THE HOFFMANN REFLEX OF HUMAN PLANTAR FOOT MUSCLES

Electrical stimulation of the tibial nerve in the popliteal fossa evok ed an M wave (10.9 ms) and a late reflex response (38.1 ms) in the pla ntar foot muscles of all 10 volunteers. The late response had a somewh at lower electrical threshold than the corresponding M wave (8.5 versu s 9 mA), and reached a maximum of amplitude when the stimulus intensit y was increased, but was strongly suppressed by further increased inte nsity. A more distal stimulation of the tibial nerve at the ankle shor tened the onset latency of the M wave and lengthened that of the late response. The reflex was facilitated by activation of synergists and i nhibited by activation of antagonists. We showed that the late respons e was contaminated neither by volume conducted activity from the soleu s muscle, as shown by intramuscular recordings from the abductor hallu cis muscle, nor by a F wave, as shown by double stimulation. In summar y, we conclude that this late response in human plantar foot muscles c orresponded to an H reflex, which may be used to assess alterations of distal motoneuronal excitability. (C) 1998 John Wiley & Sons, Inc.