Background
Whole-body electrical myostimulation (WB-EMS) is a relatively recent training methodology that has been extraordinarily lavished in recent years. WB-EMS, which is also called global-body electrical myostimulation, has emerged as the evolution of traditional electrical muscle stimulation (EMS) applied locally, since it is now possible to activate several muscle groups in a synchronized manner as a result of technological development. Using a wireless electrical stimulator that has a powerful battery, it is possible to activate up to twelve channels with a rectangular, two-phase and symmetrical current [
1]. These channels generally allow the activation of the muscles of the thighs, arms, buttocks, abdomen, chest, and low, high and lateral areas of the back with two auxiliary channels of free choice and a total area of electrodes up to 2800 cm
2 [
2]. These devices are managed by software that allows the modification of the current parameters and the intensity of each of the channels.
Local EMS is based on the application of the current to the motor point of one or two muscle groups, whereas the WB-EMS procedure is based on doing the same across a large area and along several muscle groups. On the one hand, the application of the current to a motor point during the EMS means that less energy is required to cause the involuntary contraction; therefore, the method is more comfortable [
3]. On the other hand, the application of current in a large number of muscle groups in a synchronized manner in WB-EMS makes it possible to exercise complete kinetic chains in unison and perform exercises with global positions and movements during the electrical stimulus [
4]. In the WB-EMS, the coactivation of agonist-antagonist muscles is generally observed. This feature may be an advantage given that stimulating an antagonist muscle can contribute to the improvement of aerobic strength and capacity without presenting damage to the motor pattern as shown in previous experimental studies [
5,
6].
To date, vast and extensive research has been performed in the study of the effects of local EMS [
7‐
9] that should be taken into account for WB-EMS exercise. It would not be surprising if, despite these slight differences in the two methodologies, future research demonstrates that WB-EMS offers results similar to those obtained with the local EMS for the rehabilitation of injuries [
10,
11], i.e., for the effective treatment of spasticity in subjects with neurological disorders [
12], exercise for individuals with illnesses [
13‐
15], and for strength training in healthy subjects [
4].
It has been concluded that WB-EMS could be an interesting training methodology for people who experience difficulties when exercising given the amount of effort necessary to create adaptations [
16]. WB-EMS has also been considered as an alternative with great efficiency in terms of the time-benefit ratio with a high acceptance rate even in untrained individuals [
17]. However, other studies have obtained less promising results, presenting a less optimistic position regarding the effectiveness of this type of training [
18].
EMS is capable of generating muscle tension greater than that which can occur in voluntary contraction and therefore can cause muscle degradation far superior to what traditional exercise is capable of causing [
19]. Therefore, it has been indicated that the use of WB-EMS could be a danger mainly for untrained people, arguing that increasing the number of affected muscle groups could be a risk factor. In fact, over recent years, different case reports have appeared in which rhabdomyolysis has occurred after a training session with an alarming increase in creatine kinase (CK) activity [
20‐
22].
There is a lack of consensus on the effectiveness of WB-EMS in a situation in which its use has been popularized to a large extent, thus increasing the need for a systematic review with the purpose of analyzing the results obtained from the existing research on WB-EMS and testing the level of evidence of each of the studies to understand the status of the issue and identify possible methods of investigation in the future.
Discussion
The aim of this systematic review was to determine the effects of WB-EMS. Taking into account the enormous interest in this training methodology in recent years, a review that complies and objectively verifies the knowledge obtained on the subject to date is needed to clarify the state of the matter.
Studies in the field of WB-EMS are beginning to increase. In addition, due to the enormous interest in the use of this training tool that can become very dangerous if used incorrectly, a guide for its correct use has been created in a very appropriate and timely manner by mainly appealing to common sense [
43]. However, to our knowledge, the research body is scarce, and the existing studies lack the amount of evidence necessary to draw solid conclusions about the effectiveness of training with WB-EMS and adequate technical guidelines for its use and management in different contexts and needs.
Many of the studies published to date have been performed with population groups with very determinant diseases, indicating that these studies lack a high level of external validity. Taking this limitation into account, in their systematic review on the effects of electrical myostimulation, Filipovic et al. [
9] report a high correlation between the intensity of the current and the effects of EMS. It is believed that this type of population with special needs and a delicate state of health may not be the most suitable for electrical stimulation training. Similarly, it is not believed that this type of sample is the most adequate to reproduce the physically demanding current parameters that have been typically applied given the influence of certain brands of electrical stimulators that commonly provide a frequency of 85 hz. In his review, Filipovic [
4] concludes that a current of 50 hz is sufficient for the activation of type II fibers and strength work. In fact, previous studies indicate the need to minimize the frequency of the current as much as possible given that its increase is accompanied by an increase in muscle fatigue [
44]. Therefore, it seems that the electrical stimulus chosen in some of the studies is not the most commonly recommended for people with atrophy of their muscular system and a sedentary lifestyle.
On the other hand, what has been demonstrated in studies with local EMS is the effectiveness of training with EMS as a means for functional improvement in elderly populations [
45,
46], which makes the appearance of new research on WB-EMS in these population groups necessary with parameters more adapted to their needs.
Regarding the time-to-rest electric stimulus ratio and considering what was said above, it seems that in studies of WB-EMS where populations exhibited some type of physical handicap, these populations were exposed to duty cycles of excessive density, i.e., close to 50%. In contrast, 20–25% is recommended to guarantee sufficient recovery and strength adaptation [
9]. In the study by Wirtz et al. [
18] of soccer players, a strength work and a duty cycle of 83.3% was proposed. The demanding density of these protocols, which do not guarantee a necessary rest, could perhaps be the cause of a poor evolution of the strength or the absence of improvement as an adaptation to WB-EMS training in this study.
The application of the WB-EMS is typically performed in sports centers or beauty centers where training sessions last for 20 min. This is a controversial issue. Filipóvic et al. [
9] consider that 20 min is highly advisable and a sufficient time period to increase the levels of strength and the physical skills that are derived from it, whereas other study conclude that a classic 20-min training session does not seem the most appropriate for improvement of sports skills or the rehabilitation of injuries [
47]. It must be taken into account that depending on the parameters of the current, the muscular fatigue that is generated can vary enormously [
11]. Thus, establishing such a short fixed time without remission would determine the characteristics of the training session. That a training session with EMS or WB-EMS should only contain muscle contractions combined with the current could be a common mistake. In this sense, to conceive EMS and WB-EMS as a resource among the many others available to the professional instead of converting the currents into the objective of the session would be appropriate and enriching.
Regarding the anthropometric results obtained in the studies analyzed, no statistically conclusive evolutions have been observed to date. In addition to not recording feeding control in any of the cases, the results reveal a small effect size with a large standard deviation, and the values are significant in a limited number of cases. However, it is possible that in the future, research on WB-EMS will provide more positive results in this field. It should be considered that EMS applied simultaneously to aerobic exercise can contribute to the reduction of fat tissue to a greater extent than aerobic exercise alone [
48]. However, it should be noted that this study made its assessments through the analysis of skinfolds, suggesting that research with more precise assessment techniques for the evaluation of anthropometric parameters and their evolution before training with EMS and WB-EMS is needed.
In the context of different exercises with the same level of maximum oxygen consumption (VO2), EMS causes a significant increase in lactic acid and glucose consumption, suggesting that the current increases energy consumption and the oxidation of carbohydrates to a greater degree compared with that produced by voluntary contraction [
49]. In the study by Kemmler et al. [
41], a 17% increase in energy consumption was observed during exercise performed with simultaneous WB-EMS. This minimum difference could not justify its use for this purpose although the WB-EMS involves a greater area of electrical stimulation than local EMS. The authors note that they potentially underestimated the effect of WB-EMS given that their measurement of energy consumption through VO2 is only valid in steady state situations. However, in this study, they do not indicate at any time that the participants received a familiarization session with WB-EMS prior to the experimental phase. A previous study demonstrated the need for at least one EMS session prior to the study to minimize the muscle damage produced by the current and favor the familiarization of the subjects to the electrical stimulus [
50]. It is possible that this limitation could have led to the fact that the intensity of the current with which the participants performed the exercise was substantially lower than they could sustain without risk if the participants had completed a phase of previous adaptation. Therefore, the potential effects of WB-EMS on energy consumption could have been minimized in this experimental phase.
With regard to the effects on strength, only two studies analyzed the effect of the WB-EMS in the dynamic 1RM. Kemmler et al. [
33‐
36] found a significant increase (9.5%,
p = 0.001). In the study by Filipovic et al. [
28,
29] of trained subjects, they confirmed an increase (22.42% ± 12, 79). This increase is similar to that observed by Willoughby & Simpson [
51] (26.3%) in a study that also simulated the application of currents with dynamic voluntary contractions three days a week but with local EMS. However, in other studies with similar methodology for local EMS, lower increases in the dynamic strength of the lower limbs were observed after training for three days a week for 12 weeks (+ 15.0 +/− 8.0%,
p < 0.001) [
52]. Others have found that the 1RM increased 40.2% due to local EMS with four workouts per week for four weeks [
53]. The remaining studies assessed in this review analyzed the isometric maximal strength without finding effects with a substantial effect size. In many cases, the standard deviation is greater than the effect size, and significant results are extracted in rare cases. Considering that a significant increase of 22% in the (IS) has been reported after a training period combining isometric and dynamic contractions with local EMS [
54], it is expected that with evolution and development of the technology, application protocols of WB-EMS will offer more positive results in the future.
Regarding the effects on blood parameters, none of the studies analyzed in this review reported significant changes after training with WB-EMS with the exception of total cholesterol/HDL-C and creatine kinase activity. Regarding the total cholesterol/HDL-C ratio, Kemmler et al. [
33‐
36] observed a decrease of (− 0.31 (−.15 to −.47),
p = 0.001), but the protein group experienced an even greater decrease (− 0.34 (−.21 to −.47),p = 0.001). Thus, this effect could not be attributed to WB-EMS. With regard to creatine kinase activity in blood, an increasing number of case reports describing situations of rhabdomyolysis with an alarming increase in creatine kinase immediately after exercise with WB-EMS has been reported [
20‐
22]. As Stöllberger C. and Finsterer J. [
43] indicated in their review of the side effects of the WB-EMS, rhabdomyolysis occurred after a first WB-EMS training session in most cases. In addition, the current parameters had been physically demanding, especially regarding intensity [
22] and application time [
21]. These findings indicate that the principle of load progression training was not respected with the completion of a phase of previous adaptation to the current to minimize muscle damage. It has been observed that after four sessions of WB-EMS, creatine kinase activity decreases significantly as a consequence of the adaptation of the muscular system to WB-EMS [
40]. This finding implicitly implies that exercise with WB-EMS should be always performed under the supervision and direction of a technician trained and updated in the advances of this technology to avoid unnecessary risks caused by ignorance and mere lucrative desire.
Following the analysis of existing literature on the issue, it is deduced that the emergence of new studies with rigorous and consistent methodologies and protocols is necessary to shed light on WB-EMS and to objectively prove its effectiveness. In addition, adequate protocols should be established to individualize training with currents and make it a safe practice.