Recruitment order of motor units during the stretch reflex in man.

The recruitment pattern of single motor units contributing to the short- and long-latency periods of the stretch reflex was investigated in human adults. When non-tonic, a given unit responded to stretch during the long-latency reflex period. When made tonic, the same unit showed a reflex response during the short-latency period, while a higher threshold, non-tonic unit would respond during the long-latency period. The data show an orderly recruitment of motor units during both phases of the stretch reflex in man.