Electromyographic Heterogeneity in the Human Temporalis and Masseter Muscles during Dynamic Tasks Guided by Visual Feedback

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Abstract
The complex architecture of the human jaw muscles suggests regional differences in function within these muscles. This study examines the way the temporalis and masseter muscle regions are activated when free mandibular movements with various speeds and against various external loads are carried out guided by visual feedback. Electromyographic (EMG) activity was registered in six temporalis and three masseter muscle regions with bipolar fine-wire electrodes. Recordings were made during open/close excursions, protrusion/retrusion movements, and laterodeviations. During open/close excursions and protrusion/retrusion movements, an anterior and posterior temporalis part could be distinguished, whereas during laterodeviations a more complex partitioning of this muscle was observed. During the protrusion/retrusion movements and the laterodeviations, the temporalis muscle demonstrated higher EMG peak activities than the masseter muscle, and within the masseter muscle the deep masseter showed higher EMG peaks than the superficial one. In contrast to this, during the open/close excursions the masseter showed higher peak activities than the temporalis muscle, while the superficial masseter showed higher EMG peak activities than the deep masseter. Within the deep masseter, differences were also found. During open/close excursions, the anterior deep region demonstrated higher EMG peak activities than the posterior region, whereas during protrusion/retrusion and laterodeviations the posterior deep region showed higher peaks. In general, speed had a greater effect on the EMG peak activity than external load. Only during laterodeviations did speed and load equally influence peak activity in both the deep and superficial masseter. During protrusion/retrusion movements, load showed no significant effect on EMG peak activity in the masseter muscle. A general finding was that, according to task, different regions were activated preferentially. This points to a partitioning of the excitatory command of the motoneuron pool.