Electrical Muscle Stimulation

Muscles and electricity have always had something to do with each other. The story begins with Alessandro Volta playing with frog limbs and then takes us to Russia, where scientists in the 1960s reported up to 40% force gains in athletes. In the 1970s the Russians shared these results with western establishments, prompting further inquiry into the mechanisms behind electrical stimulation of muscles, culminating in OTC electromyostimulation kits and well-documented therapeutic applications of EMS in physiotherapy and muscle training.

Also known as electromyostimulation and NMES [NeuroMuscular Electrical Stimulation], the technique basically involves passing an electrical discharge through electrodes placed on the skin near the muscle groups being stimulated. As a therapeutic tool, EMS is used to prevent muscular atrophy in cases of prolonged inactivity or musculoskeletal injuries. By causing muscles to contract repeatedly, it causes increased blood flow. It can also help retrain muscles to start contracting in response to natural electrical impulses, which helps stroke survivors who sometimes have to relearn basic motor functions. In strength training and muscle conditioning, by altering the frequency, duration, and strength of the pulses used, EMS could increase strength or endurance. Muscle groups with a higher proportion of fast-twitch fibres can be stimulated to generate more force by using higher frequency EMS signals, while muscle groups that have a greater proportion of slow-twitch fibres benefit from lower frequency pulses and show greater endurance as a result. Studies have shown that the neuromuscular adaptations that arise due to EMS are complementary/similar to the effects of voluntary resistance training, making EMS a useful tool in maximising the benefits of resistance training. The United States Food and Drug Administration has certified various EMS devices for uses including relaxation of muscle spasms, muscle re-education, increasing local blood circulation, etc.

In this review published in the European Journal of Applied Physiology, there is a description of the physiological processes involved in EMS. For muscles to contract in response to any stimulus, muscle fibres need to be recruited. In the case of a voluntary contraction, various neural pathways decide what kind of fibres get recruited to fulfil the contraction ‘request’. In the case of EMS, there is no orderly recruitment; the depth of the fibre from the electrode patch and the strength of the electrical signal received by the fibre are the basic parameters that determine whether or not a fibre is going to contract and how strongly it contracts. Due to the absence of any sequencing of muscle excitation depending on muscle unit type or size, the excitation of muscle fibres by EMS is more random and incomplete than natural excitation. Further, recruitment is superficial and spatially fixed; this results in the same set of fibres being repeatedly excited, which causes muscles to fatigue sooner.

By studying the involvement of the central and peripheral nervous systems in how muscles respond to electrical stimulation, Bergquist et al. (2011) suggested that electrically evoked contractions where the involvement of the central nervous system is higher might result in more orderly, less synchronous, and more spatially diffuse contractile responses in the muscle group being stimulated through EMS, countering the issue of muscle fatigue. Enhancing the involvement of the central nervous system in the contractile response was found to require stimulation at lower amplitudes, higher frequencies, and relatively longer duration bursts. This resulted in the wide-pulse-high-frequency EMS paradigm. Using this paradigm, muscle recruitment is made less random.

Along with the characteristics of the electrical pulse being used, it is also important to understand where to deliver the pulse. Motor points are specific regions on the skin with the highest muscle excitability; studying the distribution of motor points innervating different portions of various muscle groups, Botter et al. (2011) showed that using a multi-pad electrode with low-frequency asynchronous stimulation elicited strong, fused contractions with reduced muscle fatigue as compared to higher frequency signals using a single channel by recruiting different muscle volumes in the same muscle group.

EMS programs when done right take the above factors into account, in addition to factors like athlete preparedness, muscle composition, previous injury/trauma, chronic pain, etc., to arrive at a personalised combination of pulse timing, strength, frequency, burst duration, and electrode placement to augment muscle training routines. EMS could be used to aid muscle contraction during a rep or induce isotonic contractions; these factors are determined by the muscle groups being worked on and the purpose of the training program itself, among other things.

While there have been plenty of cases where EMS has been used to achieve improvements in contractile strength and muscle endurance, the exact pathways involved in achieving these effects and the way EMS affects these pathways remains incompletely understood. The high degree of variability in the responses that individuals have to training programs using EMS makes it difficult to establish more general results in terms of prescribed pulse characteristics or accurate motor point maps. Countering this difficulty requires trainers to themselves be well trained in the mechanisms by which EMS aids muscle training/growth/conditioning; they would also need to know what preliminary measurements must be carried out before they design an EMS program and what parameters need to be monitored during the program to aid optimisation of the program. The catch here is that such training requires a much better understanding of pathways that haven’t been studied enough. High variability between individuals also introduces the problem of replicability of results which then makes it difficult to validate results. Despite these shortcomings, EMS is a promising field of exploration that holds the potential of improving the efficiency of workout regimes and physiotherapy programmes using relatively simple electrical devices.