ORIGINAL RESEARCH article

Front. Physiol.

Sec. Exercise Physiology

Volume 16 - 2025 | doi: 10.3389/fphys.2025.1590949

This article is part of the Research TopicResponses and Adaptations to Novel Exercise ModalitiesView all 16 articles

Acute modulation of common synaptic inputs and motor unit discharge rates following neuromuscular electrical stimulation superimposed onto voluntary contractions

Provisionally accepted
  • 1Department of Exercise, Human and Health Sciences, Foro Italico University of Rome, Rome, Italy
  • 2Artificial Intelligence in Medical Imaging Lab, Department Artificial Intelligence in Biomedical Engineering, Technische Fakultät, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Bavaria, Germany
  • 3Department of Biomedical Sciences, University of Padova, Padova, Veneto, Italy

The final, formatted version of the article will be published soon.

Introduction: Superimposing neuromuscular electrical stimulation (NMES) onto voluntary contractions induces specific neuro-physiological adaptations that may have a direct effect on force related outcomes. This study investigated motor unit discharge characteristics and force steadiness following three acute experimental conditions: NMES superimposed onto isometric contractions (NMES+ISO), passive NMES, and isometric contractions only (ISO).Methods: Seventeen healthy volunteers participated in the study. Each condition involved 20 intermittent (6s contraction/6s rest) isometric ankle dorsi flexions at 20% of their maximum voluntary contraction (MVIC). NMES was delivered to the tibialis anterior (TA) during NMES and NMES+ISO. High-density surface electromyography (HDsEMG) was used to record myoelectric activity in the TA during steady force-matching contractions, at 10% MVIC, performed immediately after each experimental condition. Motor unit discharge rate (DR) and inter-spike variability (ISIvar) were analyzed from decomposed HDsEMG signals. Coherence analysis was performed to evaluate the strength of common synaptic input across different frequency bands and the proportion of common synaptic input (pCSI) received by spinal motoneurons. Force steadiness was evaluated using the coefficient of variation of force (ForceCoV).Results: NMES+ISO significantly increased motor unit DR compared to baseline and postintervention NMES. NMES+ISO also induced an increase in pCSI compared to baseline, ISO and NMES. ForceCoV was reduced after NMES+ISO compared to all experimental conditions, indicating improved force steadiness.Discussion: These results suggest that superimposing NMES onto voluntary contractions can enhance motor unit firing rate and pCSI at low force levels. These adaptations seem to positively contribute to force steadiness, likely by engaging filtering mechanisms which minimize the independent synaptic noise affecting motor control. These findings provide new perspectives on the adaptations induced by NMES exercise, highlighting some of the neuro-physiological mechanisms involved and enriching our knowledge of how the neuromuscular system responds and adapts to NMES-based interventions.

Keywords: Electrical Stimulation, motor unit, common synaptic input, force steadiness, neural drive, coherence analysis, HDsEMG

Received: 10 Mar 2025; Accepted: 09 Jul 2025.

Copyright: © 2025 Borzuola, Nuccio, Del Vecchio, Bazzucchi, Felici, De Vito and Macaluso. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Riccardo Borzuola, Department of Exercise, Human and Health Sciences, Foro Italico University of Rome, Rome, Italy

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