MOVEMENTS MODULATE THE REFLEX RESPONSES OF HUMAN FLEXOR POLLICIS LONGUS TO STRETCH

The reflex responses to brisk, ramp stretch perturbations of the human flexor pollicis longus muscle (FPL) were recorded during isometric an d slow concentric or eccentric contractions at similar levels of muscl e excitation. The subjects flexed their thumb to push down against a t humb-rest, whose position was controlled by a servo-controlled motor. In different runs, the stretch perturbations were imposed when the thu mb-rest was stationary (isometric) or was flexing or extending the int erphalangeal joint of the thumb at a constant velocity, i.e. during co ncentric or eccentric contractions of FPL. The latency of the most pro minent component of the electromyographic reflex in the isometrically contracting muscle was about 60 ms, measured from the command signal. The amplitude of this response was sharply reduced during the non-isom etric contractions. While not dependent on the direction, this modulat ion of the reflex response increased with the speed of active movement of the interphalangeal joint (flexion or extension). The response was greatly reduced during concentric or eccentric movements as slow as 1 .6 mm.s(-1) (approximately 5 degrees.s(-1) at the joint). When the for ce rather than the position of the thumb-rest was servo-controlled, th e stretch response to perturbation again diminished with speed in a se lf-paced flexion task, compared with an isometric ''hold'' condition.