The activity patterns of the masseter and the anterior temporal muscles were studied in twenty-one healthy male subjects while clenching at 10, 20, 30, 40 and 50% of the maximum clenching level. At low clenching levels the temporal muscle activity tended to dominate, at high levels the masseter muscle activity was stronger (P less than 0.001). The asymmetry in muscle activity also depended upon the clenching level (P less than 0.001), while at each level the masseter muscle asymmetry was greater than the temporal muscle asymmetry (P less than 0.05-P less than 0.025). By comparing the electromyographic activities of the left and right side within each subject it was found that the masseter muscle with the higher electromyographic activity tended to have the larger cross-sectional area (P less than 0.01) and at the 50% clenching level it tended to be on the side with the greater number of post-canine tooth contacts (P less than 0.001).

significant CMD, age and gender effects were observed in the masticatory EMG activities of a group of myogenous CMD patients and a control group.

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The EMG activities of the masseter muscles and the anterior and posterior parts of the temporalis muscles were investigated in different vertical and sagittal jaw relations. Surface EMG recordings were made. Relative muscle activities were quantified by means of the Activity Index and the Asymmetry Index. Ten subjects with a healthy masticatory system were asked to clench at 10% and 50% of their maximum voluntary clenching level. Registrations were made during clenching in the intercuspal position, on a thin stabilization splint (SSP, 1-2 mm increase of vertical dimension at first molar), on a thick stabilization splint (3 mm thicker at the incisal edge relative to SSP), and on a protrusive re-positioning splint (3 mm protrusive to SSP). The masseter EMG amplitudes were the same for all the jaw positions investigated. However, at the 10% clenching level, the temporalis muscle activity decreased after the vertical dimension was raised and decreased further after the mandible was protrusively positioned (p less than 0.01). An independent function for the posterior and the anterior parts of the temporalis muscle was not found in any of the intermaxillary jaw relations investigated. At the 50% clenching level, a decrease in anterior temporalis muscle activity was observed only with the protrusive position in relation to the other three positions. Also at this level, the index relating the activity of the anterior to that of the posterior part of the temporalis muscle was not different between jaw positions. Raising the vertical dimension and protrusive positioning of the mandible decrease the activity of the temporalis muscles. This may be a factor related to the therapeutic effect of the stabilization splint in the treatment of craniomandibular disorder patients.