The H-reflex is a reflectory reaction of muscles after electrical stimulation of sensory fibers (Ia afferents stemming from muscle spindles) in their innervating nerves (for example, those located behind the knee). The H-reflex test is performed using an electric stimulator, which gives usually a square-wave current of short duration and small amplitude (higher stimulations might involve alpha fibers, causing a M-wave, compromising the results), and an EMG set, to record the muscle response. That response is usually a clear wave, called H-wave, 50 ms after the stimulus, not to be confused with M-wave at 25-30 ms, which might appear in too intense stimulations.

H-reflex is analogous to the mechanically induced spinal stretch reflex (for example, knee jerk reflex) because both cases muscle-spindle innervating fibers are activated. Although stretch reflex gives just qualitative information about muscle spindles and relfex 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 (ms) and amplitudes (mV) of H-wave can be compared.

H-reflex is used to assess fitness of astronauts. H-reflex was the first medical experiment completed on the International Space Station.


It was first described by Paul Hoffmann (hence the name) in 1910.

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