For many years, electromyostimulation (EMS) training has been established as a proven type of training in the areas of rehabilitation and clinical intervention. After injuries, it enables athletes to reduce a decrease in performance and to support the reconstruction of muscles, as well as to create new stimuli in training routine (Seyri and Maffiuletti, 2011). A particular focus is placed on whole-body-EMS (WB-EMS) training: electrodes are attached to the skin all over the body (upper arms, gluteus, chest, abdomen, lower and upper back, thighs, shoulder) and stimulate the muscles underneath them via an externally applied stimulus. The stimulation of the motor units results in signal transduction into the muscle and thus to involuntary muscle contraction. Maximum strength increase is paramount in EMS. In a review, Filipovic et al. (2012) reported that a 3-to-6-week EMS training can result in maximum increases of up to 58.8 % in isometric and 79.5 % in dynamic maximum strength.
Planning and implementing WB-EMS training, however, need to take into account many parameters. For example impulse duration and rest intervals, impulse width, number of training units per week, regeneration times, fatigue, dynamic or static exercises have an essential impact on the effectiveness of WB-EMS (Filipovic et al., 2011; 2012; 2016). Especially the exertion intensity significantly influences the strength development of the target muscles in WB-EMS training (Binder-Macleod and McDermond, 1992). A linear interrelationship seems to exist between impulse intensity and strength development (Maffiuletti, 2010). There is also a positive relationship between a muscle's strength development during training and the resulting strength increases (Binder-Macleod and McDermond, 1992). This means that impulse intensity plays an essential role in terms of strength increase through EMS training. A further factor deemed important due to the associated muscular stimulation method is the stimulation frequency in Hertz (Hz) applied during training. Frequency is the number of impulses per second that reach the muscle via the electrode attached to the skin and trigger a contraction (Bossert et al., 2006; Wenk, 2011). An interrelationship was identified between the frequency applied and the strength development in the muscle (Binder-Macleod and McDermond, 1992). To date, numerous studies have dealt with the application of different frequencies under varied conditions (Amaro-Gahete et al., 2018b; Filipovic et al., 2011; Moreno-Aranda and Seireg, 1981). In the past, the variability of training protocols and experimental implementation often result in difficulties to compare the individual analyses. Meanwhile, a more standardized use of stimulation parameters and protocols over the last years enables a better comparability between different studies (Filipovic et al., 2011; Selkowitz, 1989). Thestimulation frequency in WB-EMS training usually ranges between 20 and 150 Hz (Vatter et al., 2016; Vogelmann, 2013). There is no consensus on the existence of an "optimal" frequency range. In their review, Filipovic et al. state that frequencies around 76 Hz lead to an optimal strength development of the musculature (Filipovic et al., 2011). Frequencies below 50 Hz have only been analyzed to a limited extent so far.
Force development during WB-EMS training positively correlates to strength increases observed in the target muscles, with the optimal choice of protocol having a significant effect on training success (Binder-Macleod and McDermond, 1992). At a frequency of 5 Hz, for example, the muscles completely relax between the contractions, whereas the individual impulses sum up with increasing frequency. This means that the muscle can no longer relax completely with increasing frequency. Summing the individual impulses results in a higher strength development in the muscles and a so-called unfused (incomplete) or fused (complete) tetanic contraction (tetanus). Furthermore, the impulse frequency seems to correlate directly with force development (Glaviano and Saliba, 2016). Bigland-Ritchie et al. (1979) were able to show that muscular stimulation at a frequency of 20 Hz generates only 65 % of the strength compared to stimulation at 50-80 Hz. Kramme (2007) attributes an optimal faradic stimulation of striated musculature to frequencies of 50 Hz (Kramme, 2007), adue to fused tetanus (Wenk, 2011). High frequencies seem to lead to faster neuromuscular fatigue and can therefore result in an earlier decrease of performance (Bigland-Ritchie et al., 1979) because the organism is subject to higher exertion (Glaviano and Saliba, 2016; Gondin et al., 2010). Apart from the strength development generated by the different frequencies, specific frequencies are attributed with an increased stimulation of specific muscle fiber types. Frequencies of 20-40 Hz mostly cause a stimulation of the slow type-I fibers (slow-twitch, ST), whereas stimulation between 50 and 120 Hz rather activate the faster type-II muscle fibers (fast-twitch, FT) (Frenkel et al., 2004; Vogelmann, 2013). For this reason, the use of an "optimal" stimulation frequency has not been unambiguously clarified. Depending on the author, the modes of action and frequency ranges and, accordingly, the applicability differs.
In today's WB-EMS training, it seems to be generally agreed that a stimulation frequency around 85 Hz represents an effective value wherefore it is usually applied most of the time (Berger et al., 2019; Brocherie et al., 2005; Filipovic et al., 2011; Micke et al., 2018). Nevertheless, the authors have no knowledge of scientific evidence pertaining to the direct comparison of the 20 Hz and 85 Hz stimulation frequencies and their impact on specific performance parameters. Therefore, the objective of this study was to determine: a) whether a 10-week EMS training might have an impact on specific sport performance parameters, and b) if any difference might occur between a control group (CON), a training group exercising at 20 Hz (T20), and a group exercising at 85 Hz (T85), under otherwise identical stimulation conditions.
The study was conducted using a randomized controlled trial (RCT). The participants were randomly assigned to one of the three groups (two training groups, one control group; groups were assigned by drawing cards).The training groups completed a 10-week training phase with 1.5 training units per week (Kemmler et al., 2016a; Kemmler et al., 2016b). They only differed due to the stimulation frequency used in WB-EMS (20 Hz or 85 Hz), all other contents and stimulation parameters were identical. CON was instructed not to engage in exercise during the period. Performance parameters (jump-, sprint- and strength parameters) were measured both before and after the intervention. This study design enabled us to compare within and between group differences to identify possible differences in the increase in performance due to the impulse frequency used.
A total of 58 persons participated in the study. Seven people did not complete the 10-week WB-EMS training or the final diagnostics, they were excluded from the data analysis (Figure 1) (Schulz et al., 2010). Thus, a total of 51 participants were included in the analysis. The anthropometric data are shown in Table 1. Inclusion criteria were an age between 18 and 40 years,
Anthropometric and performance parameters were recorded during both pre- and post-tests. Each participant performed a jump session of a total of three different jumps: counter movement jump (CMJ), squat jump (SJ), and drop jump (DJ). For all three types of jumps, proper arm positioning (hands on hips) and leg extension during the jump (no flexing to extend the jump phase, i.e., no skewing the height of the jump) were ensured. The participants could self select how deep they lowered their body (not more than 90[degrees] knee angle). To exclude an eccentric movement in the SJ, the participants had to remain in the reversal point for 2 seconds (Faude et al., 2010). Jump heights and contact times were measured by means of the Optojump Next optical measurement system (Microgate, Bolzano, Italy). For the evaluation of the DJ, a reactive strength index was calculated based on jump height divided by ground contact time.
Linear sprint diagnostics were conducted using the Witty Kit photoelectric sensor system (Microgate, Bolzano, Italy), measuring the linear sprint times over the 5 m, 10 m, and 30 m distances. The start was performed without a signal from a standing position 50 cm away from the first photoelectric sensor. The participants started at their own discretion without the influence of response time (Faude et al., 2010).
Static trunk extension and flexion (isometric strength tests) were measured by Back Check 607 (Dr. Wolff GmbH, Arnsberg, Germany). This required the participants to stand with dangling arms and slightly bent knee joints. They were fixated at the iliac crest area by one dorsal and one ventral pad in the sagittal plane. For measurement recording purposes, two pads with force transducers were placed without pressure at the sternum and between the shoulder blades. The maximum strength was recorded in both directions. The tests were performed three times (30 seconds rest between the tests) with the maximum value being used for analysis (Weissenfels et al., 2019).
All participants were randomly assigned to one of the two training groups or to the control group. Participants and the investigators were not informed about the assignment at any time during the study in order to achieve double blinding. In WB-EMS, the training groups differed only in the stimulation frequencies applied. Participants performed a familiarization session before training began. This session lasted 12 minutes and included a low-intensity impulse familiarization to prepare for the upcoming training sessions and to get to know the WB-EMS...