Jaw muscle activity in relation to the direction and point of application of bite force.

This study examined the way the masseter and the anterior and posterior temporal muscles of human subjects were activated when a bite force of constant magnitude was exerted at different teeth in different defined directions. Subjects were instructed to produce a bite force of a particular magnitude (250 N) and direction (vertical, anterior, lateral, posterior, or medial) using a three-component force transducer and a feedback method. Simultaneously, the EMG activity of the muscles was registered with use of surface electrodes. Unilateral bite forces were produced at the right canine, second premolar, and second molar. In addition, a bilateral vertical premolar bite was examined for comparison of activity levels of the muscles of both sides. The results showed that, on average, the activities of the right and left side muscles did not differ in a bilateral vertical bite. Moreover, in a unilateral vertical bite, there were no significant right-left differences. For all bite force directions, more muscle activity was required for production of a constant bite force at the anterior side of the dental arch than at the posterior side. An exception to this rule was the activity of the posterior temporal muscle in a posteriorly directed bite. There was a close relationship between the direction of bite force and jaw muscle activity. Variations in activity were small for the anterior temporal muscle and large for the posterior temporal and masseter muscles. All muscles that were studied were always active, even in directions in which they could be expected to act antagonistically.