Response to sudden torques about ankle in man. IV. A functional role of alpha-gamma linkage.

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Abstract
Torques were applied about the ankle joint of a seated subject to plantarflex the foot, preshortening the soleus muscle. After a specified delay, a dorsiflexing torque was applied to evoke a myotatic reflex in the soleus muscle. In a relaxed leg, the latency of the soleus myotatic reflex is increased by preshortening. This increase is directly proportional to the amount of preshortening and inversely proportional to the rate of restretching. These findings are consistent with the existence of at least 1 threshold within the afferent limb of the reflex arc. This threshold is shown to depend on both joint angle and to a small degree its 1st derivative. The most likely site for this threshold is within the muscle spindle. Increasing the interval between preshortening and restretching from 50-750 ms reveals only a small and incomplete recovery (reduction) in myotatic reflex latency. Voluntary tonic plantarflexion diminishes the increase in the myotatic reflex latency produced by muscle preshortening. A brisk, phasic, voluntary plantarflexion, initiated by the subject when the preshortening torque is applied, can prevent the increase in myotatic reflex latency. Voluntary contraction diminishes the effective threshold of the reflex, increasing the range of joint angles over which the reflex arc can be functional. This demonstrates 1 clear and important role for the linkage of skeletomotor and fusimotor activation in maintaining spindle feedback during voluntary shortening contractions.