Is the degree of post-activation 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 post activation depression mechanism. Volunteers participated in two experimental sessions, in which tSCS was applied at the L1-L2 level (n = 20, main experiment) or at the T10-T11 level (n = 10, additional experiment). Recruitment curves in 1s 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 post-activation 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 (P = 0.99). However, in the additional experiment the degree of post activation depression was found to be higher when a TEP was conditioned using tSCS compared to the other conditioning ratios obtained in the two experiments (for all, P < 0.03). Although both responses evoked on soleus muscle seem to be similarly sensitive to the post activation 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.