Abstract
The purpose of this paper is to investigate the functional property of the masseter muscle at the motor unit level in man. The responses of the surface EMG and the bite force were studied by averaging technique, with the following results.
1. The higher threshold units with large twitch tensions were found in the superficial layer of the muscle, the lowers were throughout the muscle.
2. Positive correlation was found between voluntary threshold force and peak twitch tension, linear correlation coefficient was 0.76.
3. The motor units with a larger twitch tension tended to have shorter contraction time, but those of smaller had the wide range.
4. There was a tendency for deeper-layer units to contribute less to the wave form of the surface EMG than did the superficial units.
5. There was a significant tendency for the units recruited with larger forces to contribute extensively to the surface EMG. The linear correlation coefficient was 0.70.
6. In 106 force records, DEM (the discrepancy between electrical activity and mechanical contraction) was observed. It seemed that DEM was similar phenomenon to the latency relaxation.
