Is the degree of postactivation depression similar between soleus responses evoked by transcutaneous spinal cord and peripheral nerve stimulation?

Transcutaneous spinal cord stimulation (tSCS) evokes responses in multiple lower limb muscles, referred to as transspinal-evoked potentials (TEPs). In certain conditions, these responses can result primarily from the activation of Ia afferents at the dorsal roots of the spinal cord and share similarities with the H-reflex elicited by peripheral nerve stimulation (PNS). The aim of this study was to compare these two responses with regard to the postactivation depression mechanism. Volunteers participated in two experimental sessions, in which tSCS was applied at the L1-L2 level (<i>n</i> = 20, main experiment) or at the T10-T11 level (<i>n</i> = 10, additional experiment). Recruitment curves in 1-s paired-pulse stimulations were constructed for the two types of stimulation. The amplitude of soleus H-reflex and TEP was matched (∼85% of maximal H-reflex), and postactivation depression was tested by conditioning the H-reflex (H_COND) or the TEP (TEP_COND) using either PNS (H_TEST) or tSCS (TEP_TEST). The results of the main experiment demonstrated no significant difference in the conditioning ratios (<i>P</i> = 0.99). However, in the additional experiment, the degree of postactivation depression was found to be higher when a TEP was conditioned using tSCS compared with the other conditioning ratios obtained in the two experiments (for all, <i>P</i> < 0.03). Although both responses evoked in soleus muscle seem to be similarly sensitive to the postactivation depression phenomenon when low-stimulation intensities are used, the electrode placement seems to exert an influence on the degree of depression induced on tSCS-evoked responses.<b>NEW & NOTEWORTHY</b> Although responses evoked by both tSCS and peripheral nerve stimulation are known to exhibit postactivation depression, a hallmark of Ia afferent activation, this study identifies specific methodological conditions under which tSCS can elicit a soleus muscle response equivalent to the classical H-reflex. When electrode placement minimizes the recruitment of Ia afferents from the vastus lateralis and low-stimulation intensities are applied, the tSCS response appears to engage the same neural pathways as peripheral nerve stimulation.