Investigation into the influence of stimulation area and coil orientation on the results of navigated transcranial magnetic stimulation (nTMS) mapping of lower limb intracortical excitability.

Background: Transcranial magnetic stimulation (TMS) is widely used to assess corticomotor excitability. Coil orientation and stimulation location are crucial for eliciting motor-evoked potentials (MEPs) and determining resting motor thresholds (RMT). Since the cortical foot area is challenging to examine, identifying the optimal coil angle and location is essential.

Method: Eleven healthy volunteers underwent navigated TMS mapping using a predefined protocol. Stimulation was applied at six locations around the tibialis anterior (TA) motor hotspot, with coil direction varied in 45° increments. Mapping was performed using the Nexstim NBS 5.0 system, and statistical analysis was conducted in RStudio 2024.

Results: TA cortical representation mapping was successful in all participants. The mean hotspot was located in the precentral gyrus, 6-13 mm lateral to the midline. The highest MEP amplitude was observed at a stimulation angle of 90°, perpendicular to the falx cerebri.

Comparison With Existing Methods: Unlike previous studies with limited coil orientations or without MRI-guided neuronavigation, our approach systematically evaluated multiple directions and locations. The findings align with prior research regarding optimal stimulation sites and angles.

Conclusion: We refined the anatomical stimulation area and preferred angle for lower-extremity TMS. These findings may improve clinical applications, especially when considering individual and pathological differences.