Response to sudden torques about ankle in man. III. Suppression of stretch-evoked responses during phasic contraction.

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Abstract
During phasic voluntary flexions of the foot about the ankle, torques were applied to perturb the movement and evoke reflex as well as longer latency corrective responses. In a 100-ms interval centered on the onset of myoelectric activity for a maximally rapid, voluntary plantar contraction, there is an increase in the gain of the soleus myotatic reflex followed by its sudden and total inhibition. Recovery of the reflex occurs after completion of the movement. Suppression of the myotatic reflex occurs to a lesser degree, even if actual shortening of the muscle is mechanically prevented. Phasic contraction produces a slower and less profound but even more prolonged reduction in the gain of the postmyotatic response. This prolongs the time it takes the subject to overcome the perturbing torque and achieve the desired foot position. Suppression of myotatic reflex is observed with movements at much slower rates. The degree of suppression is proportional to the rate of the movement and the duration always outlasts the movement. Comparable reductions in myotatic and postmyotatic responses to torque perturbations are also observed in the anterior tibial muscle during dorsiflexion. Preprogramming of voluntary movement and the absence of effective, load-compensating mechanisms during phasic movement of the foot is discussed.