Ia presynaptic inhibition after muscle twitch in the arm

Contraction of upper limb muscles in healthy subjects was used to investiga te presynaptic inhibition at spinal level. The H reflex recorded in the for earm flexor muscles in response to median nerve stimulation was depressed i n amplitude from 400 ms to 1 s after a muscle twitch induced by transcrania l stimulation, root stimulation, direct biceps stimulation, and triceps ten don tap. Stimulation of the cutaneous branch of musculocutaneous nerve, ips ilateral triceps and contralateral biceps, and biceps tendon tap did not al ter H-reflex size. Forearm flexor H-reflex amplitude is therefore related t o changes in proprioceptive inflow secondary to the biceps muscle twitch. R oot and direct muscle stimulation both failed to reduce the size of the mot or evoked potential (MEP) after transcranial magnetic stimulation, suggesti ng that the inhibition acts at presynaptic level. Attenuation of H-reflex a mplitude was related to the size of the muscle twitch and was less pronounc ed during an isometric twitch than during free joint movement. Our results suggest that the biceps muscle twitch produces long-lasting inhibition of t he la afferents from forearm flexor muscles. This is an important and a sim ple mechanism for suppressing proprioceptive input during movement. (C) 200 0 John Wiley & Sons, Inc.