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 si­ nusoidally modulated isometric contractions of elbow flexor muscles and during sinusoidal flexion/extension movements in the elbow against a preload in the exten­ sion direction. The results show 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 sinusoidal movements in the elbow increases gradually C J with frequency. The increase of the mean phase lead dur­ ing isometric contractions was very similar for all MUs and could be explained well by modeling the force pro­ duction of MUs with a second-order linear low-pass sys­ tem. For sinusoidal flexion/extension movements each MU reveals a specific, reproducible phase lead as a func­ tion of frequency. However, there is a large variability in phase behavior between MUs. Also, the modulation of the firing rate for sinusoidal isometric contractions versus si­ nusoidal movements appeared to be different for various MUs. In simultaneous recordings some MUs clearly rej vealed a larger firing rate in each burst for movements rel-