Studies on motor unit firing behavior in man by the decomposition technique are described. The decomposition technique identifies motor unit firing with 100% accuracy at force levels of greater than 80% maximal voluntary contraction (MVC). In all muscles examined, the higher the recruitment threshold of the motor unit, the lower the rate at which it fired at the target level. Smaller muscles, such as those in the hand, recruit their motor units at 0-50% MVC and rely exclusively on firing rate increases to augment force output at 50-100% MVC. Larger muscles, such as those in the leg or arm, recruit motor units at least to 90% MCV, and possibly higher. Their firing rates have a relatively smaller dynamic swing. Thus, smaller muscles rely primarily on firing rate and larger muscles rely primarily on recruitment to modulate their force. High cross-correlation functions in firing rate behavior within a muscle were observed between individual motor unit firing rates at constant force isometric contraction. Thus the nervous system does not control the firing rate of motor units individually. Instead, it acts on the pool of homonymous motoneurons in a uniform fashion. Electrical stimulation of cutaneous receptors tends to increase the recruitment thresholds of low-threshold motor units and to decrease their firing rates, while high-threshold motor units generally exhibit a decrease in recruitment threshold and an increase in firing rate.