MOTOR-UNIT BEHAVIOR IN HUMANS DURING FATIGUING ARM MOVEMENTS

1. The activity of 40 triceps brachii motor units was recorded from th e dominant arms of 9 healthy adult volunteers (age 27.8 +/- 4.3 yr, me an +/- SD) during a fatigue task that included both isometric and anis ometric contractions. The fatigue task lasted 8.3 min and consisted of 50 extension and 50 flexion movements of the elbow. Each movement (40 degrees in 0.8 s) was separated by an isometric contraction. A consta nt load resisting extension of 17.7 +/- 3.0% of maximal voluntary cont ractions (MVC) was applied throughout the task. This paradigm enabled the direct contrast of motor-unit discharge behavior during the differ ent types of fatiguing contractions. 2. Motor-unit behavior was examin ed to determine the relative contribution of two mechanisms for optimi zing force production under fatiguing conditions: recruitment of motor units and modulation of motor-unit discharge following recruitment. T hreshold torques for motor-unit recruitment thresholds were determined by ramp-and-hold isometric contractions. Motor-unit discharge was eva luated during the fatigue task by contrasting the number of motor-unit potentials (spikes) per contraction for concentric, eccentric, and is ometric contractions. 3. The fatigue task resulted in a 30 +/- 12% dec line in the mean MVC of elbow extension. Recruitment of nine new motor units (23%) was evident during the fatiguing extension movements, oft en within five to seven movements (i.e., within 25-35 s). Each newly r ecruited motor unit had the largest recruitment threshold torque in th at experiment. 4. Analysis of the motor units that were active from th e begin- ning of the fatigue task revealed that the mean number of mot or-unit spikes per contraction increased, decreased, or remained const ant as fatigue ensued, yet for the majority of motor units it increase d or remained constant. None of the newly recruited motor units demons trated decreased number of mean spikes per contraction after recruitme nt. Further, concurrently active motor units displayed different disch arge behavior in two-thirds of the subjects. It is proposed that if th e neural drive to the muscle is distributed uniformly upon the motoneu ron pool, peripheral feedback from the exercising muscle may modulate specific motoneuron discharge levels during fatigue.