ELECTROMYOGRAPHIC HETEROGENEITY IN THE HUMAN TEMPORALIS AND MASSETER MUSCLES DURING DYNAMIC TASKS GUIDED BY VISUAL FEEDBACK

The complex architecture of the human jaw muscles suggests regional di fferences in function within these muscles. This study examines the wa y the temporalis and masseter muscle regions are activated when free m andibular movements with various speeds and against various external l oads 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 laterodevi ations. During open/close excursions and protrusion/retrusion movement s, an anterior and posterior temporalis part could be distinguished, w hereas during laterodeviations a more complex partitioning of this mus cle was observed. During the protrusion/retrusion movements and the la terodeviations, the temporalis muscle demonstrated higher EMG peak act ivities than the masseter muscle, and within the masseter muscle the d eep masseter showed higher EMG peaks than the superficial one. In cont rast to this, during the open/close excursions the masseter showed hig her peak activities than the temporalis muscle, while the superficial masseter showed higher EMG peak activities than the deep masseter. Wit hin the deep masseter, differences were also found. During open/close excursions, the anterior deep region demonstrated higher EMG peak acti vities 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 ext ernal load. Only during laterodeviations did speed and load equally in fluence peak activity in both the deep and superficial masseter. Durin g 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. T his points to a partitioning of the excitatory command of the motoneur on pool.