Modulation and recovery kinetics of soleus responses to peripheral nerve and transcutaneous spinal cord stimulation under different interventions

Transspinal evoked potentials (TEPs) elicited by transcutaneous spinal cord stimulation (tSCS) share some neurophysiological similarities with the H-reflex evoked by peripheral nerve stimulation (PNS). The purpose of this study was to further compare these two responses during and/or following different external interventions, known to activate Ia afferents. Fourteen volunteers took part in two experimental sessions, where PNS and tSCS promoted the same afferent solicitation of soleus muscle (target muscle). During the first experimental session modulations of H-reflex and TEP were examined after 20 s of neuromuscular electrical stimulation delivered both at low and high frequency. During the second experimental session changes in both responses during and following local vibration and passive stretching were evaluated. Results showed no differential modulation between the two soleus responses across the four tested interventions (all P > 0.25). Both H-reflex and TEP significantly decreased following low-frequency electrical stimulation (P = 0.001), whereas no significant modulation was observed after high-frequency stimulation (P = 0.08). Similar amplitude reductions between the two responses were also observed during local vibration and passive stretching (P < 0.001). In addition to the similar modulation of soleus responses, modulations of tSCS-evoked responses in synergist muscles during the low-frequency train, as well as in both synergist and antagonist muscles during local vibration and passive stretching, have also been observed. These results provide further evidence of the similarities between H-reflex and TEP, while highlighting the potential of tSCS to concomitantly assess multiple muscles modulations.