Spinal motor preparation in humans.

This study examined the influence of preparatory 'set' on the modulation of monosynaptic reflex excitability immediately prior to the initiation of a voluntary movement. Six subjects performed 3 tasks which varied the level of preparatory set according to the availability of temporal and event anticipation. A choice reaction-time task restricted both temporal and event anticipation; a simple reaction-time task restricted temporal anticipation while allowing event anticipation; and a coincidence-timing task permitted both temporal and event anticipation. The response for all tasks performed was ankle plantarflexion. The excitability of the monosynaptic reflex was examined by eliciting H reflexes to the right soleus muscle at 50 msec intervals over the final 400 msec preceding the initiation of voluntary muscle contraction. All 3 tasks showed an initial depression and later increase in the H reflex amplitude. The onset of reflex facilitation was the same for the choice and simple reaction-time tasks, commencing 74 msec prior to voluntary muscle contraction. However, the coincidence-timing task showed an earlier onset of reflex facilitation. Reflex facilitation preceded voluntary muscle contraction by 130-337 msec during the performance of this task. Thus, it appears that the modulation of monosynaptic reflex excitability is not time-locked to the onset of voluntary muscle contraction, but rather is controlled by the preparatory set of the subject. The modulation of reflex excitability represents a separate, though necessary, stage in the organization and initiation of voluntary movement.