Superimposing neuromuscular electrical stimulation onto voluntary contractions to improve muscle strength and mass: A systematic review

Abstract

Training and rehabilitation programmes involving neuromuscular electrical stimulation superimposed onto voluntary contractions (NMES+) have gained popularity in the last decades. Yet, there is no clear consensus on the effectiveness of such intervention. The aim of this review was to evaluate the effect of chronic exposure to NMES+ on muscle strength and mass compared to conventional volitional training or passive electrical stimulation alone. Two authors conducted an electronic search to identify randomized controlled trials that investigated the effect of NMES+ training, involved healthy participants or orthopaedic patients, detailed a welldefined NMES training protocol, and provided outcomes related to muscle strength and/or mass. The authors extracted data on participants, intervention characteristics, muscle-related outcomes, and assessed the methodological quality of the studies. A total of twenty-four studies were included in the review. The majority of these reported an increase in muscle strength following NMES+ training compared to an equivalent voluntary or passive NMES training. The highest improvements were found when NMES was superimposed on sub-maximal exercises involving both concentric and eccentric contractions. Two studies reported an increase in muscle mass after NMES+, while two other studies exhibited no differences. This review indicated that chronic exposure to NMES+ determines muscle strength improvements greater or equal compared to volitional training alone. However, differences in the methodological characteristics of the stimulation and the type of exercise associated with NMES + revealed significant discrepancies in the results. A deeper understanding of the neurophysiological adaptations to NMES+ is crucial to fully explain the muscle-related enhancement resulting from such intervention. Highlights. NMES+ consists of simultaneously applying neuromuscular electrical stimulation while voluntarily contracting the stimulated muscle. . Although a growing number of studies have suggested that intervention based on NMES+ have a strong potential in enhancing as well as preserving muscle function, there is still no clear consensus on the effectiveness of such technique. . This review revealed that training based on NMES+ can induce a significant improvement of muscle strength in both healthy and orthopaedic individuals.

The effect of neuromuscular electrical stimulation on the human skeletal muscle transcriptome

“…Intriguingly, the combination of NMES on one leg and EX on the other leg resulted in a higher gene activation on each of the legs as compared to the groups that had unilateral intervention, suggesting synergetic effects. Previous studies have shown that NMES and EX at the same time, in the same leg with the same muscle, results in a greater muscle strengthening effect than EX alone 7,10,26,69,70 . Interestingly, this study demonstrated that the NMES and EX can potentiate each other even when performed in opposite extremities.…”

“…Previous studies have that NMES and EX at the same time, in the same leg with the same muscle, results in a greater muscle strengthening effect than EX alone. 7,10,26,69,70 Interestingly, this study demonstrated that the NMES and EX can potentiate each other even when performed in opposite extremities. These observations suggest involvement of either systemic hormonal mechanisms and/or effects mediated through the central nervous system, for example, recruitment and activation of additional motor units.…”

“…Such mechanisms could be beneficial for patients, for example, after an injury or during partial immobilization, when they are not able to recruit and activate motor units fully during EX. 7,70 Our observation that both NMES and EX resulted in similar DEGs in the control leg, that is, a "cross-over" effect in the untrained limb, suggest similar underlying mechanism that could be explored in immobilized patients using NMES. Our findings are supported by previous studies showing that unilateral training can increase contralateral strength, both during regular exercise [71][72][73] and NMES.…”

“…These observations suggest involvement of either systemic hormonal mechanisms and/or effects mediated through the central nervous system, for example, recruitment and activation of additional motor units. Such mechanisms could be beneficial for patients, for example, after an injury or during partial immobilization, when they are not able to recruit and activate motor units fully during EX 7,70 …”

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

“…In recent years, exercise protocols based on neuromuscular electrical stimulation (NMES) have provided a considerable number of functional and physiological benefits in both healthy and individuals with neurological or orthopedic disorders ( Maffiuletti, 2010 ). Most commonly, studies have reported an increase in skeletal muscle strength following training protocols involving NMES, with the greatest effectiveness obtained by delivering NMES while voluntarily performing movement exercises (NMES+), as described in two recent reviews ( Paillard, 2018 ; Borzuola et al, 2022 ). Interventions based on NMES + have shown the potential to improve muscle force generating capacity even to a greater extent than NMES only or voluntary exercise training alone ( Paillard et al, 2005 ; Herrero et al, 2010 ; Bezerra et al, 2011 ; Labanca et al, 2018 ; Labanca et al, 2022 ; Dörmann et al, 2019 ).…”