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The activity of the single motor units in the human anterior digastric muscle was recorded in 7 subjects with a monopolar fine wire electrode. The force generated by jaw opening was simultaneously recorded with a mechanical transducer. The results obtained were as follows: (1) Increasing the force gradually, a few motor units were recruited orderly and the firing rate of each unit reached the peak frequency rapidly. These findings suggest that the control of the muscle force caused by jaw opening movements might depend more on the orderly recruitment of the motor units than on the rate modulation. (2) The contraction time, measured by the spike-triggered averaging technique varied from 25.6 to 132.4 ms. It was suggested that the anterior digastric muscle consisted of both the fast twitch motor units and the slow twitch motor units. (3) The motor units that have a higher threshold force tended to have a larger twitch tension and have a shorter twitch contraction time than that of the lower threshold motor units. This means that the anterior digastric muscle has a relatively simple role during the active jaw opening movements. (4) Fatigue can be estimated as the change in the firing rate. The motor units of a higher threshold force tended to be fatigue during the continuous jaw opening movements.

Physiological properties of motor units in the human anterior digastric muscle.