ELECTROMYOGRAPHIC HETEROGENEITY IN THE HUMAN TEMPORALIS AND MASSETER MUSCLES DURING STATIC BITING, OPEN CLOSE EXCURSIONS, AND CHEWING

The human temporalis and masseter muscles are not activated homogeneou sly during static bite force tasks. In this study, we studied the poss ible existence of regional differences in these muscles under dynamic conditions. Electromyographic (EMG) activity was recorded by means of bipolar fine-wire electrodes. Six electrodes were inserted into the te mporalis muscle and three into the masseter muscle. Recordings were ma de during maximal-effort intercuspal and incisal static clenches, open \close excursions from both the intercuspal and incisal positions, and unilateral gum and licorice chewing on right and left sides. The EMG peak amplitudes and the peak occurrences were compared. During the sta tic clenches and the open\close excursions, no differences could be de monstrated between the regions of the temporalis muscle. However, duri ng the chewing tasks, the anterior and posterior regions behaved diffe rently. Throughout almost all tasks, both superficial and deep parts c ould be distinguished in the masseter muscle. A further division of th e deep masseter was task-dependent. In both the temporalis and massete r muscles, maximal activity (100%) was reached during intercuspal clen ches. The average activity declined to 35% of the maximal activity in the temporalis muscle, to 47% in the deep, and to 86% in the superfici al masseter during incisal clenches. During all chewing tasks, the EMG peak activity of the anterior temporalis and the superficial masseter muscles was higher in the working than in the balancing condition. Th e general finding was that different regions were preferentially activ ated, according to task. The detailed regional specialization previous ly observed during static bite force tasks could not be demonstrated i n the present study.