Acute effects of neuromuscular electrical stimulation on cortical dynamics and reflex activation

Abstract

Superimposing neuromuscular electrical stimulation (NMES) on voluntary muscle contractions has shown the potential to improve motor performance even more than voluntary exercise alone. Nevertheless, the neurophysiological and neurocognitive mechanisms underlying this technique are still unclear. The aim of this study was to investigate the acute responses in spinal excitability, and brain activity following three conditions: NMES superimposed on isometric contractions (NMES+ISO), passive NMES, and voluntary isometric contractions (ISO). Each condition involved fifteen intermittent ankle plantar-flexions at submaximal level. Before and after each condition, tibial nerve stimulation was used to elicit H-reflexes, which represent a measure of spinal excitability, and somatosensory evoked potentials (SEPs), which index the activity of subcortical and cortical somatosensory areas. H-reflex amplitudes increased following NMES+ISO and decreased following passive NMES compared to baseline values. Subcortical lemniscal activity remained unaltered after the conditions. Activity in both primary and secondary somatosensory cortices (S1 and S2) increased after the NMES+ISO and decreased after the ISO condition. At later stages of S2 processing, cortical activity increased also after NMES, however the NMES+ISO effect was greater than that produced by NMES alone. These findings indicate that the beneficial effects of the NMES may be mediated by potentiation of the reflex pathways at the spinal level. At the brain level, peripheral input representation in the brainstem was not influenced by the experimental conditions which, conversely, altered cortical activity by affecting the synaptic efficiency through the somatosensory pathway. While the ISO effect was suppressive, the NMES was excitatory, especially if combined with voluntary contractions.

Adjustments in the motor unit discharge behavior following neuromuscular electrical stimulation compared to voluntary contractions

“…Participants were allowed a recovery period of 10 min between all conditions. The number and duration of contractions used in this protocol was selected to prevent development of muscle fatigue ( Gueugneau et al, 2017 ; Grosprêtre et al, 2018 ), while at the same time inducing neural adjustments as demonstrated in previous literature ( Lagerquist et al, 2012 ; Borzuola et al, 2020 ; Borzuola et al, 2023 ). All the procedures were performed on participants’ dominant leg as described by Botter et al (2011) .…”

Neuromuscular Electrical Stimulation Does Not Influence Spinal Excitability in Multiple Sclerosis Patients

“…During each experimental condition, a target force of 20% of MVIC was set based on MVIC measurements. Previous studies involving the application of NMES demonstrated that this force level could affect spinal excitability without inducing muscle fatigue [39][40][41][42]48,58]. In this regard, MVIC was evaluated again at the end of the whole experimental session to determine if muscle fatigue had arisen.…”

“…According to previous studies [39,41,56], the stimulus intensity was selected to evoke an H-reflex (H test ) on the ascending limb of the RC with an amplitude corresponding to 80-85% of the Hmax, as illustrated in Figure 4. To ensure stimulus consistency and repeatability, a small M-wave (M test ) corresponding to the H test was selected and monitored throughout the entire experiment [39,40]. If the M-wave amplitude lay within 5% of the selected M test , the corresponding H-reflex measure was accepted.…”

“…The Hoffmann reflex (H-reflex), which is a spinal reflex elicited by electrically stimulating a peripheral mixed nerve, has been extensively used to investigate some of the neurophysiological mechanisms underlying NMES intervention in both healthy young and elderly individuals [39][40][41][42][43]. As a monosynaptic spinal reflex, the H-reflex could offer an effective tool for studying the modulation of spinal excitability under certain conditions [44].…”

“…Several research works have revealed that NMES stimulation modulates H-reflex responses in healthy individuals [39,[41][42][43]45,46]. Some authors have demonstrated that a single bout of NMES superimposed onto voluntary isometric contractions of the ankle plantar flexor muscles increased soleus H-reflex amplitude [39][40][41]. On the contrary, recent evidence has indicated a sudden reduction in H-reflex amplitude following passive NMES [39,41,43,[47][48][49].…”