VOLUNTARY AND REFLEXIVE RECRUITMENT OF FLEXOR CARPI RADIALIS MOTOR UNITS IN HUMANS

The order of recruitment of flexor carpi radialis (FCR) motor units was studied during voluntary and reflexive activation of the motoneuron pool for 2 adult subjects. During slow voluntary activation, recruitment threshold for tonic motoneuron firing was determined; the twitch profile of the motor unit was computed by the spike-triggered averaging technique. A positive correlation (r = 0.79 and 0.68 for the 2 subjects, respectively) between recruitment threshold and twitch amplitude implies that recruitment of FCR motoneurons during slow ramp isometric contractions proceeds in order of increasing size. The accompanying paper describes the behavior of single motor units during the short and long latency periods of the stretch reflex. When the effects of sufficient voluntary facilitation (preload) combined with a sufficiently large torque step were just adequate to cause a motor unit to fire during the stretch reflex, its response was virtually always confined to the long-latency period. In addition, the 1st unit to begin responding to muscle stretch always had the lowest voluntary recruitment threshold relative to other units visible at that recording site. By making this unit tonic, the reflex response to the same load increased substantially during the short-latency reflex periods, while a 2nd unit increased its reflex response probability during the long-latency period. The voluntary recruitment order of 2 or more motor units is preserved during the stretch reflex and is maintained within first the long-latency and then short-latency components of this reflex.