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

Abstract

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+.

Can Neuromuscular Electrical Stimulation Enhance the Effect of Sprint Interval Training?

“…The notion that VOLES enhances the discharge rate of motor units, particularly in fast-twitch muscles during high-intensity exercise, is supported by recent research 19 . The discharge rate of motor units increases during high-intensity exercise following acute low-intensity exercise involving NMES, but not following either low-intensity exercise alone or NMES alone, compared to the discharge rate during high-intensity exercise following resting conditions 19 . Given the known association between fast-twitch muscle contractions and blood lactate generation 12 , the enhanced neural drive induced by VOLES during the second Wingate test could indeed contribute to a more pronounced increase in [La] b during the recovery phase, distinguishing it from the VOL protocol.…”

“…The observed higher [La] b during the recovery phase in the VOLES protocol compared to the VOL protocol suggests that the neural drive induced by VOLES during the 2 nd Wingate test may play a significant role. The notion that VOLES enhances the discharge rate of motor units, particularly in fast-twitch muscles during high-intensity exercise, is supported by recent research [19]. The discharge rate of motor units increases during high-intensity exercise following acute low-intensity exercise involving NMES, but not following either low-intensity exercise alone or NMES alone, compared to the discharge rate during high-intensity exercise following resting conditions [19].…”

Assessing the Impact of Neuromuscular Electrical Stimulation-Based Fingerboard Training versus Conventional Fingerboard Training on Finger Flexor Endurance in Intermediate to Advanced Sports Climbers: A Randomized Controlled Study

“…Especially, the electrical stimulus has the potential to induce neuronal adaptations of the muscles, such as frequency and recruitment synchronization. These subtle adaptations may confer significant advantages for athletes, so that small intramuscular adaptations of the muscle fibers (frequency, recruitment, synchronization) already lead to decisive advantages for the athletes [33,68]. However, to date, this remains a hypothesis that has yet to be confirmed.…”

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.…”