MOTOR UNIT FIRING BEHAVIOR IN HUMAN ARM FLEXOR MUSCLES DURING SINUSOIDAL ISOMETRIC CONTRACTIONS AND MOVEMENTS

Simultaneous recordings of action potentials (APs) of multiple single motor units (MUs) were obtained in brachialis and biceps (caput breve) muscles during sinusoidally modulated isometric contractions of elbow flexor muscles and during sinusoidal flexion/extension movements in t he elbow against a preload in the extension direction. The results sho w that MUs typically fire in one short burst for each sinusoidal cycle . The mean phase lead of the bursts of APs relative to a sinusoidally modulated isometric torque in the elbow joint or relative to sinusoida l movements in the elbow increases gradually with frequency. The incre ase of the mean phase lead during isometric contractions was very simi lar for all MUs and could be explained well by modeling the force prod uction of MUs with a second-order linear low-pass system. For sinusoid al flexion/extension movements each MU reveals a specific, reproducibl e phase lead as a function of frequency. However, there is a large var iability in phase behavior between MUs. Also, the modulation of the fi ring rate for sinusoidal isometric contractions versus sinusoidal move ments appeared to be different for various MUs. In simultaneous record ings some MUs clearly revealed a larger firing rate in each burst for movements relative to isometric contractions, whereas other MUs reveal ed a smaller firing rate. This suggests that some MUs are preferential ly activated during movements whereas others are preferably activated during isometric contractions. The results demonstrate task-dependent changes in the relative activation of MUs within a single muscle for s inusoidal isometric contractions and movements.