Power spectral analysis of the surface electromyogram of human jaw muscles during fatigue.

Muscle fatigue in the anterior temporal and masseter muscles of 13 normal subjects was induced by maximum clench in intercuspal position. Frequency analysis using a fast Fourier transform algorithm to obtain the power-spectral density function and the power spectrum of the electromyogram signal indicated that the power spectra obtained during fatigue were statistically significantly shifted to lower frequencies and narrower than those obtained at the beginning of the clench. The shift was due to a significant increase of the power in the lowfrequency range and a significant decrease of that in the high-frequency range. The powerspectrum shift to lower frequencies had an exponential time course. The most pronounced shift occurred in the first 25 per cent of the total clenching time. Decrease of the conduction velocity of the action potential along the muscle fibre seems to be the main cause of the shift.