Electromyographic heterogeneity in the human temporalis muscle.

This study examined the possible existence of regional differences in activation of the temporalis muscle. Since the muscle is fan-shaped, its fibers pull in different directions. This suggests regional specialization for different motor tasks. EMG activity was registered by six bipolar fine-wire electrodes inserted anteroposteriorly across the muscle belly. Muscle signals were recorded during different static bite tasks for which both the direction and magnitude of bite force were specified. The results showed that the ratio of activities of the six muscle regions changed with the direction of bite force. This indicates a partitioning of the excitatory command to the muscle's motoneuron pool. Alteration in activity with changing bite-force direction was generally the smallest in the anterior-most region of the muscle, the largest in the posterior-most region, and the intermediate in the interjacent regions. Generally, all muscle regions exhibited the highest EMG activity when the bite force was in an approximately posterolateral direction. The muscle was activated uniformly only for bites in this preferential direction. Activity in the regions appeared to be scaled up or down in a linear way according to the desired bite-force level. The results indicate that the direction of pull of the muscle and the maximal force it can produce are not fixed, but depend on the direction of bite force.