MOTOR UNIT RECRUITMENT AND DISCHARGE BEHAVIOR IN MOVEMENTS AND ISOMETRIC CONTRACTIONS

The purpose of this study was to contrast the discharge patterns of th e same motor units during movements and during isometric contractions that were produced with comparable torque-time characteristics. Subjec ts performed elbow flexion and extension movements with predetermined acceleration characteristics. The average acceleration and deceleratio n profiles for the movements were reproduced in the isometric setting by presenting the kinematic profiles as templates for torque productio n. Trained subjects were able to match the first agonist (AG1) and ant agonist (ANT) electromyographic (EMG) bursts, but tended to produce a smaller second agonist burst (AG2) in the isometric contraction. Twent y-five motor units from triceps brachii were studied. The same motor u nits (with one exception) were recruited and subsequently discharged i n a similar fashion in both the isometric and movement tasks in the AG 1 and ANT EMG bursts, with fewer motor unit discharges in the AG2 burs t in the isometric contraction. The central control mechanisms appear to be the same for the acceleration phase of movement and isometric co ntraction, but differ during the deceleration phase. (C) 1997 John Wil ey & Sons, Inc.