Undulations and bending in peripheral nerves benefit coil positions projecting transverse fields

For peripheral magnetic stimulation it is widely accepted that the field components parallel to the nerve are responsible for stimulation. However, experimental findings have often suggested that transverse field components contribute as well or are even dominant. A reason for that discrepancy could be undulations or curving of the nerve. As a consequence, the question of ideal coil placement for magnetic stimulation is still not conclusively answered. To identify beneficial coil positions, we quantified the impact of undulation and nerve bending in this study. First, we performed neuronal simulations with different extent of fascicle and fibre undulations inside the field distribution of a figure-of-8 coil. Second, we simulated median nerve stimulation using an anatomical model of the forearm to include the contribution of nerve bending. Third, we conducted median nerve stimulation on healthy subjects with different wrist positions to manipulate undulations. Our simulations suggested both fascicle and fibre undulations cause transverse field components to cause lower thresholds than parallel ones. Simulations on median nerve stimulation showed that the position of the coil in relation to the nerve course has more impact than the orientation itself. Finally, the experimental validations confirmed that transverse coil positions produce smaller stimulation thresholds. Further, we saw that bending the wrist has a potential influence on thresholds, possibly due to undulations. We conclude that placing a round coil centrally above the nerve yields the lowest thresholds.