Tibial motor nerve conduction studies: An investigation into the mechanism for amplitude drop of the proximal evoked response

INTRODUCTION: The amplitude of the compound muscle action potential (CMAP) of abductor hallucis (AH) shows the largest drop with proximal stimulation of any routinely studied motor nerves. The cause has not been established. METHODS: Four experiments of tibial motor nerve conduction in several healthy control subjects were performed using far-field recordings, collision, H-reflex, and intramuscular recordings of foot muscles. RESULTS: The proximal CMAP showed a mean peak-peak amplitude of 66% (range 57-79%) compared with the distal response. Collision and H-reflex recordings in AH did not show evidence of a contribution from the tibial-innervated calf muscle. Needle electrode recordings of CMAPs showed consistently different latencies between different foot muscles. CONCLUSION: Our experiments indicate that temporal dispersion and phase cancellation between the distal tibial-innervated foot muscles recorded by the E2 (i.e., reference) electrode can explain the drop in amplitude between the proximal and distal tibial evoked CMAP.