POST-ACTIVATION DEPRESSION OF POSTERIOR ROOT MUSCLE RESPONSES EVOKED BY TRANSCUTANEOUS SPINAL CORD STIMULATION AT CERVICAL AND LUMBOSACRAL LEVELS IN HEALTHY HUMANS

Transcutaneous spinal cord stimulation (tSCS) has emerged as a promising tool to facilitate functional motor recovery, likely by modulating motor output through the activation of large-diameter afferents in the posterior roots. However, it remains controversial whether posterior root motor responses (PRM-responses) can be treated as purely reflexive—transsynaptically converging on motor pools in the same manner as the H-reflex. To address this question, we evaluated whether PRM-responses exhibit the same inhibitory characteristics as H-reflexes by comparing their post-activation depression in seventeen healthy adults (20.2 ± 1.6 years). Cervical tSCS was delivered via a cathode over C5–C6, and lumbosacral tSCS over T11–L1, with anodes placed bilaterally over the iliac crests. H-reflexes were evoked via electrical stimulation of the tibial nerve at the popliteal fossa for the soleus and the median nerve at the elbow for the flexor carpi radialis. Post-activation depression was assessed using paired pulses at interstimulus intervals (ISI) of 0.5, 1, 2 and 4 seconds, with test responses recorded every 10 seconds. We found that in the soleus, post-activation depression was similarly enhanced at shorter ISIs for both H-reflexes and PRM-responses. In contrast, PRM-responses in the flexor carpi radialis exhibited significantly weaker depression compared to H-reflexes. These results suggest that whereas lumbosacral PRM-responses reflect afferent-mediated pathways, cervical tSCS is more susceptible to direct efferent recruitment. These results highlight the importance of optimizing stimulation parameters to ensure the selective recruitment of afferent pathways across different spinal levels.