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Peripheral and central mechanisms of reciprocal inhibition between antagonist muscles in the forearm have been studied in ten human subjects. H reflexes were evoked in flexor muscles by stimulating the median nerve with single shocks at around motor threshold intensity. Peripheral inhibition of the flexor H reflex was produced by motor threshold stimulation with a single shock of the radial nerve supplying the extensor muscles. The conditioning radial nerve stimulus produced inhibition of the flexor H reflex consisting of three phases. In some individuals, an H reflex could be evoked in extensor muscles of the forearm. Stimulation of the median nerve produced inhibition of the extensor H reflex with a similar time course to that from extensors to flexors. The first phase of inhibition was apparent when the test median nerve shock was given from 1 ms before to 3 ms after the conditioning radial nerve shock. It was abrupt in onset and short in duration and could be evoked with a conditioning stimulus intensity as low as 0.75 X motor threshold. The second and third phases of inhibition were evident when the conditioning radial nerve stimulus preceded the median nerve test shock by 5 to 50, and 50 to 500 ms respectively. The characteristics of these later phases of inhibition are to be the subject of a separate report. The difference in timing of the peak initial short-latency inhibition from extensor to flexor and from flexor to extensor muscles enabled an estimate to be made of the central synaptic delay of the inhibitory process. This method yielded a central delay of 0.95 ms in excess of that of the H reflex. We conclude that the first phase of inhibition is mediated via large group I afferents acting through a single inhibitory interneurone . Central inhibition of the flexor H reflex was demonstrated with the radial nerve anaesthetized by injection of local anaesthetic at the elbow. Subjects were asked to try to contract the paralysed extensor muscles. Under this condition, attempted voluntary wrist extension inhibited the flexor H reflex even though no movement occurred. A shock was delivered to the radial nerve at a site proximal to the anaesthetic block. When the shock was applied in conjunction with an attempted voluntary contraction of the paralysed extensor muscles, the depth of inhibition was greater than that predicted from the effect of either a shock or a willed contraction acting independently.(ABSTRACT TRUNCATED AT 400 WORDS)
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