Surface EMG frequency dependence on force in the masseter and the anterior temporal muscles.

Myoelectric power spectrum analysis was used to study isometric contractions, 10-15 s long, of the masseter muscles and the anterior temporal muscles. Surface EMG activity was recorded from these muscles for nine females while biting on a bite force transducer up to maximal effort (100% maximal voluntary contraction; MVC). The mean frequency (MF) was calculated as a single estimate of the myoelectric power spectrum. Regression analyses were made of MF versus bite force (0-100% MVC). The mean MF values of all females' masseter muscles increased up to 55-60% MVC. For the anterior temporal muscles no increase in mean MF was found above 20-25% MVC. The increase in MF was possibly dependent on recruitment of type I fibers and low-pass tissue filtering effects. The slope of regression for the force level 0-60% MVC was intraindividually steeper for the masseter muscles than for the anterior temporal muscles. A similar decrease in mean MF was found for the masseter muscles and the anterior temporal muscles for the force level 60-100% MVC. The possibility of muscular fatigue was discussed.