Recruitment threshold force and its changing type of motor units during voluntary contraction at various speeds in man.

The motor unit discharges in human hand muscles were recorded during voluntary isometric contraction. Bipolar wire electrodes were inserted in the right adductor pollicis muscle (AP) and first dorsal interosseous muscle (FDI) for the recordings. Motor unit activities from these muscles were investigated when subjects exerted voluntary force to each target force at various speeds. On 27 motor units of AP and 28 motor units of FDI, ramp RTF and ballistic RTF were compared. Furthermore, RTF changes of motor units with the different force speeds were investigated. It is clear that the RTF of the motor units during ballistic contractions were lower than those during ramp contraction. The processes of motor unit RTF changes were classified into 3 types: RTF of type I motor units were relatively low and decreased slightly even when force speed increased widely; RTF of type II motor units were relatively high and decreased irregularly; and RTF of type III motor units, which appeared rarely, decreased their relatively high RTF with increases of force speed increment. It is suggested that the volitional outflow related with the force speed change to the spinal motor pool might cause the different RTF change effects in the various motor units.