Discharge patterns in human motor units during fatiguing arm movements.

The purpose of this study was to determine whether short interspike intervals (ISIs of <20 ms) would occur naturally during voluntary movement and would increase in number with fatigue. Thirty-four triceps brachii motor units from nine subjects were assessed during a fatigue task consisting of fifty extension and fifty flexion elbow movements against a constant-load opposing extension. Nineteen motor units were recorded from the beginning of the fatigue task; the number of short ISIs was 7.1 +/- 4.1% of the total number of ISIs in the first one-third of the task (unfatigued state). This value increased to 11.8 +/- 5.9% for the last one-third of the task (fatigued state). Fifteen motor units were recruited during the fatigue task and discharged, with 16.4 +/- 6.0% of short ISIs in the fatigued state. For all motor units, the number of short ISIs was positively correlated (r2 = 0.85) with the recruitment threshold torque. Short ISIs occurred most frequently at movement initiation but also occurred throughout the movement. These results document the presence of short ISIs during voluntary movement and their increase in number during fatigue.