Modulation of reflex activity of motor units in response to stretch of a human finger muscle.

Changes in stretch reflex responses were examined in two types of motor task, force control and position control, by applying a small quick stretch to the middle finger extensor digitorum communis (EDC) muscle at an unpredicted time and observing activities of single motor units. Subjects were asked to maintain a constant extending isometric force at the metacarpophalangeal (MP) joint for force control, and to maintain a constant middle finger position against an applied force for position control. No significant differences in the tonic activities of EDC motor units were seen between the two types of motor task when the same force was exerted about the MP joint. Tonic activities of the EDC muscle and its antagonists were thus similar for both types of motor task. Ten of the eighteen motor units investigated showed obvious reflex responses (increase in firing rate) with latencies of 30-60 msec after the stretch. This reflex response was greater for position control than for force control, given the same operating conditions of tonic force, finger position, and activities of motor units. Enhancement of the stretch reflex for position control was also observed in surface electromyograms of the same muscle.