Electrical stimulation for neuromuscular testing and training: state-of-the art and unresolved issues

Contrary to other widespread forms of electrical stimulation, such as transcutaneous electrical nerve stimulation (TENS) and functional electrical stimulation (FES), neuromuscular electrical stimulation (NMES) is generally delivered to the muscle in static conditions (without functional movement occurring) and at sufficiently high current intensities to evoke visible muscle contractions (beyond motor threshold). NMES has received increasingly attention in the last few years, because it has the potential to serve: - as a strength training tool for healthy subjects and athletes, since its chronic use may induce neuromuscular adaptations similar/complementary to voluntary strength training; - as a rehabilitation and preventive tool for partially- or totally-immobilized patients, since its chronic use may preserve muscle mass and function during prolonged periods of reduced muscular use; - as a testing tool for evaluating the neural and/or muscular function in vivo, since it is able to induce standardized muscle contractions whose electrical (EMG) and mechanical (torque) properties could be easily quantified; - as a post-exercise recovery tool for athletes, since its acute application may increase muscle blood flow and therefore metabolites washout which could in turn accelerate recovery kinetics during and after exercise. Portable NMES units are widely available to the general population, however commercial claims often go far beyond the existing scientific evidence regarding NMES use. Moreover, due to the lack of general consensus in the scientific community about the main physiological and methodological features of NMES, end users are faced with confusion regarding its usage and effectiveness, so that they often prefer not to apply NMES or to apply it with exaggerated caution. This cluster of articles is dedicated to selected topics relative to the physiology, methodology and applications of NMES (Figure 1), with a state-of-the art scientific review on motor unit recruitment, acute muscular effects, neural and muscular adaptations induced by NMES training, optimization of the stimulation paradigm to increase NMES effectiveness, and NMES use as a tool for strength training, neuromuscular testing and post-exercise recovery. The rationale behind the cluster is that, in order to be able to improve/optimize neuromuscular function by means of NMES (applications in Fig. 1), we need first to improve our knowledge on NMES (physiology in Fig. 1), and then improve the actual use of NMES (methodology in Fig. 1). This series of article is therefore addressed to practitioners and researchers from different fields (exercise training, exercise physiology, sport medicine, and even general medicine) whose main interest is the optimization of skeletal muscle function through NMES. After an overview of the group of articles, a series of unresolved issues and recommendations for future NMES research are presented in this Editorial.