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The H-reflex is a reflectory reaction of muscles after electrical stimulation of sensory fibers in their innervating nerves. The H-reflex test is performed using an electric stimulator, which gives usually a square-wave current of short duration and small amplitude, and an EMG set, to record the muscle response. That response is usually a clear wave, called H-wave, 28-35 ms after the stimulus, not to be confused with an F-wave. An M-wave, an early response, occurs 3-6 ms after the onset of stimulation. The H and F-waves are later responses. As the stimulus increases, the amplitude of the F-wave increases only slightly, and the H-wave decreases, and at supramaximal stimulus, the H-wave will disappear. The M-wave does the opposite of the H-wave. As the stimulus increases the M-wave increases. There is a point of minimal stimulus where the M-wave is absent and the H-wave is maximal. H-reflex is analogous to the mechanically induced spinal stretch reflex. "The primary difference between the H-reflex and the spinal stretch reflex is that the H-reflex bypasses the muscle spindle, and, therefore, is a valuable tool in assessing modulation of monosynaptic reflex activity in the spinal cord." Although stretch reflex gives just qualitative information about muscle spindles and reflex arch activity; if the purpose of the test to compare performances from different subjects, H-reflex should be used. In that case, in fact, latencies and amplitudes of H-wave can be compared.
U.S. National Library of Medicine
A monosynaptic reflex elicited by stimulating a nerve, particularly the tibial nerve, with an electric shock.
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