Recruitment of motor units in human flexor carpi ulnaris

In 1949, Denny-Brown reported that the motoneuron pool of the human flexor carpi ulnaris (FCU) muscle was fractionated into subpopulations, each subpopulation being activated during a different voluntary motor task. The following report presents data on motor unit recruitment in the human FCU muscle for the tasks of isometric flexion and ulnar deviation, cocontraction of the forearm muscles and non-isometric flexion of the wrist. These observations show that every FCU motor unit tested reliably, contributed to all four tasks, that is, no separate subpopulations were observed for any of the contractions tested. Furthermore, the order of recruitment was the same for all four tasks.