Objective - We investigated the effect of magnetic coil orientation on
the excitation of the median nerve in healthy subjects. Methods - An
8-shaped coil, 3.2 cm in outer diameter, was used. The median nerve wa
s stimulated at the elbow while the compound muscle action potentials
(CMAPs) of abductor pollicis brevis muscle were recorded at 4 differen
t directions of the induced current: orthodromic, antidromic, medio-la
teral and latero-medial. Results - We found that the amplitude of the
CMAP was the greatest in a medio-lateral (ML) direction. We also measu
red the induced electric field in the saline tank that mimicked the fo
rearm. The induced electric field and its spatial gradient were the gr
eatest in the ML direction among 4 directions. Conclusion - The fact t
hat the forearm is a restrictive volume conductor may result in the di
fferent effects of coil orientation on the excitation of the median ne
rve at the elbow.