Effect of bite force on the power spectrum of the surface electromyogram of human jaw muscles.

The frequency content of the surface electromyogram (EMG) of the anterior temporal and masseter muscles of 14 normal adults was obtained during maximum clench in intercuspal position, as well as at different bite force levels. Frequency analysis was carried out by computer using a fast Fourier transform algorithm to obtain the power-spectral density function and the power spectrum of the EMG signal. Bite force was monitored by a force transducer placed intra-orally. The power spectra of the anterior temporal muscle during maximum clench in intercuspal position were significantly wider than those of the masseter muscle, indicating that the EMG of the former muscle contained more power at high frequencies than that of the masseter muscle. Increase in bite force produced, in both muscles, a power-spectrum shift to lower frequencies. The shift was probably caused by recruitment of larger motor units and by increase in synchronization between them. Different contents of A, B and C fibres and/or differences in thickness of skin covering the muscles could account for the differences between the temporal and masseter muscle power spectra.