Abstract
Purpose: The purpose of the study was to examine the effects of scalp-cortex distance on motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) in the lower limb motor cortex.
Methods: Ten healthy adults participated in the study. The MEPs evoked by TMS of the left primary motor cortex of the lower limb were assessed. The stimulation intensity was set at 100%, 110%, 120%, 130%, and 140% of the resting motor threshold (rMT). MEPs were recorded from the right tibialis anterior muscle. The distance between the scalp and the left precentral gyrus was calculated from T1-weighted magnetic resonance images of the brain. We examined the correlations between the scalp-cortex distance and the values of rMT, MEP amplitude, and the root mean square value (RMS) of electromyogram signal in a 100 ms window between 20 ms and 120 ms after TMS using Spearman’s rank correlation coefficients.
Results: There were significant correlations between the scalp-cortex distance and the MEP amplitude at 140% rMT, and between the scalp-cortex distance and RMS at 140% rMT.
Conclusion: Our findings suggest that the scalp-cortex distance affects the TMS evaluation of the lower limb motor cortex excitability.
