EMS is also called as Neuromuscular Electrical Stimulation (NMES) or “electromyostimulation". Muscles which perform different tasks, in addition to having different muscle architecture, respond to different electrical input. For example, skeletal muscles that play a postural role, and thus have a high proportion of slow fibers, are physiologically activated at low frequencies. Conversely, muscles with a very high proportion of fast fibers may be activated only intermittently with high frequency bursts of electrical activity. The fact that electrical activity and muscle properties seem to be intimately interrelated, provides an experimental basis for understanding muscle plasticity".
Regarding the claim of strengthening muscles by EMS, per Nicola A. Maffi Uletti in 2006, there is no doubt that it is possible to increase muscle strength with an ES training program.
ES-training-induced strength and power gains for unimpaired muscles are similar (and complementary) to, but not greater than, those that can be achieved with normal voluntary training.
Scientific analysis by Andre Filipovic et.al. in 2011 revealed that EMS is effective for developing physical performance."It is pretty well established that electric muscle stimulation works, there are still a lot of unknowns with regards to what works best".
With regard to the level of fitness, the analysis shows that trained and elite athletes, despite their already high level of fitness, are able to significantly enhance their level of strength to same extent as is possible with untrained subjects. The EMS offers a promising alternative to traditional strength training for enhancing the strength parameters and motor abilities.
An OTC EMS device cleared by FDA for toning, strengthening and firming abdominal muscles is Slendertone Flex marketed by BMR NeuroTech Inc.
Regarding the claim of weight loss and minimizing body fat using EMS devices, per FDA, "While an EMS device may be able to temporarily strengthen, tone or firm a muscle, no EMS devices have been cleared at this time for weight loss, girth reduction, or for obtaining "rock hard" abs". Also per FDA, "Applying electrical current to muscles may cause muscles to contract. Stimulating muscles repeatedly with electricity may eventually result in muscles that are strengthened and toned to some extent but will not, based on currently available data, create a major change in your appearance without the addition of diet and regular exercise. The effect of using these devices is primarily to help a patient recover from impaired muscle function due to a medical condition, not to increase muscle size enough to affect appearance."
Regarding efficacy of EMS using EMS devices, approval pending EMS devices listed here have a 510(k) number which refers to a premarket submission made to FDA to demonstrate that the device to be marketed is at least as safe and effective, that is, substantially equivalent, to a legally marketed device that is not subject to premarket approval. Submitters must compare their devices to one or more similar legally marketed devices and make and support their substantial equivalency claims when they submit the 510(k) application.
Use of FDA approved EMS devices is recommended as safe since per FDA, "Using a product that has not met FDA requirements isn't necessarily unsafe or dangerous. But it could be. FDA has received reports of shocks, burns, bruising, skin irritation, pain, and interference with other critically important medical devices (e.g., pacemakers) associated with the use of unregulated products. Unregulated devices also may have safety problems associated with cables and leads that can lead to accidental shock and electrocution by users and other household members, including children."
Recent research into EMS indicates that,
1. Neuromuscular electrical stimulation for enhancing performance is not convincingly supported by evidence.
Per systematic review by Malone, John K et.al. in 2014, evidence is not convincing to support NMES for enhancing subsequent performance.although there may be some subjective benefits for postexercise recovery.
The meta-analysis showed no evidence in favor of NMES vs. active (ACT) and mixed evidence vs. passive (PAS) recovery for BLa. In conclusion, although there may be some subjective benefits for postexercise recovery, evidence is not convincing to support NMES for enhancing subsequent performance.
2. NMES is a training modality resulting in changes in oxidative enzyme activity, skeletal muscle fiber type and skeletal muscle fiber size but a future meta-analysis of NMES is need to have definitive conclusions.
Per a systematic review by Maurice J. H. Sillen et.al. in 2013 on metabolic and structural changes in lower-Limb skeletal Muscle following Neuromuscular Electrical Stimulation (NMES), it seems to be a training modality resulting in changes in oxidative enzyme activity, skeletal muscle fiber type and skeletal muscle fiber size. However per the authors, a future meta-analysis of NMES will help to have definitive conclusions about the effects of stimulation frequencies on muscular changes.
NMES seems to be a training modality resulting in changes in oxidative enzyme activity, skeletal muscle fiber type and skeletal muscle fiber size. However, considering the small sample sizes, the variance in study populations, the non-randomized controlled study designs, the variance in primary outcomes, and the large heterogeneity in NMES protocols, it is difficult to draw definitive conclusions about the effects of stimulation frequencies on muscular changes. This systematic review, however, will help generate discussion in the field that would lead to a consensus in study design that would permit a meta-analysis in the future.
3. NMES should be considered in combination with resistance training for optimum improvement of muscle function.
Per Gondin J et.al's overview of the currently acknowledged benefits and limitations of neuromuscular electrical stimulation (NMES) training programs in both healthy individuals and in recreational and competitive athletes regarding muscle performance in 2011.
NMES cannot be considered as a surrogate training method, but rather as an adjunct to voluntary resistance training. The combination of these two training modalities should optimally improve muscle function.