Response to sudden torques about ankle in man: V effects of peripheral ischemia

Abstract

Sudden dorsiflexions and plantar flexions of the foot were imposed by a torque motor while blood flow to the lower leg was occluded by a sphygmomanometer cuff. Seated subjects were instructed to resist the torques and restore the foot to its original position as rapidly as possible. Measurements of the first two electromyographic (EMG) responses were made in the soleus (SOL) and anterior tibial (TA) muscles. These are the myotatic reflex at about 40 ms and the postmyotatic response at about 120 ms. In the anterior tibial muscle, the myotatic component often occurs at 60- to 90-ms latency and is continuous with the postmyotatic component. After about 20 min of ischemia, there is a rapid and eventually complete loss of the myotatic component of the response in the stretched muscle. Concurrent with the loss of the myotatic reflex there is usually a reduction of the postmyotatic component. The latency of postmyotatic component remains unchanged, even after the myotatic reflex is abolished. Voluntary restoration of the foot to its original position in opposition to the motor torque is delayed and slowed by the time that the myotatic reflex has vanished. By measuring electromyographic responses to visually triggered reactions, we demonstrate that the reduction of the postmyotatic response is not due to failure of the efferent pathway. On this basis we discuss the possible contributions made by myotatic mechanisms to the tasks of load compensation and maintenance of muscle tone. Hoffmann reflexes were evoked in similar experiments with stimulation of the posterior tibial nerve at the popliteal fossa, distal to the cuff. The maximal direct motor response and muscle twitch are unaffected by loss of the H-reflex. Myotatic and Hoffmann reflexes were both evoked with torque perturbations and with a stimulating electrode proximal to a below-the-knee cuff. In this case, the myotatic reflex failed while the maximal H-reflex was slightly facilitated. The loss of the myotatic reflex was accompanied by a modest reduction in the direct motor response to electrical stimulation and a somewhat greater reduction in the postmyotatic response to torque perturbation. The data are interpreted to support a dual role for the primary afferent pathway in the control of voluntary movements. It provides the fastest path for the activation of a stretched muscle to initiate a load-compensating contraction. It also provides a form of servo assistance in modulating descending control signals. The degree of this assistance is estimated. In this simple experimental paradigm, functional contributions of the reflex arc are demonstrated.