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.