Neural mechanisms underlying leg muscle responses during cervical transcutaneous spinal cord stimulation

Objective. Transcutaneous spinal cord stimulation (tSCS) is increasingly used as a neuromodulation technique to improve motor function after neuromotor disorders. Cervical tSCS has been reported to influence lumbar networks, leading to the hypothesis that leg muscle recruitment may occur via activation of long-range spinal connections between cervical and lumbar circuits. However, it remains unclear whether such responses reflect central network engagement or electrode-dependent local activation.Approach. We investigated the neural mechanisms underlying leg muscle responses during cervical tSCS in unimpaired participants (N= 12). We compared cervical tSCS electrode configurations in which the return electrodes were placed over the iliac crests versus over the clavicles. We used lumbar tSCS as a control for leg muscle recruitment via the posterior root-muscle reflex.Results. Leg muscle responses during cervical tSCS were observed primarily when the return electrodes were placed over the iliac crests and were mostly absent when the anodes were placed over the clavicles. Compared to lumbar tSCS, cervical-iliac responses exhibited shorter onset latencies and minimal post-activation depression, consistent with direct efferent recruitment rather than afferent-mediated reflex activation.Significance. These findings indicate that leg muscle activation during cervical-iliac tSCS originates from neural structures near the distal return electrodes rather than from long-range cervico-lumbar pathways. Return electrode placement therefore critically determines the origin of evoked responses during cervical stimulation and must be considered when interpreting cervical tSCS mechanisms and rehabilitative effects that extend caudally from the cervical spinal cord.