Author(s): SORDET, J., MARTIN, A., PAPAIORDANIDOU, M., Institution: INSERM U1093, Country: FRANCE, Abstract-ID: 469

INTRODUCTION:
Transcutaneous spinal cord stimulation (tSCS) is a non-invasive stimulation technique presenting the advantage of eliciting responses in multiple muscles of the lower or upper limb, known as transspinal evoked potentials (TEPs) (Knikou et al., 2014). Because of this capability, tSCS is widely used in clinical practice (Megia-Garcia et al., 2020) and has recently gained increasing interest in neurophysiological research as a non-invasive approach to study the neuromuscular system, offering an alternative to the classical H-reflex elicited by peripheral nerve stimulation (PNS). Indeed, recent studies have demonstrated that the H-reflex and TEP exhibit similar modulation patterns associated with presynaptic inhibitory mechanisms at rest, including post-activation depression and D1 presynaptic inhibition (Sordet et al., 2025). Whether these similar behaviors are preserved during voluntary muscle contraction remains unclear. Therefore, the aim of the present study was to compare these modulations at rest and during voluntary contractions performed at 25% and 75% of maximal voluntary contraction (MVC).
METHODS:
Participants took part in two experimental sessions, one focusing on post-activation depression (Experiment 1) and the second investigating D1 presynaptic inhibition (Experiment 2). These mechanisms were evaluated at rest and during voluntary contractions at 25% and 75% MVC of the plantar flexor muscles. During both experiments, soleus TEP was evoked by tSCS applied via electrodes positioned at the L1–L2 spinal level and beneath the umbilicus, while the soleus H-reflex was elicited by peripheral nerve stimulation of the posterior tibial nerve. Post-activation depression was assessed using paired PNS or tSCS pulses separated by a 1-s interstimulus interval, whereas D1 inhibition was induced by conditioning the H-reflex or TEP with prior stimulation of the common peroneal nerve.
RESULTS:
Both the soleus H-reflex and TEP displayed similar modulation patterns across contraction intensities (P < 0.36). Post-activation depression decreased at 25% MVC relative to rest and was absent at 75% MVC for both responses, whereas a comparable level of D1 inhibition was observed at rest and during low and high contraction intensities for the two responses.
CONCLUSION:
These findings demonstrate that responses evoked by PNS and tSCS exhibit comparable sensitivity to presynaptic inhibitory mechanisms at rest and during voluntary contraction. This convergence suggests that the two responses engage common neural pathways and supports the use of tSCS as a reliable tool to probe spinal excitability during voluntary contraction.