Modulation of spinal excitability following neuromuscular electrical stimulation superimposed to voluntary contraction

Purpose. Neuromuscular electrical stimulation (NMES) superimposed on voluntary muscle contraction has been recently shown as an innovative training modality within sport and rehabilitation, but its effects on the neuromuscular system are still unclear. The aim of this study was to investigate acute responses in spinal excitability, as measured by the Hoffmann (H) reflex, and in maximal voluntary contraction (MVIC) following NMES superimposed to voluntary isometric contractions (NMES+ISO) compared to passive NMES only and to voluntary isometric contractions only (ISO). Method. Fifteen young adults were required to maintain an ankle plantar-flexor torque of 20% MVC for 20 repetitions during each experimental condition (NMES+ISO, NMES and ISO). Surface electromyography was used to record peak-to-peak Hreflex and motor waves following percutaneous stimulation of the posterior tibial nerve in the dominant limb. An isokinetic dynamometer was used to assess maximal voluntary contraction output of the ankle plantar flexor muscles. Results. H-reflex amplitude was increased by 4.5% after the NMES+ISO condition (p < 0.05), while passive NMES and ISO conditions showed a decrease by 7.8% (p < 0.05) and no change in reflex responses, respectively. There was no change in amplitude of maximal motor wave and in MVIC torque during each experimental condition. Conclusion. The reported facilitation of spinal excitability following NMES+ISO could be due to a combination of greater motor neuronal and corticospinal excitability, thus suggesting that NMES superimposed onto isometric voluntary contractions may provide a more effective neuromuscular stimulus and, hence, training modality compared to NMES alone

Neural and muscular mechanisms of electrically induced fatigue in patients with spinal cord injury

International audienceStudy design: Intervention study. Objectives: The present study aimed at examining whether spinal and/or peripheral alterations are in the origin of neuromuscular fatigue development induced by intermittent neuromuscular electrical stimulation (NMES) in subjects with complete spinal cord injury (SCI). Setting: Neurological Rehabilitation Center CMN Propara, Montpellier, France. Methods: Thirteen volunteers with complete SCI participated in the study. The right triceps surae muscle was fatigued using a 30-Hz NMES protocol (2 s ON-2 s OFF) composed of three series of five trains. Spinal excitability (assessed by the H-reflex), muscle excitability (assessed by the M-wave), muscle contractile properties (assessed by mechanical response parameters) and torque evoked by NMES were tested before and after each five-train series. Results: NMES-evoked torque significantly decreased throughout the protocol (P<0.001). This decrease was accompanied by a significant increase in M-wave amplitude (P<0.001), whereas H-reflex and the Hmax/Mmax ratio were not significantly modified. The amplitude of the mechanical response was significantly decreased at the end of the protocol (P<0.05). Conclusion: The results indicate significant fatigue development, which was attributed to impaired cross-bridge force-generating capacity, without modification of spinal excitability nor muscle excitability