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.

Objective: This study aimed at evaluating the acute effect of the combined and single use of two orthotic devices (neck balance system [NBS] and lumbar support [LS]) on muscle activity of neck and back muscles during typical computer working tasks. Background: An excessive activation of neck muscles could threaten the balance between agonist and antagonist muscles, resulting in a lower stability of the head and possibly leading to neck pain. At present, no study evaluated the effect of a specific orthotic device in reducing neck muscles activation. Methods: Surface electromyography (sEMG) from neck flexor (sternocleidomastoid [SCMD]) and extensor muscles (semispinalis capitis [SPC]) and back extensor muscles (erector spinae [ERS]) of 20 healthy individuals was recorded during three computer working tasks performed with the NBS, with NBS and LS, with the LS, and without devices (ND). Results: In the NBS condition, the SPC showed a reduced activation (NBS = 3.97%; NBS + LS = 4.49%; LS = 4.48%; ND = 4.61% of the maximal voluntary contraction) compared to the other conditions. Conclusions: The use of the NBS promotes a reduction of neck extensor muscles, possibly due to the inertial mass added in the occipital part of the head, producing an external neck extensor moment that cooperates with that produced by neck extensor muscles. Application: Orthotic devices such as the NBS may be used by computer workers to reduce the activation of their neck extensor muscles and possible risks of developing neck pain.

Introduction: The application of neuromuscular electrical stimulation superimposed on voluntary muscle contractions (NMES+) has demonstrated a considerable potential to enhance or restore muscle function in both healthy and individuals with neurological or orthopedic disorders. Improvements in muscle strength and power have been commonly associated with specific neural adaptations. In this study, we investigated changes in the discharge characteristics of the tibialis anterior motor units, following three acute exercises consisting of NMES+, passive NMES and voluntary isometric contractions alone.Methods: Seventeen young participants participated in the study. High-density surface electromyography was used to record myoelectric activity in the tibialis anterior muscle during trapezoidal force trajectories involving isometric contractions of ankle dorsi flexors with target forces set at 35, 50% and 70% of maximal voluntary isometric contraction (MVIC). From decomposition of the electromyographic signal, motor unit discharge rate, recruitment and derecruitment thresholds were extracted and the input-output gain of the motoneuron pool was estimated.Results: Global discharge rate increased following the isometric condition compared to baseline at 35% MVIC while it increased after all experimental conditions at 50% MVIC target force. Interestingly, at 70% MVIC target force, only NMES + led to greater discharge rate compared to baseline. Recruitment threshold decreased after the isometric condition, although only at 50% MVIC. Input-output gain of the motoneurons of the tibialis anterior muscle was unaltered after the experimental conditions.Discussion: These results indicated that acute exercise involving NMES + induces an increase in motor unit discharge rate, particularly when higher forces are required. This reflects an enhanced neural drive to the muscle and might be strongly related to the distinctive motor fiber recruitment characterizing NMES+.