Changes in the excitability of the H-reflex in wrist flexors related to the prone or supine position of the forearm in man

The effect of the forearm position, prone vs. supine, on the excitability of the H-reflex in flexor carpi radialis (FCR) muscle was tested in nine adult volunteers by comparing the recruitment profiles of the H and M waves. The H-reflex size, normalized to the maximal M response, was lower when the forearm was supine than when it was prone, with an average reduction of about 50% over most of the H-recruitment curve. In three wrist positions, intermediate between prone and supine, the amount of reflex attenuation was related to the prono-supination angle. Control experiments excluded that the changes in the H reflex excitability were due to displacements of the stimulating or recording electrodes.