Cervicothoracic trans-spinal magnetic stimulation: effect of coil position and orientation on spinal motor evoked potentials.

Stroke is a leading cause of long-term adult disability, and over 80% of survivors have persistent upper limb motor impairment. Spinal electrical stimulation has shown therapeutic potential after neurological disorders. Post-stroke applications have largely relied on invasive implants that limit clinical adoption, and non-invasive high intensity electrical stimulation may be intolerable for those with residual sensory function. Trans-spinal magnetic stimulation offers a non-invasive alternative capable of modulating residual sensorimotor pathways. Fundamental parameters, such as coil position and orientation, remain under-investigated. Two experiments were conducted to fill this gap. Experiment 1: 19 healthy adults underwent spinal mapping involving 30 cervicothoracic sites (3×10 grid). Responses were recorded from 6 upper limb muscles bilaterally at suprathreshold intensity using a rostral-left induced current direction. Friedman ANOVAs revealed significant medial-lateral and rostral-caudal variation in amplitude of spinal motor evoked potentials (sMEPs): ipsilateral>midline>contralateral, with arm responses peaking rostrally, wrist at mid-rows, and hand caudally. Experiment 2: 20 healthy adults received stimulation at three horizontal cervicothoracic sites using eight current directions spaced 45° apart. Friedman ANOVAs indicated a bilateral mirror-symmetrical pattern. The largest responses from left-sided muscles were elicited with a rostral-left current direction achieved by rotating the coil handle 45° counterclockwise from the caudal direction. Likewise, the largest right-sided muscle responses were elicited with a rostral-right current direction achieved by rotating the coil handle 315° counterclockwise. Coil position and orientation are critical determinants of sMEP amplitude. These results provide a foundational framework to further investigate cervicothoracic magnetic stimulation as a potential non-invasive neuromodulatory intervention.