The purpose of this study was to elucidate the fatigue characteristics of the motor units in the human masseter muscle. Eight asymptomatic subjects were asked to maintain isometric clenching of fatiguing tasks at various force levels, and electromyographic signals were taken from the surface of the right masseter muscle. The endurance time was measured, and the following parameters were calculated. The results were as follows:
1) At 10% of maximum voluntary contraction force level, all subjects could complete their tasks. The endurance time become shorter as the force levels became higher.
2) While the mean frequency decreased steadily at the end of a sustained isometric contraction, no significant difference was observed between the root mean square values at the beginning and the end.
3) Turns per second decreased significantly at the end of contraction, while the mean amplitude per turn increased significantly, especially at the higher force levels.
4) It has been considered that turns per second reflect firing rates of a toral number of motor units, and that the mean amplitude per turn reflects the largest peak-to-peak amplitude among motor units recruited. Therefore, it was assumed that the firing rates of the motor units decreased and larger spikes were recruited in voluntary isometric contractions of the fatigued human masseter, and that the motor units were frequently synchronized at the higher force levels.