Reaction time of elbow flexion during passive movements

The effect of passive movements of the elbow on biceps EMG reaction time (RT) was studied in 10 normal subjects. Each subject was required to perform a simple RT task, flexing the left elbow to a tone stimulus, under three different conditions: (a) during passive flexion (PFLX) of the elbow, (b) during passive extension (PEXT), and (c) without passive movements (STAT). A forearm-rotator was used to produce a constant movement with angular velocity of 20 deg/sec, and in each condition the tone stimulus was always presented at the same angle of the elbow joint. Significant differences in mean EMG-RTs were found between the three conditions; the shortest EMG-RT of all was observed in the PFLX condition (77.0 +/- 15.0 msec), followed by that in STAT (93.1 +/- 11.8 msec) and lastly in PEXT (116 +/- 23.1 msec). Neurophysiological mechanisms underlying these phenomena are discussed.