating the pain eliminates the muscle spasm.

In this study, the method used to change pain levels was the non-narcotic analgesic, ibuprofen. Other observations in the laboratory indicated that the correlation holds even when using different methods of changing pain levels. Preliminary data suggest that the regression equation is reliable with mechanical therapy to… For example, MPD pain can be alleviated by fitting the subject with a dental appliance to increase vertical dimension, or in other words, change the length of the masticatory muscles so that they are operating in a more optimal range

In summary, the present study has indicated a close correlation between changes in perceived muscle pain and changes in EMG parameters in uncomplicated MPD patients.

In a controlled clinical study in the Department of Physiology at the University of Florence, Pantaleo et al, electromyographically compared the postural activity and maximal activity of patient versus control subjects before and after treatment.

The authors conclude that in control subjects “the EMG analysis did not show any activity either in the masseter or temporalis muscles at rest…”. “The MPD patients showed involuntary muscular activity at rest in the anterior portion of the temporalis muscle…”. “The correction of occlusal position by acrylic splints was able to induce a persistent reduction of abnormal EMG activity at rest and a good relief of pain”.

Maximal (bite) activity of normal subjects “was fast and complete” in contrast to the patients that exhibited “low maximum level that was not maintained”. After occlusal therapy which accompanied improvements in symptoms, the patients showed “higher levels of EMG during maximal biting in the intercuspal position”.

Pantaleo, T., Prayer-Galletti, F., Pini-Prato, G., and Prayer-Galletti, S. An electromyographic study in patients with myofacial pain- dysfunction syndrome, Bulletin Group. Int. Rech. sc. Stomat. et Odont., 26: 167- 179, 1983.

In normal subjects, in an upright position with the lips slightly in contact but the teeth not together, during the voluntary muscle relaxation, there is virtually no activity in the masticatory muscles investigated (Vitti, M. & Basmajian, J. V. 1975 and 1977), except for the subjects with occlusal interferences.
The relationship between the electromyographic (EMG) activity of the masticatory muscles and some types of malocclusion has been investigated by earlier researchers (Moyers, R. E.,1949; Liebman, F. M. & Cosenza, F., 1960; Moss, J. P. & Greenfield, B.E., 1965; Grosfeld, 0., 1965; Ahlgren, J., 1966; Ahlgren, J. et al., 1973; Moss, J. P. & Chalmers, C. P., 1974; Moss, J. P., 1975; Pancherz, H., 1980), chiefly regarding temporalis and masseter muscles.

METHODS
The subjects studied were 5 healthy volunteers (3 males and 2 females) aging from 19 to 38 years) and 11 MPD syndrome patients (6 males and 5 females ranging in age from 20 to 42 years). .
The EMG activity was studied both at rest and during the maximal biting in the intercuspal position

CONTROL SUBJECTS
The EMG analysis did not show ant activity either in the masseter or in the temporalis muscles at rest; (maximal biting in the intercuspal position) was fast and complete.

PATIENTS WITH MPD SYNDROME
The EMG study revealed an involuntary muscular activity at rest in the anterior position of the temporalis muscle in all rested positions
When the occlusion was corrected with acrylic splints the EMG patterns also improved: the involuntary EMG activity greatly decreased and sometimes disappeared after the splint was applied.

All MPD patients displayed abnormal EMG patterns during the maximal biting in the intercuspal position, i.e., the EMG activity during the voluntary contraction increased slowly to a low maximum level that was not maintained (progressive decrease of activity).

After the correction of the occlusal malrelation with splints, EMG patterns improved and appeared more similar to those of control subjects.

DISCUSSION
The decrease of the abnormal EMG activity after the correction of the occlusal malrelation with acrylic splints confirms this interpretation and points out that an occlusal discrepancy can be the cause of an abnormal EMG activity and of the consequent MPD syndrome.

SUMMARY
An electromyographic (EMG) study of ispsilateral masseter and temporalis muscles was undertaken in healthy volunteers and in patients with MPD syndrome, with the aim of getting further insight into the pathophysiology of this disease. Unlike controls, patients had abnormal MKG features and displayed unvoluntary sustained EMG activity at rest, chiefly in the temporalis muscle.

The correction of occlusal position by acrylic splints was able to induce a persistent reduction or a suppression of the abnormal EMG activity at rest and a good relief of pain; moreover, after the correction, higher levels of EMG activity were found during maximal biting in the intercuspal position.

Mechanisms underlying these effects were discussed and in particular it was suggested that abnormal afferent activity from periodontium and jaw muscles may contribute to the establishment of sustained contraction leading to muscular pain, which in turn may cause reflex muscle activity in a vicious circle.

Cooper et al., in a 1986 study of 476 patients conducted at New York Medical College, investigated the symptoms, clinical examination findings, and EMG of postural activity of masseter and temporalis muscles. The authors demonstrated that only 11% of subjects had normal postural activity prior to treatment. Following 3 months of therapy, 88% of subjects reported that improvement or cure of some symptoms had occurred.” The improvement in symptoms accompanied reduced postural EMG activity of the masseter and temporalis muscles.

Cooper, B.C., Alleva, M., Cooper, D., and Lucente, F.E. Myofacial pain dysfunction: Analysis of 476 patients. Laryngoscope, 96: 1099-1106, 1986.

ABSTRACT
Myofacial Pain Dysfunction (MPD) is a musculoskeletal dysfunction involving malrelationship among the neuromuscular system, temporomandibular joints, and dental occlusion. The illness affects children and adults of all ages and both sexes. Patients complain of pain and or dysfunction in the mandible, temporomandibular joints, ears, oral cavity, head, and neck. Electronic measurement of mandibular movement and associated muscle function now provide reproducible data with which the parameters of this illness and therapy can be designed and monitored.
In this study, data are presented on 476 MPD patients. Included are statistics on the most commonly occurring symptoms, clinical examination findings, and electronic test data before and following treatment Electromyography (EMG) is used to analyze the resting status of mandibular muscles and the functioning in the occlusal position.

The data show a positive correlation between the clinical symptoms of MPD and unhealthy mandibular position at occlusion, accompanied by specific unhealthy muscle activity. There is a strong positive correlation between a therapeutic change in the dental occlusion to a neuromuscularly healthy person using a precision orthotic appliance and the relief of symptoms within 1 month as expressed by 88% of the patients. A similar correlation exists at 3 months and long-term.

In this study, data is presented on symptoms and clinical examination findings of 837 MPD patients. With the use of bioelectronic instrumentation, fixed parameters of a single disease entity (MPD) have been established and are used to evaluate patients, design therapy, and monitor results objectively. Electronic test data before and after treatment are presented on 476 patients from the initially examined subject group.

MATERIALS AND METHODS
From 1979 through 1985, 837 patients (661 females and 176 males) between the ages of 7 and 76 were referred for diagnosis of MPD.
A subgroup of 476 subjects underwent complete testing and therapy.

The Electromyograph (models EM1R and EM2) utilizes bipolar surface electrodes to measure the electrical activity in the muscle fibers beneath the electrode. This instrument measures both the resting levels of muscles and their compressive capacity at work in this study, middle masseter and anterior temporalis muscle fibers were monitored.

PHASE I CLINICAL DIAGNOSIS
Following review of a patient?s questionnaire, which included a listing of symptoms and history, a clinical examination was performed to determine whether a tentative clinical diagnosis of MPD could be established. Lateral transcranial radiographs of the temporomandibular joints were taken to rule out joint abnormalities.

PHASE II ELECTRONIC DIAGNOSIS
An electromyographic study of electrical resting levels of the anterior temporalis and masseter muscles bilaterally was performed.

PHASE III THERAPY
Within the first 3 weeks after insertion of the appliance, patients returned for adjustments . . . one to three times. They were asked to reevaluate the status of their symptoms 1 month after initiation of therapy.

PHASE IV REEVALUATION AND MONITORING THERAPY
Three months after initiation of treatment, patients were retested with MKG EMG to determine whether the orthotic appliance being worn accurately provided neuromuscular occlusion Patients, were again requested to compare their current symptoms with their original presenting symptoms.

PHASE V LONG-TERM TREATMENT
Long-term therapy was recommended to perpetuate the neuromuscular occlusion.

RESULTS
After 1 month, 79% indicated that they had experienced significant improvement or cure of symptoms as a result of therapy.
Following 3 months of therapy, 88% of subjects reported that improvement or cure of some symptoms had occurred.
Electromyographic recordings made on the anterior temporalis and middle masseter muscle fibers bilaterally were used to determine the resting status of those muscle groups . . . . “Rest” . . . . is defined . . . . on the EM2 instrument as ?? 2.5 mV. 17.1% of subjects had 3 of the 4 muscles rested and 11.2% had all four muscles rested.

The treated group of 476 received electromyographic testing which substantiated the hypothesis that almost all individuals attempt to posture their mandibles in an accommodative position. This is done by hyperactivity in the muscles responsible for movement.

The importance of the establishment of a neuromuscular occlusal position is that it corresponded directly with the patient evaluation of significant improvement or cure of symptoms at each of the same stages of treatment. At 3 months under therapy, 88% of patients reported improvement or cure, and of those undergoing long-term treatment, 100% acknowledged significant improvement or cure.

In a carefully controlled and quantitative longitudinal study of 31 patients with signs and symptoms of functional disorders, Sheikholeslam et al,, investigated clinically and electromyographically the long-term effects of occlusal splint therapy over a 7 year period.

The extensive statistical tests of quantitative EMG data indicated that postural activity of the patients decreased significantly in the masseter and temporalis muscles as signs and symptoms improved, following the splint therapy. Additionally, the treatment coincided with a more symmetric pattern of postural activity.

The 27 patients whose signs and symptoms had improved were asked to stop using the splint 3-6 months after treatment. 78% of these patients had recurring signs and symptoms during the 7 years of follow-up. The recurring signs and symptoms accompanied significant increases of EMG postural activity in the masseteric and temporal muscles.

Sheikholeslam, A., Holmgren, K., and Riise, C. A clinical and electromyographic study of the long-term effects of an occlusal splint on the temporal and masseter muscles in patients with functional disorders and nocturnal bruxism. Journal of Oral Rehabilitation, 13: 137-145, 1986.

SUMMARY
The postural activity of the temporal and masseter muscles in thirty-one patients with signs and symptoms of functional disorders were studied: before during and after 3-6 months of occlusal splint therapy. The fluctuating signs and symptoms, as well as the postural activity of the temporal and masseter muscles were significantly reduced after treatment. Further, the coefficients of correlation within pairs of postural activity of the right and left muscles increased significantly. After cessation of the splint therapy the signs and symptoms recurred to the pre-treatment level within 1-4 weeks in about 80% of the patients. The results indicate that an occlusal splint can eliminate or diminish signs and symptoms of functional disorders and re-establish symmetric and reduced postural activity in the temporal and masseter muscles, which can facilitate procedures, such as functional analysis and occlusal adjustment.

INTRODUCTION
The occlusal splint is one of the most generally accepted devices for treatment of signs and symptoms of functional disorders in the stomatognathic system. Its effectiveness has been questioned by Greene & Laskin (1972) who reported that there was a large placebo component in the subjective response of patients undergoing such therapy. Nevertheless, there is evidence that the use of the occlusal splint leads to improvement of dysfunctional signs and symptoms (Posselt & Wolff, 1963; Fuchs et al., 1972; Fuchs, 1975; Kovalevksi & De Boever, 1975;
Clark et al., 1979; Zarb & Speck, 1979; Beard & Clayton, 1980; Wedel et al., 1981; Dahlstrm, Carlsson & Carlsson, 1982; Hamada et al., 1982).

In short-term investigations Jarabak (1956) reported that in patients with spasm in the temporal muscle, the postural activity decreased immediately after insertion of an occlusal splint. As the activity returned to the pre-treatment level 5 mm after the removal of the splint Jarabak (1956) concluded that the occlusion plays at least some part in the aetiology of temporal muscle spasm. Roura & Clayton (1975) found that 1 month of occlusal splint therapy in patients with TMJ dysfunction, there was relief of most of the clinical symptoms and reduction of postural activity in the elevator muscles.

In a previous report (Holmgren, Sheikholeslam & Riise, 1985), the immediate effects of occlusal splint therapy were described. Little is known, however, about the therapeutic mechanisms underlying the long-term effects of an occlusal splint. The purpose of the present study, the second one in a series performed during a period lasting up to 7 years, was to investigate both clinically and electromyographically the long-term effects of occlusal splint therapy in patients with functional disorders and nocturnal bruxism.

MATERIALS AND METHODS
Thirty-one patients (26 female, 5 male, 18-27 years, median 27) with signs and symptoms of functional disorders and nocturnal bruxism.

ELECTROMYOGRAPHY
The postural activity in the anterior temporal and masseter muscles was recorded bilaterally, with the patient seated upright without head support and looking straight forward and teeth apart. The activity was picked up by bipolar surface electrodes.

TREATMENT
The patients were examined in almost 2 week intervals, and the splint adjusted if necessary during the treatment period (3-6 months).

ELECTROMYOGRAPHY
On the average, the occlusal splint therapy resulted in a significant reduction of the postural activity during mandible at rest. Pairing the data of the postural activity in the right and left temporal muscles of twenty-seven patients whose signs and symptoms improved after splint therapy showed that the coefficients of correlation (r) within pairs of postural activity in the right and left temporal muscles increased from r = + 0.48 before, to r = + 0.77 after treatment (zo = 1.86, 0.05<P<0.10).

Further, the postural activity was significantly reduced (P<0.05) in the masseter muscle. In addition the coefficients of correlation (r) within pairs of postural activity in the right and left masseter muscles of twenty-seven patients whose signs and symptoms improved after splint therapy increased from r = + 0.353 before, to r = +0.686 after treatment (zo = 1.768, 0.05<P<0.10).

In a recent electromyographic study, Sheikholeslam, Moller & Lous (1982) reported that after conventional treatment of patients with functional disorders (occlusal splints, occlusal adjustments, etc.) the postural activity in the elevator muscles was reduced simultaneously with the improvement of signs and symptoms. Further, in a short-term study, Riise & Sheikholeslam (1982) reported that in less than 48 h after insertion of an artificial occlusal interference in the intercuspal position in subjects without a history of functional disorders, signs and symptoms of these disorders developed concomitantly with an increase of postural activity and improvement of the signs and symptoms. Thus, the occlusion is likely to be a co-factor in the development of muscular hyperactivity.

When the patients in this study were instructed to stop wearing the splint, the signs and symptoms of nocturnal bruxism returned to the pre-treatment level within a period of 1-4 weeks in 80 % of the patients. This is in line with Beard & Clayton (1980), Solberg et al., (1979), reporting that when the splint was removed in patients with habitual nocturnal bruxism, the cumulative EMG of the masseter muscles during sleep immediately increased top the pre-treatment level, Thus the splint therapy in most cases must be regarded as a symptomatic treatment.

Rugh & Solberg (1975) stated that nocturnal bruxism is closely related to the psychic stress level of the previous day. However, in the present study there were no attempts to treat or change the patients psychic state. Therefore the results suggest that the reduction of postural activity and improvement of signs and symptoms in at least 80% of the patients during the splint therapy, were not due to changes in the patient?s daily psychological condition as stated in the questionnaires. Moreover, had it been so, one would expect that there should be no recurrence of signs and symptoms when the patients stopped wearing the occlusal splint.

The increase in the coefficients of correlation (r) within pairs of postural activity in the right and left temporal and masseter muscles after treatment was due to a more symmetric pattern of postural activity. This is consistent with the findings of Ramfjord (1961a) who reported that a well-balanced muscle activity during the rest position was recorded after successful treatment of patients with habitual bruxism. Recently, Sheikholeslam et al., (1982) reported that symmetry of the relative loading of the right and left anterior temporal muscle (assessed in term of coefficients of correlation by correlating postural activity as a percentage of maximal activity) increased after treatment. Moreover, as the mandible at rest in the upright position is controlled by muscular activity (Lund, Nishiyania & Moller, 1970; Moller, Sheikholeslam, & Lous, 1971; Moller, 1971; Rugh & Drago, 1981), the reduced and more symmetric postural activity in both temporal and masseter muscles after occlusal splint therapy in the present study may even be able to influence the position of the mandible. Kovalevski & De Bover (1975) found that in nine out of eleven patients with signs and symptoms of functional disorders, the position of the mandible in relation to the maxilla changed toward the side of the painful joint after one month of occlusal splint therapy. Further, Mejias & Mehta (1982) reported that there was a definite shift in the maxillomandibular relationship in favour of a more distal position of the mandible after splint therapy which is also in agreement with the findings of Riise & Sheikholeslam (1985) on patients after occlusal adjustment therapy.

The occlusal splint may be very valuable in the examination and treatment of patients with mandibular dysfunction. It is a simple device which can reduce signs and symptoms and establish a more symmetric muscular rest activity. The splint also enhances procedures such as functional analysis and occlusal adjustment, and is a valuable tool which can be used for periods of several years, especially in patients where other forms of treatment are difficult to perform.

CONCLUSION
As the splint can eliminate signs and symptoms of functional disorders and create symmetric and reduced postural activity in the temporal and masseter muscles, it may enhance procedure where functional analysis and occlusal adjustment are involved.

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