In five male and one female volunteers, the electromyogram (EMG) for sustained isometric exercises of the jaw openinq and jaw closing muscles was analyzed using power spectrum.
The power spectra obtained from jaw closing muscle (masseter muscle) during isometric contraction (clenching) in the maximum intercusspal position were shifted to lower frequencies and the bandwidth became narrower than these obtained at the beginning of this exercise. Although during clenching accompanying activity was observed from the antagonist (digastric muscle), there was no evidence which suggests the reciprocal and/or synchronized firing of the two muscles in the frequency analysis (cross spectrum and coherence) .
Fatigue of the digastric muscle was also investigated. The isometric contraction of the digastric mucle was sustained by supporting the Jaw by hands. Although the digastric muscle contains much more fatigue resistant fibers than the masseter muscle, some shift had been expected in the power spectra. However, the power spectra obtained muscle during isometric contraction of the jaw opener remained as those observed at the beqinninq of this exercise.
The results suggest that the power spectrum shift observed during isometric contraction is mainly due to a slowing of the conduction velocity of the action potential along the muscle fiber.

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