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The relation between integrated electromyographic activity and computed biting force during voluntary isometric contraction was evaluated in the masticatory muscles of healthy subjects. The slopes of the curves relating integrated electromyographic activity to computed biting force in masseter muscles were steeper on the non-preferred chewing side than on the preferred chewing side, and they progressively became steeper during the course of continuous isometric contraction of a given biting force.

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An electromyographic study of the masseter and anterior temporal muscles was performed on fifteen bruxism patients with clinical symptoms such as muscle pain, fatigue and tenderness. The slopes of the voltage tension (V/T) curves (calculated as the value of the relative inclination between the integrated electromyogram and the biting force) were compared before and after treatment with an occlusal splint. Following use of the splint, the steep slopes of the V/T curves, common in bruxism, became gentle and similar to those of healthy subjects. The difference found in the slopes of different types of bruxism (grinding, clenching, grinding and clenching) might be useful as a diagnostic tool. Follow-up recording of the slopes was considered to be a useful indicator of treatment effectiveness.

The present study investigated the asymmetry of masticatory muscle activity during maximal intercuspal clenching in healthy subjects and subjects with stomatognathic dysfunction syndrome. Stomatognathic dysfunction syndrome, unilateral mastication and the asymmetry of masticatory muscle activity appear to be related to each other. The asymmetry of masseter muscle activity was greater as stomatognathic dysfunction became increasingly severe. Because fatigue and pain are produced more quickly by unilateral clenching than bilateral clenching, clenching under conditions of left and right muscular imbalance can further aggravate stomatognathic dysfunction. It is suggested the asymmetry of masseter muscle activity during maximal clenching correlates with the onset of the stomatognathic dysfunction syndrome. Stomatognathic dysfunction syndrome is closely related to the asymmetry of masseter muscle activity and only slightly related to the asymmetry of temporal muscle activity. The asymmetry of anterior temporal muscle activity appears to have little clinical significance.