During each exercise, temporal distribution of contraction modes was standardized per repetition by an acoustical signal at start and end positions of the exercise. The intensity of each exercise set was controlled by Borg rating of perceived exertion (RPE) (Tiggemann et al., 2010). If a set was no longer exhaustive (RPE < 16 "hard"), the impulse intensity was raised for S+E during the training period. S enhanced the intensity for (1) Bulgarian split squat by adding free weights ≤20 kg, for (2) Nordic curl by adding softer rubber bands (gold to

black, Gymstick; Ludwig Artzt GmbH, Dornburg, Germany), and for (4) side abs by adding a heavier medicine ball (Fitness-Mad, Worcestershire, United Kingdom; ≤4 kg). A TRX (Fitness Anywhere, San Francisco, United States) was used as sling system for (3) knee tucks. The intensity could have been increased by stabilizing only one instead of two legs in the TRX. There was no need for this variation during the training period (RPE > 16).

The sessions focusing on sprinting and jumping involved five exercises for both groups: (1) skipping: knee lever runs against a rubber band fixed around the hips, (2) heeling: single leg heels with fore-swinging of the lower leg and active foot attachment when returning the swing leg and hands remaining in the akimbo position, (3) side jumps: single leg side jump from one leg to the other about a 20-cm hurdle, (4) box jumps: single leg box jump after a two-step start on a 38-cm box with 1-m jump distance, as well as (5) drop jumps: drop jumps from a 38-cm box with hands remaining in the akimbo position (exercise characteristics are presented in **Table 3**, pictures of each exercise are presented in the **Supplementary Table S2**). These exercises followed the recommendations of Young (2006), Brughelli et al. (2008), and Stojanovic et al. (2017) ´ to increase SSS and CODS as well as VJ and HJ. The intensity of each sprinting and jumping exercise set was controlled by RPE, too. Both groups enhanced the intensity by a maximum movement frequency or an explosive movement speed. The impulse intensity for S+E had been adjusted at 70% of the iPT.

The warm-up consisted of 5-min cycling before each session. The rest between the exercises was 2.5 min. Thus, the total contact time for S lasted 30 min for the strength sessions as well as 25 min for the jumping and sprinting sessions. S+E had a 7-min (5-min application of the electrodes plus 2-min verification of impulse intensity) longer total contact time for each session.

### Testing Procedure Sprint Testing

Sprint testing involved a T-run for CODS (**Figure 2**) and a 30-m sprint for SSS. Both tests were performed with a selfinitiated standing start with no hopping or backward movement before the start. The split times were measured for 30-m sprint at 5, 10, and 20 m. The split time values for T-run were measured after the first change of direction at cone A (**Figure 2**). Double infrared photoelectric barriers (DLS/F03, Sportronic, Leutenbach-Nellmersbach, Germany) were used to measure time. The sprinting time as the best of two attempts was used for subsequent analysis. The participants had 2-min rest between the trials.

Moreover, a tapping test about 5 s was conducted with the OptoJump system (Microgate, Bolzano, Italy). It is based on measurements of optical light emitting diodes. The participants were instructed to complete as many steps as possible in 5 s and to start on their own. The system automatically counted the total number of steps for 5 s from the first step. The parameter as the best of two trials was total steps (n). The participants had 2 min rest between the trials.

FIGURE 2 | T-run: Cone A, B, and C had to be reached after one another by a forward sprint (1–5). Marks on the floor must be crossed at each corner. After cone C (6), participants could run as fast as possible across the finish line [adapted from McBride et al. (2002) and Reiman and Manske (2009)].

#### Jump Testing

After one familiarization jump trial, the participants performed three trials of each jump variation in a fixed order: (1) standing long jump (SLJ), (2) counter movement jump (CMJ), (3) squat jump (SJ), as well as (4) drop jump (DJ). For the SLJ (1), the participants were instructed to start jumping from an upright standing position, squatting down to an adequate momentum in order to jump as long as possible. The jump distance was measured from the start line to the participants' heel. The attempts were invalid if the participant stepped back or forward after landing. For the CMJ (2), participants were instructed to start jumping from an upright standing position, squatting down to a knee angle of approximately 90◦ in order to jump as high as possible. For the SJ (3), participants were instructed to start jumping from a static semi-squatted position holding the knees at 90◦ without any preliminary movement. The DJ (4) started from a 38-cm box. The participants were instructed to drop down from the box and then to jump as high as possible after a short contact time on the ground. The OptoJump system (Microgate, Bolzano, Italy) was used to verify jump height and contact time using the flight time method. Thereby, hands remained in the akimbo position for the entire movement of each jump to minimize the influence of arm swing. The jump with the greatest height or distance for each variation was subsequently used for analysis. The DJ performance was evaluated by the highest reactive strength index (RSI) (Flanagan and Comyns, 2008; Prieske et al., 2018; Ramirez-Campillo et al., 2018), which was calculated by dividing the jump height by the corresponding ground contact time (Healy et al., 2018).

#### Strength and Power Testing

Strength and power diagnostics took place on the leg curl (LC), the leg extension (LE), and the leg press (LP) machine (Edition-Line, gym80, Gelsenkirchen, Germany). Those were equipped with the digital measurement equipment Digimax (mechaTronic, Hamm, Germany). It enabled the measurement of the peak

force Fmax and the peak power Pmax (5-kN strength sensor type KM1506, distance sensor type S501D, megaTron, Munich, Germany) employing the software IsoTest and DynamicTest 2.0. The sensors were installed in line with the steel belt of the machines that lifts the selected weight plates.

Diagnostic procedures consisted of three isometric trials for each machine. Isometric attempts were conducted at an interior knee angle of 120◦ for LE and LP as well as of 150◦ for LC. The instruction was to press as forcefully and as fast as possible against the fixed lever arm. This enabled to determine knee joint angle-dependent force–time curve during explosive maximum VC and to calculate the parameter Fmax (N) as the isometric peak force. Moreover, diagnostic procedures consisted of six isoinertial trials for LE and LC as well as three isoinertial trials for LP. The isoinertial test attempts were conducted with an additional load (AL). AL was individually calculated as a percentage of Fmax in a further isometric test with the same angle as the starting position of the isoinertial test (LE and LP 90◦ ; LC 170◦ ). Three attempts were conducted with 40% AL for LE and LC as well as three attempts with 60% AL for LP, LE, and LC. Concerning isoinertial tests, the participants were introduced to move the lever arm as forcefully and as fast as possible over the complete concentric range of motion. This enabled to examine knee joint angle-dependent power–load curve during explosive maximum voluntary LE for LP, knee extension for LE, or knee flexion for LC and to calculate the parameter Pmax as the concentric dynamic peak power. The concentric range of motion corresponded to 90– 180◦ for LP and LE as well as to 170–80◦ for LC (interior knee angle). The rest design was 60 s between the trials and 3 min between the strength machines, respectively. The parameters Fmax (N) and Pmax (W) were calculated for statistical analysis and data presentation as the best of three trials.

#### Statistical Analysis

The data of the 22 participants were reported as mean value ± SD. All data were normally distributed for all groups except for Fmax for LC at posttest (p = 0.010), Pmax with 40% AL for LE at retest (p = 0.050), Pmax with 60% AL for LP (p = 0.033), SSS 10-m split time (p = 0.032) and SJ (p = 0.002) at pretest for S+E as well as Pmax with 40% for LC at pretest (p = 0.050) for S, as assessed by Kolmogorov–Smirnov test (p > 0.05). With a closer look at the quantile–quantile plots and according to Berkovits et al. (2000), the data were continued without applying measures. There was homogeneity of the error variances, as assessed by Levene test (p > 0.05). To determine the effect of the training interventions, a separate 3 × 2 (time × group) mixed ANOVA with repeated measures was conducted. ANOVA assumption of homogenous variances was tested using Maulchy test of sphericity. If a violation of Mauchly's test was observed, Greenhouse–Geisser correction was used. Partial eta-square (η 2 p ) values were reported as effect size for significant main effects of the ANOVA with η 2 p ≥ 0.01 indicating small, ≥ 0.059 medium, and ≥ 0.138 large effects (Cohen, 1988). If 3 × 2-mixed ANOVA revealed a significant time or time × group interaction effect on any variable, this effect was further investigated carrying out Bonferroni-corrected post hoc pairwise comparison (p < 0.05). In this context, standardized mean differences (SMD) were calculated between pre-, post-, and retests. Thresholds for small, moderate, and large effects were 0.20, 0.50, and 0.80, respectively (Cohen, 1988). SPSS 25.0 (IBM R , Armonk, NY, United States) was used for all statistical procedures.

Reliability was determined by the coefficient of variation (CV) and the intraclass correlation coefficient (ICC) for Fmax (CV < 8%; ICC 0.95–0.97) and for Pmax (CV < 9%; ICC 0.84–0.97) during a week-long test–retest procedure.

# RESULTS

## Sprint Testing

Sprint values for both groups are provided in **Table 4**. A statistically significant and large interaction between time × group was observed for split time of CODS (p = 0.002; η 2 p = 0.25). Only S showed a significantly higher performance for T-run split time between pre- and retests following post hoc analyses (p = 0.001).

Significant main effects of time were found for SSS for total time (p < 0.001; η 2 p = 0.32), 5-m (p < 0.001; η 2 p = 0.55), 10-m (p < 0.001; η 2 p = 0.48), and 20-m split time (p < 0.001; η 2 p = 0.41) as well as for CODS for total time (p = 0.008; η 2 p = 0.22). Total steps of tapping test showed significant main effects of time (p = 0.005; η 2 p = 0.31), too. Bonferroni-adjusted post hoc analysis revealed a significant improvement for both groups between preand retests, respectively.

#### Jump Testing

Jump values for both groups are provided in **Table 5**. There was a significant and large interaction between time × group for contact time of DJ (p = 0.043; η 2 p = 0.15). Only S showed a significantly higher performance for contact time of DJ between pre- and posttests (p = 0.007) as well as pre- and retests (p = 0.004).

Significant main effects of time were observed for SLJ (p < 0.001; η 2 p = 0.57), SJ (p < 0.001; η 2 p = 0.35) and CMJ (p < 0.001; η 2 p = 0.42), and RSI (p = 0.011; η 2 p = 0.20). Bonferroni-adjusted post hoc analysis revealed a significantly higher SLJ and CMJ performance between pre- and posttests as well as pre- and retests for both groups. Both groups showed a significantly higher SJ performance between pre- and retests and a significantly higher RSI between pre- and posttests in the post hoc comparison.

#### Strength and Power Testing

Strength and power values for both groups are provided in **Tables 6**, **7**.

Significant main effects of time were found for LC for Fmax (p < 0.001; η 2 p = 0.38), Pmax with 40% (p < 0.001; η 2 p = 0.56) and 60% (p < 0.001; η 2 p = 0.42) AL, for LE for Fmax (p < 0.001;

Whole-Body Electromyostimulation in Female Exercisers

fphys-10-00728 June 26, 2019 Time: 15:44 # 7

TABLE 4 | Changes in 30-m linear sprint (LS) and in T-run for total time (TT) and split time (ST) as well as in tapping test for total steps in group S (strength training) and S +E (strength training with superimposed WB-EMS) during pre-, post-, and retests.

Values are presented as mean ± SD. Moderate to large standardized mean differences (SMD) have been highlighted in bold. Significance level for time effects for both groups to pretest was set at p ≤ 0.05 ∗. Significance level for group∗time interaction effects was set at p≤0.05◦ .

TABLE 5 | Changes in standing long jump (SLJ), squat jump (SJ) and counter movement jump (CMJ), as well as in drop jump (DJ) for length, height, contact time, and reactive strength index (RSI) in group S (strength training) and S+E (strength training with superimposed WB-EMS) during pre-, post-, and retests.

Values are presented as mean ± SD. Moderate to large standardized mean differences (SMD) have been highlighted in bold. Significance level for time effects for both groups to pretest was set at p ≤ 0.05∗. Significance level for group∗time interaction effects was set at p≤0.05◦.

TABLE 6 | Changes in maximal strength (Fmax) and power (Pmax) with 40 and 60% additional load for leg curl (LC) and leg extension (LE) in group S (strength training) and S+E (strength training with superimposed WB-EMS)duringpre-,post-,andretests.

Values are presentedas mean±SD. Moderate to large standardized mean differences (SMD) have been highlighted in bold. Significance level for time effects for both groups to pretest was set at p≤0.05∗.

Dörmann et al. Whole-Body Electromyostimulation in Female Exercisers

η 2 p = 0.48) and Pmax with 40% AL (p < 0.001; η 2 p = 0.38) as well as for LP for Pmax with 60% AL (p = 0.010; η 2 p = 0.20). The significantly higher performances were shown for both groups for LE and LC between pre- and posttests as well as pre- and retests in the post hoc comparison. The significant increases for LP exclusively occurred between pre- and retests.

#### DISCUSSION

This study compared the effects of short-term strength training with and without superimposed WB-EMS on (1) SSS and CODS, on VJ and HJ, as well as on (2) strength and power parameters in female strength trained sport students. It was hypothesized that short-term strength training with submaximal superimposed WB-EMS improves physical fitness in physically active females more than short-term strength training without superimposed WB-EMS.

There is a lack of studies dealing with submaximal superimposed WB-EMS on sprinting and jumping performance, especially using dynamic strength exercises in combination with sprinting and jumping exercises. Moreover, there are no available WB-EMS studies in female athletes.

Against the hypothesis, the findings of this study indicated no advantageous effects for short-term strength training in favor to submaximal superimposed WB-EMS (S+E) in comparison with strength training alone (S) on physical fitness in physically active females. Both groups, S as well as S+E, significantly increased the parameters Fmax for LC and LE as well as Pmax for LC, LE, and LP over time. Moreover, both groups transferred these strength and power gains into a significantly greater performance of the primary endpoints like total time of CODS, split and total time of SSS, as well as VJ height, RSI, and HJ length over time. Thus, both training methods, S and S+E, confirmed the results of existing meta-analyses in this context (Perez-Gomez and Calbet, 2013; Seitz et al., 2014). According to Young (2006) and Seitz et al. (2014), the transfer of the strength increases (S: 19.0%; S+E: 18.2%) into jumping performance (S: 11.7%; S+E: 13.2%) was higher than into sprinting performance (S: 2.8%; S+E: 2.3%), too. Moreover, the findings of S+E are in line with the two dynamic WB-EMS studies with males. They improved sprint time with change of directions by 5.5% at 15 m (Filipovic et al., 2016) and 2.4% at 30 m (Wirtz et al., 2016) as well as SJ performance by 8.1% (Filipovic et al., 2016) and 8.7% (Wirtz et al., 2016), respectively. Following Kots and Chwilon (1971), a higher number of motor units are recruited during exercises with superimposed EMS in comparison with dynamic VC alone. Moreover, EMS increases the activation levels at different muscle length and during different contraction modes, especially during eccentric work phases as Willoughby and Simpson (1998) hypothesized. According to Paillard (2018), EMS superimposed onto VCs in a submaximal task could result in greater muscle fibers recruitment than with voluntary stimulation alone. However, no greater gains of motor output could be generated in comparison with the present strength training regime (S) after the short-term training period, in particular for DJ ground contact time and CODS split time.

fphys-10-00728 June 26, 2019 Time: 15:44 # 10

In contrast, the main findings of this study revealed a significant time × group interaction effect on split time of CODS and contact time of DJ for S in post hoc analysis. The split time significantly decreased at 5 m for S (4.2%) in comparison with S+E (0.4%) between pre- and retests (p < 0.002) and the contact time of DJ for S (11.6%; 10.6%) in comparison with S+E (5.7%; 0.0%) between pre-, post-, and retests (p < 0.043) in post hoc analysis. The two available WB-EMS studies investigated CODS at 15 m (Filipovic et al., 2016) and at 30 m (Wirtz et al., 2016) but no split time at 5 m. Moreover, Filipovic et al. (2016) analyzed RSI with positive effects for the WB-EMS intervention group but no height or contact time of DJ performance. Therefore, the present result cannot be placed in a larger context of WB-EMS. However, these initial results offer a practical recommendation. Concerning a large effect size (SMD = 1.21) for CODS at 5 m and a moderate to large effect size (SMD = 0.70–0.90) for contact time of DJ, dynamic strength training combined with jumping and sprinting exercises without superimposed WB-EMS (S) is currently to be preferred for physically active females. Perez-Gomez and Calbet (2013) reviewed training methods to improve vertical jump performance. In comparison with local EMS, vibration training or strength training alone, the combination of strength and plyometric training seemed to be the most effective method. It likely took advantage of the enhancement of maximal dynamic force through strength training and the positive effects of ploymetric training on speed and force of muscle contraction through its specific effect on type II fibers and high-threshold alpha-motoneurones. However, the present S+E intervention showed a moderate effect size (SMD) for CODS at 5 m (0.50) and for contact time of DJ (0.63) at posttest. As mentioned before, both interventions, S and S+E, significantly improved the overall performance like the total time of CODS at 15 m and RSI of DJ as well as the height of CMJ and SJ. On the one hand, these results do not suggest negative consequences by the artificial muscle activation by submaximal WB-EMS. EMS does not facilitate learning the specific coordination of complex movement like jumping or sprinting, especially during maximal local isometric stimulation (Perez-Gomez and Calbet, 2013; Paillard, 2018). According to Babault et al. (2007) and Bezerra et al. (2011), a low voluntary movement control exists at maximum stimulation intensities, and only submaximal contractions enable an efficient movement control with superimposed EMS. Perez-Gomez and Calbet (2013) also recommend applying EMS concomitantly with plyometric training or practice of sports. This confirms the approach of the present study by a submaximal WB-EMS superimposed on strength, sprinting, and jumping exercises. On the other hand, the observation of Bezerra et al. (2009) could not be confirmed. VC + EMS caused no additional training effect compared with VC training. They hypothesized that it would activate the same neural pathways that are normally used in voluntary exercise, with additional afferent inputs (centrally integrated) provoked by the electrostimulation.

With a closer look at the results of SSS and the training method EMS, the present dynamic strength training intervention with sprinting and jumping exercises superimposed by submaximal WB-EMS significantly decreased total time (2.3%) of a 30-m linear sprint as well as 5-m (6.3%), 10-m (3.7%), and 20-m (2.9%) split time between pre-, post-, and retests. So far, SSS performance over distances ≥ 30 m could not be improved, neither by the two WB-EMS studies (Filipovic et al., 2016; Wirtz et al., 2016) nor by local EMS studies in male athletes (Herrero et al., 2006, 2010a; Babault et al., 2007; Billot et al., 2010). The diagnostics of force and power parameters as secondary endpoints of this study supported the results of SSS. Superimposed WB-EMS during the Bulgarian split squat and the Nordic curl resulted in increases of 18.2% of biceps femoris force to 12.6% of quadriceps femoris force. According to Mero et al. (1992) and Delecluse (1997), biceps femoris showed the highest EMG activation during the maximum speed of SSS and quadriceps femoris showed the highest EMG activation during the acceleration phase of SSS. Ebben et al. (2009) demonstrated a < 0.5 EMG activation ratio between biceps femoris and quadriceps femoris for the exercises squat and split squat. The 2 dynamic WB-EMS studies used the squat exercise as strength training intervention. Wirtz et al. (2016) applied ALs during superimposed WB-EMS. They showed increases of 8% of biceps femoris force, but they enhanced quadriceps femoris force by 28.6% without a transfer to SSS. Filipovic et al. (2016) used an explosive movement velocity during superimposed WB-EMS. They showed a significant decrease for 5-m split time after 7 weeks. Thus, the acceleration phase was influenced by an improved quadriceps femoris force of 19.9% but not the maximum speed phase of SSS. Moreover, Herrero et al. (2010a) who applied local EMS at quadriceps femoris during knee extension showed no effects on 20-m sprint time, too. This exercise reached 8.6% of maximum VC of biceps femoris instead of 86.9% of quadriceps femoris during EMG analysis (Ebben et al., 2009). On the contrary, three local EMS studies showed positive effects on SSS over distances ≤20 m (0.8–2.3%) (Herrero et al., 2006, 2010b; Voelzke et al., 2012). They used isometric EMS of quadriceps femoris in combination with plyometrics. Plyometrics reached a higher biceps femoris to quadriceps femoris activation ratio of 1.01 in precontact and 0.55 in post-contact (Ebben et al., 2010). Thus, a strength exercise selection superimposed by WB-EMS with a high biceps femoris activation and a positive activation ratio to quadriceps femoris like the Nordic curl appear reasonable to enhance SSS, especially over distances ≥ 30 m. Moreover, in contrast to CODS, SJ or CMJ performance, additional jumping and sprinting exercises or an explosive movement velocity during strength exercises seem to be crucial in the application of EMS or WB-EMS to improve SSS.

Some limitations of the present study have to be mentioned for further research on WB-EMS. Six dropouts, three in each group without a coherent statement of reasons, occurred. Thus, it seemed to be independent of the intervention with or without WB-EMS. The final 22 participants, sufficient according to the a priori power analyses, improved strength, power, and jumping performance without a detraining phase. This indicated that strength trained female sport students were able to cope with the physical requirements. In particular, the WB-EMS training at 70% iPT seemed to be a beneficial compromise to achieve strength and power adaptations as well as to have an appropriate exertional tolerance. A detraining period of 2 to 6 weeks is common in several EMS studies (Filipovic et al., 2011). However, this should be further verified with additional

physiological parameters like creatine kinase, questionnaires about physical habits, and recovery–stress states. Moreover, a control, blinding, or placebo intervention group could verify the attribution of environmental influences, expectations, or learning effects on performance gains. With reference to Sommi et al. (2018), a broad acceptance of superimposed WB-EMS by female athletes is decisive for further development of athletic programs. Due to the lack of WB-EMS studies in female athletes, the results of this study were often compared with studies in male athletes.  However, the subjective tolerance to current intensity remains a key limiting factor of EMS, regardless of sexes (Reed, 1997).

Finally, the conclusion of our investigation is that superimposed submaximal WB-EMS during dynamic strength, sprinting, and jumping exercises could serve as a reasonable but not superior alternative to classic training regimes to improve CODS and SSS, VJ and HJ, as well as strength and power parameters in physically active female. The present WB-EMS approach at 70% of iPT seems to intensify dynamic strength exercises equal but not higher than ALs corresponding to the 8 to 10 repetition maximum. Thus, against the hypothesis, it leads to comparable but no greater improvements in physical fitness. Therefore, it remains to be considered whether the effort of a submaximal superimposed WB-EMS short-term intervention is remunerative. Whether it provides perspectives for female athletes with little experiences or insufficient technique to incorporate strength routines without using moderate to high AL, which is necessary to improve sprinting performance or to provide injury prevention and joint stability, has to be verified in further studies. Moreover, training regimes concentrating on contact time of DJ or CODS at 5 m should be executed without superimposed WB-EMS in physically active females concerning to the present results and has to be verified in further WB-EMS studies, too. Additionally, an improvement of SSS performance over a distance of ≥30 m

### REFERENCES

occurs for the first time for superimposed local EMS or WB-EMS. To improve SSS at maximum speed by superimposed WB-EMS, our results offer a combination of jumping and sprinting exercises with strength exercises that have a high biceps femoris activation and a positive activation ratio to quadriceps femoris. In this context, a higher transferability of physically active females than males and the adaptations of WB-EMS over time need to be further researched, as well as concepts for periodization in high-performance sports need to be developed.

# ETHICS STATEMENT

The study protocol was approved by the "Ethics Committee of the German Sport University Cologne" in December 2013 and complied with the Declaration of Helsinki.

# AUTHOR CONTRIBUTIONS

UD, NW, and FM conceived and designed the research. UD, NW, FM, and MM conducted the experiments. UD analyzed the data and wrote the manuscript. HK, NW, FM, and LD revised the manuscript. All authors read and approved the manuscript.

## ACKNOWLEDGMENTS

The authors would like to acknowledge the time and effort of Nicole Hiermayr, Jens Müller, and Christopher Born as well as of all the participants involved in this investigation.

### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fphys. 2019.00728/full#supplementary-material

oxygen consumption in patients with chronic heart failure: results from the exEMS study comparing different stimulation strategies. Clin. Res. Cardiol. 102, 523–534. doi: 10.1007/s00392-013-0562-5

Young, W. B. (2006). Transfer of strength and power training to sports performance. Int. J. Sports Physiol. Perform. 1, 74–83.

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Dörmann, Wirtz, Micke, Morat, Kleinöder and Donath. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Adjustment Effects of Maximum Intensity Tolerance During Whole-Body Electromyostimulation Training

*Joshua Berger1 \*, Stephan Becker1 , Marco Backfisch1 , Christoph Eifler 2 , Wolfgang Kemmler 3 and Michael Fröhlich1*

*1 Department of Sports Science, Technische Universität Kaiserslautern, Kaiserslautern, Germany, 2 Department of Applied Training Science, German University of Applied Sciences for Prevention and Health Management (DHfPG), Saarbrücken, Germany, 3 Institute of Medical Physics, Friedrich Alexander University of Erlangen-Nürnberg, Erlangen, Germany*

#### *Edited by:*

*Shi Zhou, Southern Cross University, Australia*

#### *Reviewed by:*

*Francisco J. Amaro-Gahete, University of Granada, Spain Luis Manuel Rama, University of Coimbra, Portugal Conor M. Minogue, University College Dublin, Ireland*

#### *\*Correspondence:*

*Joshua Berger<EMAIL_ADDRESS>
#### *Specialty section:*

*This article was submitted to Exercise Physiology, a section of the journal Frontiers in Physiology*

*Received: 21 January 2019 Accepted: 08 July 2019 Published: 24 July 2019*

#### *Citation:*

*Berger J, Becker S, Backfisch M, Eifler C, Kemmler W and Fröhlich M (2019) Adjustment Effects of Maximum Intensity Tolerance During Whole-Body Electromyostimulation Training. Front. Physiol. 10:920. doi: 10.3389/fphys.2019.00920*

Intensity regulation during whole-body electromyostimulation (WB-EMS) training is mostly controlled by subjective scales such as CR-10 Borg scale. To determine objective training intensities derived from a maximum as it is used in conventional strength training using the one-repetition-maximum (1-RM), a comparable maximum in WB-EMS is necessary. Therefore, the aim of this study was to examine, if there is an individual maximum intensity tolerance plateau after multiple consecutive EMS application sessions. A total of 52 subjects (24.1 ± 3.2 years; 76.8 ± 11.1 kg; 1.77 ± 0.09 m) participated in the longitudinal, observational study (38 males, 14 females). Each participant carried out four consecutive maximal EMS applications (T1–T4) separated by 1 week. All muscle groups were stimulated successively until their individual maximum and combined to a whole-body stimulation index to carry out a possible statement for the development of the maximum intensity tolerance of the whole body. There was a significant main effect between the measurement times for all participants (*p* < 0.001; 2 = 0.39) as well as gender specific for males (*p* = 0.001; 2 = 0.18) and females (*p* < 0.001; 2 = 0.57). There were no interaction effects of gender × measurement time (*p* = 0.394). The maximum intensity tolerance increased significantly from T1 to T2 (*p* = 0.001) and T2 to T3 (*p* < 0.001). There was no significant difference between T3 and T4 (*p* = 1.0). These results indicate that there is an adjustment of the individual maximum intensity tolerance to a WB-EMS training after three consecutive tests. Therefore, there is a need of several habituation units comparable to the identification of the individual 1-RM in conventional strength training. Further research should focus on an objective intensity-specific regulation of the WB-EMS based on the individual maximum intensity tolerance to characterize different training areas and therefore generate specific adaptations to a WB-EMS training compared to conventional strength training methods.

Keywords: adjustment effects, familiarization, intensity tolerance plateau, specific adaptations, whole-body electromyostimulation

# INTRODUCTION

Electromyostimulation training (EMS training) has been used since the early 1970s as a high-intensity training technology in high-performance sports, physical therapy, and rehabilitation (Selkowitz, 1985, 1989; Duchateau and Hainaut, 1988; Binder-MacLeod and McDermond, 1992; Filipovic et al., 2011, 2012; Kemmler et al., 2018). Electrodes attached to the skin lead to involuntary contraction of the muscles underneath the electrodes while in conventional strength training, voluntary muscle contraction against a resistance takes place. In conventional strength training, differentiated intensities are derived to control training and generate specific adaptations of the musculature to athletic exercise. These intensities are often based on the one-repetition-maximum (1-RM). The 1-RM has been established as a simple, economic, and adequately valid option to determine training intensity levels. It is considered the gold standard of muscle strength evaluation in non-laboratory conditions (LeSuer et al., 1997; Ritti-Dias et al., 2005; Levinger et al., 2009; Kenney et al., 2015). Due to the complexity of movement and neuronal adaptation mechanisms, strength training beginners are often not able to exploit their absolute maximum capabilities the first time they exercise. Only after repeated units, the personal best may be reached at all (Rutherford and Jones, 1986; Mayhew et al., 1989; Braith et al., 1993; Ritti-Dias et al., 2005, 2011; Wirth et al., 2012). This means that athletes require consecutive stimuli until they are able to reach a certain plateau or their individual maximum. This individual learning and familiarization phase is said to be unexplored in EMS training. Furthermore, there is insufficient knowledge in the fields of deriving percentage training intensities and determining individual maximum intensity tolerances or an equivalent to the 1-RM.

The individual training fields for EMS training are usually specified by assessing the degree of perceived exertion based on the Borg RPE (rating of perceived exertion) scale (Borg, 1998; Kemmler et al., 2012, 2016b,c,d; Kemmler and von Stengel, 2012; Amaro-Gahete et al., 2018). To date, objectified training intensities have not been measured. In their work, Alon and Smith already examined the adaptation to an EMS application after several consecutive sessions and found that the maximum intensity tolerance continuously increases (Alon and Smith, 2005). However, they focused on one muscle only (m. quadriceps femoris) and the combination of an EMS application and maximum voluntary isometric contraction (MVC). Familiarization and adaptation effects regarding a whole-body EMS training have not been identified yet. Training guidelines recommend an 8- to 10-week adaptation phase in order to avoid unwanted metabolic effects. This, however, does not allow any statement on the individual maximum intensity tolerance (Kemmler et al., 2016a).

Therefore, the aim of this study is to determine whether a maximum intensity tolerance plateau occurs after multiple consecutive whole-body electromyostimulation (WB-EMS) application sessions.

# MATERIALS AND METHODS

#### Study Design and Subjects

A total of 59 test subjects participated in the longitudinal, observational analysis in a panel design. Four consecutive tests were performed at the same time of day with a 1-week interval. Due to health complications and further personal reasons (not related to the WB-EMS application), seven persons did not perform four consecutive tests, so that in the end, 38 male and 14 female participants (*n* = 52) were entered in the study. Participants' characteristics are shown in **Table 1**.

All participants were EMS beginners, recruited via e-mail distribution lists, flyers, and personal contact. Exclusion criteria were acute or chronic diseases, infections or limitations to the musculoskeletal system, EMS experience in the past, and open skin lesions, which would have inhibited EMS application. The subjects had to be between 18 and 40 years of age with a BMI < 30 kg/m2 , and they were not allowed to take any medication that might have affected the examination (pain medication, beta blockers, etc.). Before study start, a detailed anamnesis questionnaire had to be completed in order to exclude any relative and/or absolute contraindications for EMS training (Vatter et al., 2016; Kemmler et al., 2016a). Furthermore, the course of the study, the testing design, and potential risks were explained in detail. Before entering the study, all participants provided written informed consent to the experimental procedure and written consent pertaining to data use. The study was conducted based on the current Declaration of Helsinki guidelines (World Medical Association, 2013) and approved by the ethics commission of the German University of Applied Sciences for Prevention and Health Management (project number 02/17).

#### Materials

The WB-EMS application was carried out with the miha bodytec 2 EMS device from Miha Bodytec. The device includes one main controller to manage the possible maximal output of the device and 10 subcontrollers to dose the specific muscle groups (**Figure 1**). In this investigation, we just used the eight prepared muscle groups with no additional muscle groups using the canals 9 and 10. The muscle groups were stimulated using an electrode vest including five pairs of electrodes (lower back, latissimus, upper back, abdomen, and chest), a hip belt to stimulate the glutaeus muscle and one

TABLE 1 | Anthropometric data for the entire group (*n* = 52) and the gender subgroups, shown in mean value ± standard deviation; BMI = body mass index.

belt pair each for the thighs and the upper arms. To avoid the direct contact of the electrodes to the skin, a special functional EMS lingerie was used. All the electrodes (vest and belts) were moistened before use to guarantee a better conductivity from the electrodes to the participants' body. The EMS application and the way of using the workout clothes were in line with the manufacturer's instructions (Miha Bodytec, Augsburg, Germany). Body fat was determined using the Tanita BC-418 body composition analyzer with the software GMON v.3.2.3. 10. The participants were instructed to drink 1 litre of water up to 60 min before the bioimpedance analysis to reduce the variability of the results.

#### Tests

The tests were conducted based on common EMS application stimulation parameters, i.e., a frequency of 85 Hz, an impulse width of 350 μs, and a bipolar impulse without impulse increase (rectangular impulse) with interchanging 4-s load and 4-s break intervals (Filipovic et al., 2011; Kemmler et al., 2012, 2018).

Each muscle group was strained in intervals up to their individual, subjective maximum. The maximum was determined by the point at which the test person gave the signal to stop the strain due to the highest degree of strain that could be endured. The last stimulation value output by the EMS device was therefore recorded as the maximum intensity tolerance. This is, however, just a numerical value that cannot be equated with a specific value in milliamperes (mA). All maximum intensity tolerances refer to the stimulation of a specific muscle. The device was set up so that a maximum output and thus maximum device load were possible [maximal value of the main level controller (99)]. All muscle groups were strained consecutively in this way. During the tests, the test persons were never informed about the stimulation value or the maximum intensity tolerances of any of the previous tests. This resulted in a maximum intensity tolerance range of 0–99 (device-specific unit, 0–75 mA) for each muscle group. For a better understanding, **Figure 1** shows the miha bodytec 2 device including the main level controller as well as the specific muscle groups. The individual muscle groups were combined into an unweighted, additive index in order to come to a statement on the development of the entire body and not only a specific muscle group. After a preceding impulse adaptation, each test person took part in four consecutive tests in 1-week intervals. Four tests were selected for better comparability with the approach to the determination of 1-RM in conventional strength training. A separation of 1 week was used to guarantee complete regeneration after each session. In order to exclude time-of-day effects, the tests were performed at the same time of day for each test person. In addition, the participants were always examined by the identical research staff. Before each test, a new anamnesis questionnaire on the current condition was completed in order to exclude spontaneously occurring contraindications (intake of pain medication or alcohol, muscle aches, liquid receptive before the training, etc.).

#### Statistics

Sample size was calculated using G\*power <IP_ADDRESS> (university of Düsseldorf, Germany). With an effect size of 0.25, total sample size was calculated by 36 participants (Faul et al., 2007). With regard to possible drop-outs, more participants were admitted to the study. Statistical evaluation and graphics generation were executed using IBM SPSS (SPSS Version 25.0, Chicago, IL, USA). The normal distribution was verified by means of the Shapiro-Wilk test. Because of this criteria, to check the maximum intensity tolerance index development, the four test times T1–T4 were analyzed by means of

Berger et al. Adjustment Effects During WB-EMS Training

repeated-measures analysis of variance. Since ANOVA is known to be robust when it comes to infringements of the normal distribution, it was applied every time (Schmider et al., 2010). *Post hoc* comparisons including Bonferroni correction were performed to detect differences between the measurement times. As the Mauchly test indicated a violation of sphericity, the degrees of freedom corrected according to Huynh-Feldt were used for the further calculation of *p* [because epsilon by Greenhouse Geiser >0.75 (Girden, 1992; Field, 2009)]. Furthermore, the effect size 2 was calculated (Cohen, 1988). The significance level was set to *p* < 0.05.

#### RESULTS

Since the device-specific maximum was reached for eight test persons during the four test dates, the number of valid data records was reduced to 44. **Table 2** represents the index values of the individual test days. Between the measurement times, a significant main effect *F* (2.46; 105.96) = 26.95, *p* < 0.001, 2 = 0.39 was identified, which categorizes the effect size as large according to Cohen. To differentiate between genders, the degrees of freedom corrected according to Greenhouse Geiser were applied. For the male test persons, a significant main effect was identified *F* (2.21; 77.41) = 7.66, *p* = 0.001, 2 = 0.18. Also for the female test persons, a significant main effect was found between the measurement times with *F* (1.84; 22.07) = 15.82, *p* < 0.001, 2 = 0.57. Significant main effects were also identified pertaining to the gender factor: [*F* (1; 45) = 12.09, *p* = 0.001, 2 = 0.21]. There were no significant interaction effects for gender multiplied by measurement time [*F* (3; 135) =1.00, *p* = 0.394, 2 = 0.02].

There were significant differences between the measurement times T1 to T2 and T2 to T3. There were no significant differences between T3 and T4. The maximum intensity tolerance index values for the various measurement times (including confidence interval for men and women) as well as the percentage increases over the measurement times are included in **Tables 2** and **3**. For improved understanding, the boxplots for all test person data and gender specifics are shown in **Figure 2**.

### DISCUSSION

The results of this study show that a plateau develops after the third test session of consecutive measurements of the maximum intensity tolerance. A possible reason could be the habituation to the external electrical stimulation on the one hand and the muscular-coordinative adaptations on the intramuscular level on the other hand. Also neuronal adaptation caused by the maximal stimulation could be important. Therefore, the maximum intensity tolerance should be determined based on several consecutive test units in order to exclude the named influencing factors in the interpretation of results. This approach seems to be similar to that pertaining to the determination of the 1-RM, for which a certain number of familiarization and adaptation sessions is required to reach the individual maximum (Reynolds et al., 2006; Ritti-Dias et al., 2011). In conventional strength training, usually 2 weeks of learning and adaptation training with four measurement sessions precede the actual 1-RM determination so that the training intensity can be derived based on the 1-RM. Similarly, the determination of the individual maximum intensity tolerance through preceding adaptation sessions could serve as a preparation for the wholebody EMS training (Ritti-Dias et al., 2011). So far, one habituation session to an EMS training is recommended, which seems not sufficient to account all habituation and learning effects to a maximaum electrical intensity.

Previous analyses did not identify any intensity tolerance adaption in consecutive tests. For example, Alon and Smith (2005) conducted six NMES (neuromuscular electrical stimulation) sessions with 21 test persons within 2 weeks to see whether an adaptation of the maximum intensity tolerance would occur. During these examinations, they did not identify any adaptations. This could have been due to the isolated stimulation of the right-hand side m. quadriceps femoris in contrast to our whole-body index of maximum intensity tolerance (Alon and Smith, 2005). It remains to be clarified with more test persons whether there is a difference between beginners and experienced subjects at the point of plateau like in the 1-RM bench pressing and squats tests by Ritti-Dias et al. (2011). First studies by Hortobágyi et al. (1992) with 12 test persons point to a difference in maximum intensity tolerance between trained and untrained subjects. The trained persons exhibited a strength value for the m. biceps brachii that was up to 29% higher than that of the untrained participants. With 31.3 mA measured for the trained and 21.9 mA for the untrained subjects, the tolerated intensity strength was clearly higher in the trained subjects (Hortobágyi et al., 1992). The difference of the sensory threshold between males and females, i.e., the response to

TABLE 2 | Maximum intensity tolerance as mean value ± standard deviation and 95% confidence interval [CI] of the measuring times T1–T4 for the entire collective and men and women.

Berger et al. Adjustment Effects During WB-EMS Training

an external impulse, also seems to have been confirmed in previous tests. Maffiuletti et al. (2008) analyzed this with the help of 20 men and 20 women. They identified gender differences at the sensory threshold: the values of the female test persons were 41% lower than those of the male test persons (Maffiuletti et al., 2008). This is a result that we were able to corroborate based on the gender-specific differences in the maximum intensity tolerance (men were 13% higher than women). The results by Alon and Smith are thus confirmed. They found a significantly higher intensity tolerance in males (31.5 ± 8.6 μC) than in females (16.9 ± 8.0 μC). However, the percentage increase between the first and last unit conducted turned out to be mostly identical (male: 47.2%; female: 50.8%) (Alon and Smith, 2005). Similar genderspecific differences were observed by Fehr in pilot studies, with a difference of the maximum intensity tolerance of 35.9 mA in males and 22.3 mA in females (Fehr, 2011). These studies support our results because the intensity tolerance when developing the plateau is also higher in males in our tests. The course beyond the testing times, however, does not seem to be subject to any gender-specific differentiation.

TABLE 3 | Comparison of pairs of testing sessions including significance and increase in %.

In order to be able to perform the training studies with the appropriate intensity in the future and to derive conclusions from conventional strength training for EMS, it may be useful to specify a 1-RM equivalent to establish objective intensity ranges. The key question as to the extent to which percentage values of the individual intensity tolerances actually represent individual training areas such as maximum strength training, muscle hypertrophy, or rather strength-endurance training also remains to be clarified in future studies. Since strain intensity (i.e., the intensity in WB-EMS training) is just a control parameter for training adaptations, the methodology to be applied to WB-EMS training itself still needs to be defined. Due to a methodology's essential influence on intensity perception and perceived exertion, it may actually represent a key factor in the establishment of training intensity based on its individual maximum.

### CONCLUSION

In summary, after multiple consecutive EMS application sessions with EMS beginners, a maximum intensity tolerance plateau was reached after three adaptation sessions. Therefore, future studies should plan for at least three adaptation sessions preceding the actual whole-body EMS training. We recommend that follow-up studies take the maximum intensity tolerance findings as a basis for defining percentage training areas in order to enable objective training control in the future. It is also recommended that the increase of the individual maximum intensity tolerance over a prolonged period of time and its potential changes caused by training adaptation should

be verified. Also at a sub-maximum level, training intensity development should be observed in order to be able to derive relevant conclusions. Even if intensity is only one of several measures of training control in EMS training, it could be a key value to define training intensity, similar to 1-RM.

#### ETHICS STATEMENT

This study was carried out in accordance with the recommendations of the Declaration of Helsinki with written informed consent from all subjects. The protocol was approved by the ethics commission of the German University of Applied Sciences for Prevention and Health Management (project number 02/17).

# REFERENCES

#### AUTHOR CONTRIBUTIONS

JB, SB, and MB participated in the design of the study, carried out the experiments, and performed data analyses. JB wrote the manuscript. MF and SB helped write the manuscript and supervised statistical analysis. MF, CE, and WK contributed to the study design and supervised the entire project. All authors read and approved the final manuscript.

### ACKNOWLEDGMENTS

The authors wish to thank their students for their help with data acquisition and Monika Schutz for translating the manuscript.

maximum test. *J. Strength Cond. Res.* 25, 1418–1422. doi: 10.1519/ JSC.0b013e3181d67c4b

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Copyright © 2019 Berger, Becker, Backfisch, Eifler, Kemmler and Fröhlich. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

# Effects of Whole-Body Electromyostimulation on the Energy-Restriction-Induced Reduction of Muscle Mass During Intended Weight Loss

*Sebastian Willert1 \* , Anja Weissenfels1 , Matthias Kohl2 , Simon von Stengel1 , Michael Fröhlich3 , Heinz Kleinöder4 , Daniel Schöne1 , Marc Teschler5 and Wolfgang Kemmler1*

*1 Institute of Medical Physics, Friedrich-Alexander University of Erlangen-Nürnberg, Erlangen, Germany, 2 Department of Medical and Life Sciences, University of Furtwangen, Villingen-Schwenningen, Germany, 3 Department of Sports Science, University of Kaiserslautern, Kaiserslautern, Germany, 4 Institute of Training Science and Sport Informatics, German Sport University Cologne, Cologne, Germany, 5 Institute of Rehabilitation Sciences, University of Witten/Herdecke, Witten, Germany*

#### *Edited by:*

*Yurdagül Zopf, University Hospital Erlangen, Germany*

#### *Reviewed by:*

*Giovanni Messina, University of Foggia, Italy Emiliano Cè, University of Milan, Italy*

*\*Correspondence: Sebastian Willert<EMAIL_ADDRESS>
#### *Specialty section:*

*This article was submitted to Exercise Physiology, a section of the journal Frontiers in Physiology*

*Received: 25 February 2019 Accepted: 23 July 2019 Published: 12 August 2019*

#### *Citation:*

*Willert S, Weissenfels A, Kohl M, von Stengel S, Fröhlich M, Kleinöder H, Schöne D, Teschler M and Kemmler W (2019) Effects of Whole-Body Electromyostimulation on the Energy-Restriction-Induced Reduction of Muscle Mass During Intended Weight Loss. Front. Physiol. 10:1012. doi: 10.3389/fphys.2019.01012*

Purpose: Overweight and obesity are an increasing problem worldwide. However, most studies that focus on weight reduction by energy restriction and/or aerobic exercise reported considerable loss of muscle mass as well. Increased protein intake and/or resistance exercise might inhibit this detrimental effect during a negative energy balance. Whole-body electromyostimulation (WB-EMS), a time effective, joint-friendly, and highly customizable training technology, showed similar hypertrophic effects compared with high-intensity resistance training. The aim of this study is to evaluate the effect of WB-EMS on body composition during negative energy balance with maintained/increased protein intake in overweight premenopausal women.

Patients and Methods: Ninety premenopausal, 25–50-year-old, overweight women were randomly assigned to three groups (*n* = 30 each). (1) Negative energy balance (−500 kcal/day) by energy restriction with compensatory protein intake (CG). (2) Negative energy balance (−500 kcal/day) by energy restriction (−250 kcal/day) and increased physical activity (−250 kcal/day) with increased protein intake (PA). (3) Negative energy balance (−500 kcal/day) due to energy restriction and increased physical activity with increased protein intake plus WB-EMS. The duration of the intervention was 16 weeks. Participants underwent restrictions in kcal per days and supplementation of protein (CG: 1.2 or PA/WB-EMS: 1.7 g/kg body mass/day) where needed. Bipolar WB-EMS was applied 1.5× week for 20 min (85 Hz; 350 μs; intermittent 6 s impulse, 4 s rest; rectangular). The primary study endpoint "lean body mass" (LBM) and secondary endpoint body fat mass (BFM) were assessed by bio-impedance analysis (BIA).

Results: LBM decreased in the CG and PA group (CG: −113 ± 1,872 g; PA: −391 ± 1,832 g) but increased in the WB-EMS group (387 ± 1,769 g). However, changes were not significant (*p* > 0.05). Comparing the groups by ANOVA, no significant differences were observed (*p* = 0.070). However, pairwise adjusted comparisons determined significant differences between WB-EMS and PA (*p* = 0.049). BFM decreased significantly ( *p* < 0.001) in all groups (CG: −2,174 ± 4,331 g; PA: −3,743 ± 4,237 g; WB-EMS: −3,278 ± 4,023 g) without any significant difference between the groups (ANOVA: *p* = 0.131).

Conclusion: WB-EMS is an efficient, joint-friendly, and highly customizable training technology for maintaining muscle mass during energy restriction and can thus be considered as an alternative to more demanding resistance exercise protocols.

Keywords: electromyostimulation, energy restriction, weight loss, lean body mass, body composition, protein supplementation

#### INTRODUCTION

Overweight and obesity represent an increasing problem worldwide. According to newer statistics, more than 2.1 billion people worldwide are overweight or even obese (Smith and Smith, 2016). This development is of course primarily a health problem for the person affected: overweight and obesity are crucial risk factors for hypertension, fat metabolism disorders, diabetes mellitus type 2, cardiovascular diseases, and some types of cancer (Wilson et al., 2002; Villareal et al., 2005; Renehan et al., 2008). Further, the direct and indirect health costs for the treatment of secondary diseases, for example, and the higher risk of inability to work constitute an immense burden for the healthcare system (Lehnert et al., 2015). Smith and Smith (2016), for example, report several hundred billion dollars in costs caused by obesity in the United States.

Different strategies for generating a negative energy balance promise sustainable success in the fight against overweight and obesity. As has been proved, energy restriction leads to a loss of body mass. For health reasons, however, this reduction of body mass should focus on the loss of body fat, not muscle mass (Biesalski and Grimm, 2007). Creating a negative energy balance *via* pure restriction of calorie intake without looking at the macronutrients seems to result in a loss of lean body mass of up to one third of the total loss of body mass (Miller et al., 1997). However, there is evidence that supplementation with protein might reduce the loss of muscle mass during energy restriction (Pasiakos et al., 2013, 2015).

In addition to energy restriction alone, an increase in energy turnover through greater physical activity in daily life, particularly exercise training, also contributes to a negative energy balance. Endurance- and resistance-exercise training (RT) generate a sharp increase in energy consumption and thus contribute to a loss of body fat (Wilmore et al., 1999; Dietz et al., 2012). Apart from the acute effect of exercise, particularly intense bouts of resistance exercise affect energy demands through two further mechanisms. Adaptation and repair processes significantly increase energy demands up to 48–72 h post-exercise (Melby et al., 1993; Gillette et al., 1994; Paschalis et al., 2010). Furthermore, in contrast to endurance or mixed exercise protocols, hypertrophic effects induced by adequate resistance exercise increase the resting metabolic rate (RMR; Byrne and Wilmore, 2001; Hunter et al., 2008). Correspondingly, retention of lean body mass and consequently RMR in periods of weight reduction (Goran and Poehlman, 1992; Hunter et al., 2008) is of high relevance.

Thus, the combination of strength training and high protein intake during phases of energy restriction might be a reasonable intervention for decreasing body mass predominately or even exclusively by a reduction in body fat mass (Wycherley et al., 2010).

However, a lack of time might be the primary obstacle to start conventional exercise programs that focus on weight loss (Rütten et al., 2009). In addition, orthopedic problems due to overweight or simple aversion to heavy exercise might prevent a subject's participation in exercise programs. Thus, time efficiency, joint friendliness, and high customization should be considered as a crucial aspect for effective interventions particularly for cohorts with low experience with or enthusiasm for physical exercise.

Whole-body electromyostimulation (WB-EMS), a novel timesaving, joint-friendly, and highly customizable training technology, might be a perfect candidate for cohorts with low time resources and/or low propensity for exercise (Kemmler et al., 2018). Recent studies have confirmed that WB-EMS has a comparable effect on hypertrophy, muscle strength, and fat loss as high intensity resistance training.

With regard to a personal-effective approach, the digital monitoring of training targets for increased physical activity in everyday life with the help of so-called activity trackers is of key importance. With the help of these trackers and the corresponding online training diary, a single trainer would thus have the opportunity to support his clients simultaneously without direct and prolonged contact with them and to check the activity specification. This combination of time-saving WB-EMS training, protein supplementation, and digital monitoring could be an effective, useful, and elegant solution in the setting of weight loss programs.

In summary, the aim of this study is to evaluate the effects of WB-EMS on body composition during energy restriction, increased physical activity in everyday life, and increased protein intake.

Our primary hypothesis is that WB-EMS, applied under energy restriction (ER), increased habitual physical activity in everyday life (PA) and high protein intake (PI), generates significantly more favorable changes on lean body mass than (1) isolated ER with moderate PI or (2) ER, high PI, and increased habitual physical activity in everyday life. Our secondary hypothesis is that (1) all three study arms generate significant reductions in absolute body fat mass (2) without significant group differences.

# MATERIALS AND METHODS

#### Trial Designs

The **W**eight-**R**eduction and **E**lectromyostimulation **P**lus **P**rotein (WREPP) project is a randomized, controlled, clinical intervention study (RCT) with three study arms in a parallel group design. The study was designed and conducted by the Institute of Medical Physics (IMP), Friedrich-Alexander University Erlangen-Nuremberg (FAU). The study follows the Helsinki Declaration "Ethical Principles for Medical Research Involving Human Subjects" and was approved by the Ethical Committee of the FAU (ethics application no. 19\_16b). The project was fully registered under NCT03746977. All women participating were intensively informed about the study procedures. Written informed consent was obtained before each subject's participation in the trial. We strictly followed the CONSORT 2010 guideline for randomized studies with a parallel group design (Moher et al., 2010).

#### Participants

Using citizen registers provided by the municipality, 1,000 women, 25–50 years old, living in the area of Erlangen-Nuremberg, Germany, were contacted by personal letters that already included the most important eligibility criteria. Two hundred women responded and were informed in more depth *via* a hotline and successive joint information events. Based on the eligibility criteria (1) 25–50 years old; (2) premenopausal status; (3) no diseases or pharmacological therapy with relevant influence on muscle mass and body fat (e.g., glucocorticoids >5 mg/day); (4) no pregnancy or acute breastfeeding period; (5) no conditions that exclude WB-EMS application (e.g., cardiac pacemaker); (6) no cardiovascular events (e.g., stroke, coronary infarction); and (7) no absence of more than 2 weeks during the study period, the eligible women (*n* = 110) were invited to determine obesity criteria by BIA (total body fat rate > 28%). Finally, 90 women fulfilled these criteria and agreed to participate in the study. Stratified according to body fat rate (%), participants were randomly allocated to three balanced intervention groups: (1) negative energy balance due to energy restriction with compensatory protein intake (CG: *n* = 30); (2) negative energy balance due to energy restriction and increased habitual physical activity in everyday life with increased protein intake (PA: *n* = 30); and (3) negative energy balance due to energy restriction and increased physical activity in everyday life with increased protein intake plus whole-body electromyostimulation (WB-EMS: *n* = 30). The flow chart of the study is shown in **Figure 1**, and the baseline characteristics of all the participants are illustrated in **Table 1**.

#### Intervention

The interventions were applied for 16 weeks in all the groups (February 2018 until July 2018).

#### Group CG: Negative Energy Balance Due to Energy Restriction With Compensatory Protein Intake

Based on individual nutritional analysis (Freiburger Nutrition Protocol, Nutri-Science, Hausach, Germany) and nutrition consultations, participants had to achieve the 500 kcal/day energy reduction through a corresponding reduction of carbohydrates (i.e., ≈125 g/day). Multi-component protein powder (Hansepharm, Roth, Germany) was supplemented to ensure a total protein intake of 1.2 g/kg body mass/day. Women in the CG were asked to maintain their normal daily activity and exercise habits.

#### Group PA: Negative Energy Balance Due to Energy Restriction and Increased Habitual Physical Activity With Increased Protein Intake

As with the CG, participants had to reduce the amount of carbohydrates by 250 kcal/day (i.e., ≈62.5 g/day). Based on the dietary protocol, multi-component protein powder (Hansepharm, Roth, Germany) was supplemented to ensure a total protein intake of 1.7 g/kg body mass/day. In order to generate a negative energy balance of −500 kcal/day, the women were asked to increase their normal daily activity by 250 kcal/day by executing more daily steps at their habitual gait speed. For this purpose, a specific individual target of steps was calculated. Therefore, the mean value of pedestrian cadence (115.2 steps/ min) under natural conditions (Tudor-Locke and Rowe, 2012) and the body mass of each participant were taken as a reference. Steps were monitored by a fitness tracker (Polar Loop 2, Polar Electro Oy, Kempele, Finland). The information, transmitted electronically to the study management, was checked regularly to ensure compliance with the training target. Nutritional and exercise guidelines should accordingly result in a reduction of energy balance of 500 kcal/day.

#### Group Whole-Body Electromyostimulation: Negative Energy Balance Due to Energy Restriction and Increased Habitual Physical Activity With Increased Protein Intake Plus Whole-Body Electromyostimulation

In addition to the treatment of the PA group, we applied a WB-EMS training 1.5 × 20 min/week (every Tuesday and every second Friday) using devices from miha bodytec (Gersthofen, Germany). Eight muscle groups (upper arms, chest, abdomen, latissimus, upper back, lower back, buttocks, thighs) were addressed by the WB-EMS (standard) application (85 Hz; 350 μs; intermittent; 6 s impulse phase, 4 s rest; rectangular; bipolar). Two sets of six to eight repetitions of easy movements with a small range of motion such as moderate squats, butterfly movements, and trunk flexions were carried out during the impulse phase. Sessions were supervised and guided by two certified instructors each responsible for two participants. In order to gradually guide the participants to the intended training load, the duration of the

FIGURE 1 | Flow diagram of the study intervention. *n*, numbers; CG, pure energy restriction with compensatory protein intake; PA, energy restriction and increased physical activity with increased protein intake; WB-EMS, energy restriction and increased physical activity with increased protein intake plus whole-body electromyostimulation; ITT, intention-to-treat.

TABLE 1 | Baseline characteristics of all groups (WB-EMS, PA, and CG; *n* = 30 per group).

*WB-EMS, energy restriction and increased physical activity with increased protein intake plus whole-body electromyostimulation; PA, energy restriction and increased physical activity with increased protein intake; CG, pure energy restriction with compensatory protein intake; p, significance; d, day; kcal, kilocalories; g, gram; n, number; min, minutes. a Assessed by baseline questionnaire.*

*b Measured by stadiometer.*

*c Measured by bio-impedance analysis (DSM-BIA, InBody 770, Seoul, South Korea).*

*d Analyzed by Freiburger Nutrition Protocol (Nutri-Science, Hausach, Germany).*

*e Measured by fitness tracker (Polar Loop 2, Polar Electro Oy, Kempele, Finland).*

*f Assessed by baseline questionnaire with a scale from 1 "very satisfied"/"very low" to 7 "very dissatisfied"/"very high."*

WB-EMS session was successively increased by 2 min from the first unit of 12 min until the full 20 min were reached in the fifth session. The intensity of training was based on the Borg CR 10 scale (Borg and Kaijser, 2006). The participants were instructed to realize a perceived exertion of between "5" (hard) and "7" (very hard). Trainers asked every 2 or 3 min, if they could raise the intensity of stimulation to adjust for the accustoming effect. The energy turnover of a single bout of WB-EMS (120 kcal/20 min session) (Kemmler et al., 2012) was considered in the calculation of the energy restriction (−225 kcal/day).

#### Outcomes

Primary endpoint:

▪ Changes in lean body mass from baseline to 16-week follow-up.

Secondary endpoint:

▪ Changes of total body fat mass from baseline to 16-week follow-up.

#### Assessments

All measurements were conducted at the IMP, FAU by dedicated researchers and research assistants. To ensure adequate standardization, the same test assessor using the same procedure carried out the dedicated baseline and follow-up measurement. All measurements were performed at similar times of the day (±1 h).

Calibrated devices were used for all assessments. Body height was measured barefoot using a stadiometer (Holtain Ltd., Crymych Dyfed, Great Britain). Body mass was assessed using bio-impedance technique (InBody 770, Seoul, South Korea), described in detail below. Body mass index (BMI) was calculated body mass (kg)/body height (m2 ).

All participants recorded their habitual diet as accurately as possible for 5 representative days (four weekdays and one weekend day) before and during the last week of the intervention. Using the corresponding evaluation software (Freiburger Nutrition Protocol, Nutri-Science, Hausach, Germany), the average volume of carbohydrate, fat, protein, and alcohol intake per day was calculated. Corresponding dietary changes and specification were discussed in detail with the participants by the principal investigator (SW). Based on the dietary analysis, the amount of carbohydrate intake was limited, by 56, 62.5, and 125 g/day, respectively, in order to generate a negative balance of 225 kcal/day (WB-EMS), 250 kcal/day (PA), and 500 kcal/day (CG). On the other hand, the intake of all the other macro-nutritional components was maintained. In parallel, protein supplements were provided to ensure a daily protein uptake of 1.2 g/kg body mass/day in the CG and 1.7 g/kg body mass/day in the PA and WB-EMS groups. We used a multicomponent protein powder (Hansepharm Power Eiweiß Plus, Roth, Germany), i.e., a mix of a whey, casein, soy, and egg protein blend with a high leucine (10.3%) and carnitine (1.7%) content. The same protocols and analysis were applied to check participants' compliance with the dietary requirements, during the sixth and the last week of the intervention.

The calculation of the daily step target was based on the habitual walking volume of participants as determined by a fitness tracker (Polar Loop 2, Polar Electro Oy, Kempele, Finland). Trackers were worn for 2 weeks prior to the start of the intervention. Participants were instructed to maintain their normal daily activity and use representative days for the monitoring of their physical activity in everyday life. Considering the body mass of the participants, the daily walking volume in minutes required to consume 250 kcal of energy was calculated using an online calculator. Using the average number of steps per minute (115 steps/min) reported for "adult pedestrians" (Tudor-Locke and Rowe, 2012), the number of steps per day equivalent to an energy expenditure of 250 kcal/day was calculated. Of importance, all the tracked information were transmitted every fifth day to an online platform and submitted to the principal investigator (SW) who checked compliance of the participants and, if necessary, asked participants to increase (or decrease) their daily step number in order to properly comply with the physical activity protocol.

Direct segmental, multi-frequency BIA-technique (Inbody 770, Seoul, South Korea) was applied to evaluate body composition including the primary (LBM) and secondary (BFM) study outcome. The device separately measures trunk, arms, and legs using a tetrapolar eight-point tactile electrode system by means of six different frequencies (1, 5, 50, 250, 500, and 1,000 kHz). Among the participants of the present study, intra class correlation (ICC, test-retest) of LBM as assessed by the Inbody 770 was 0.90, and ICCs for percent body fat was comparably high (0.88).

#### Sample Size

Based on a 5% difference in lean body mass between the WB-EMS group and the PA group (MV, SD: 6.5%), 27 persons/ group had to be included to generate at least a power of 80% based on an *α* = 0.05 (*t* test based sample size calculation). Anticipating a "loss to follow-up" rate of 10%, we included 30 women/group in order to perform an adjuvant per protocol analysis.

#### Randomization

Stratified for body fat rate (%) and consistently supervised by the principal investigator (SW), the participants drew lots and allocated themselves to the three conditions. Lots were put in opaque plastic shells and were drawn by the participants from the same bowl in the order of their appearance. Of importance, neither participants nor researchers knew the allocation beforehand. After the assignment into three equal groups (*n* = 30/each), each woman was informed in detail by the principle investigator (SW) about dos and don'ts.

#### Blinding

Due to the study design, we were unable to properly blind participants with respect to their allocation. However, researchers conducting the tests or analysis were not aware of the participants' group status and were not allowed to ask, either.

#### Statistical Analyses

Analyses were carried out using the statistical software R (R Development Core Team, 2018) and SPSS 25 (SPSS Inc., Chicago, IL, USA). Data were reported using mean values (MV) and standard deviation (SD). We applied an intention to treat analysis with multiple imputation for missing values, using Amelia II, which is implemented in the R package "Amelia" (Honaker et al., 2011). Following the recommendations of Honaker et al. (2011), the complete data set was used for imputation. In order to obtain stable results, 100 imputed data sets were generated. The "overimpute" plot was used to control the imputations. The plots showed that the imputations for the variables to the primary and secondary endpoints had worked very well. Statistical (Shapiro-Wilk test) and graphical (QQ plots) tests indicated a normal distribution for the measured endpoints. To evaluate intra-group changes dependent *t* test were applied. Inter-group differences were analyzed using ANOVA. In case of relevant differences (*p* < 0.150), pairwise differences between the groups were checked using independent *t* tests, adjusted for multiple testing according to the method of Barnard and Rubin (Barnard and Rubin, 1999). Nominal scaled data, reported in **Table 1**, were analyzed by *χ*2 tests. All tests were two tailed, and statistical significance was accepted at *p* < 0.05.

#### RESULTS

Baseline characteristics are given in **Table 1**. As stated, no significant between-group differences were determined (**Table 1**).

Nine participants (WB-EMS: *n* = 1 vs. PA: *n* = 3 vs. CG: *n* = 5) were lost to follow-up. Reasons given for withdrawal were (1) lack of motivation (*n* = 2); (2) pregnancy (*n* = 2); (3) lack of time (*n* = 2); and (4) aversion to protein powder (*n* = 1). Two women apparently lost interest and did not give reasons for their withdrawal.

Due to the possibility to catch up missed sessions, the attendance rate in the WB-EMS group was 100% (i.e., 24 sessions). None of the participants reported any adverse effects or complaints caused by the training. Further, based on our logs, compliance with the prescribed protein (powder) dose was high. Personal interviews with the participants (SW) showed a strict compliance with the protein supplementation protocol and confirmed this finding. However, slight decreases in dietary protein intake were observed, thus none of the study groups fully realized the intended total protein intake (WB-EMS: 1.67 ± 0.11 vs. PA: 1.62 ± 0.12 vs. CG: 1.16 ± 0.13 g/kg body mass/day).

Five-day dietary intake protocols conducted by all the participants indicated that average energy reduction by carbohydrate restriction, i.e., 500 kcal/day in the CG, and 225 and 250 kcal/day for the WB-EMS and PA group, respectively, was significantly exceeded (*p* ≤ 0.007) in all the groups. However, considerable differences among the participants of all three groups were observed. Energy reduction reported by participants in the CG averaged 678 ± 505 kcal/day (range: 110 to −1,433 kcal); corresponding energy reduction was 528 ± 477 kcal/day (range: 301 to −1,261 kcal/day) in the PA and 462 ± 399 kcal (range: 97 to −1,177 kcal/day) in the WB-EMS group. We further observed that the WB-EMS (198 ± 40 kcal/day) and the PA (180 ± 38 kcal/day) groups failed to generate the intended physical activity level corresponding to 225/250 kcal/day. Thus, overall reductions of net energy balance were 678 ± 505 kcal/day in the CG, 708 ± 519 kcal/day in the PA, and, considering the energy cost of the WB-EMS application, 660 ± 501 kcal in the WB-EMS group. Using ANOVA, significant between-group differences were not observed for the latter parameter (*p* = 0.660).

The primary study endpoint "lean body mass" (**Table 2**) decreased in the CG (−0.2 ± 2.9%) and PA groups (−0.8 ± 1.8%) but increased in the WB-EMS group (0.5 ± 2.5%). However, none of these intra-group changes (*p* ≥ 0.11) were statistically significant. In parallel, no significant differences between the three groups were determined by ANOVA (*p* = 0.070) (**Table 2**). However, pairwise comparisons adjusted for multiple testing indicated a borderline significant difference between WB-EMS and PA (*p* = 0.049), while other comparisons, i.e., CG vs. PA (*p* = 0.424) or CG vs. WB-EMS (*p* = 0.390) were far from being significant (**Table 2**). Thus, we confirm our primary hypothesis (2) that WB-EMS generated significantly more favorable changes on lean body mass than a similar intervention, however without EMS-application (i.e., PA group). However, we have to reject our primary hypothesis (1) that WB-EMS, PA (250 kcal/day), energy restriction (225 kcal/day), and high protein intake (1.7 g/kg body mass/day) generated significantly more favorable changes on LBM than a control group with isolated energy restriction (500 kcal/day) and moderate protein intake (1.2 g/kg/body mass/day).

Total body fat mass decreased significantly (*p* < 0.001) in all the groups (CG: −6.3 ± 7.8%; PA: −10.7 ± 8.7%; WB-EMS: −8.3 ± 7.4%) without significant group differences (*p* = 0.131) as determined by ANOVA (**Table 2**). After adjusting for multiple testing, pairwise comparisons did not result in significant differences between the groups (CG vs. PA: *p* = 0.142; CG vs. WB-EMS: *p* = 0.331; PA vs. WB-EMS: *p* = 0.578). Thus, we fully confirm our secondary hypothesis that (1) all three groups generated significant reductions in total body fat mass (2) without significant group differences.

Changes in parameters that might confound our results were not detected. None of the participants reported any new disease or change of relevant medication or smoking habits during the study period. Physical activity and exercise outside the study increased in all the groups (CG: *p* = 0.034 vs. PA: *p* = 0.257 vs. WB-EMS: *p* = 0.076), however, without significant between-group differences (*p* = 0.800).

#### DISCUSSION

The general purpose of the study was to evaluate the effect of WB-EMS on body composition, under specific consideration of LBM changes, during negative energy balance, but increased protein intake in overweight 25- to 50-year-old premenopausal women. In summary, all of the interventions favorably affect LBM (<10%). Compared with the 25–30% LBM contribution to total weight loss reported by others (Ballor et al., 1988; Miller et al., 1997; Weinheimer et al., 2010), WB-EMS in combination with high protein intake seems to be the most suitable method for maintaining muscle mass during intended, mild decreases of net energy balance by caloric restriction and/or increased physical activity. Addressing changes in total body fat mass, all the groups significantly reduced fat mass during the 16-week intervention period (−6.3 ± 7.8 to −10.7 ± 8.7%).

This study is the first trial to determine the effect of WB-EMS on body composition during intended and structured negative energy balance. In summary, the present study provided evidence for introducing WB-EMS in treating overweight and obesity. Considering the hypertrophic effects of WB-EMS, a recent study demonstrates positive effects in the range of a highintensity resistance training (HIT-RT). Applying both types of exercise for 16 weeks in untrained men, 30–50 years old (*n* = 46), Kemmler et al. (2016) reported comparable (*p* = 0.395), highly significant increases in LBM in the WB-EMS (0.63 ± 0.77 kg) and HIT (0.86 ± 0.97 kg) study arm of its randomized controlled trial (RCT). Of importance for the present topic, most WB-EMS studies reported reductions in fat mass that considerably exceeded the increases in muscle mass, resulting in a net weight loss (Kemmler et al., 2018). However, in contrast to HIT-RT, WB-EMS is a joint friendly and safe training method (Kemmler et al., 2016) feasible for all people unable or unwilling to join conventional (resistance) exercise programs. Due to the lack of other WB-EMS studies in this field, however, we now discuss results of RT trials to allow the reader to consider the relevance of our results.

Reviewing the literature, several studies indicated that isolated energy restriction protocols reduce body fat mass and LBM

*WB-EMS, energy restriction and increased physical activity with increased protein intake plus whole-body electromyostimulation; PA, energy restriction and increased physical activity with increased protein intake; CG, pure energy restriction with compensatory protein intake; p, significance; MV, mean value; SD, standard deviation. \*\*\*p < 0.001.*

(Ballor et al., 1988; Miller et al., 1997; Weinheimer et al., 2010). Applying a hypocaloric diet in 40 obese people, Ballor et al. (1988) reported that 25% of total body mass loss can be attributed to LBM reductions. This result was confirmed by the systematic review of Weinheimer et al. (2010) that included trials examining overweight postmenopausal women under caloric restrictions of 9–52 weeks and calculated an average decrease in fat-free mass of about 25% of weight loss. In the "diet only" study arm of their meta-analysis (493 studies) with overweight adults, 2–90 weeks long, Miller et al. (1997) reported a body-mass reduction that constituted two thirds fat and one third fat-free mass.

Revisiting promising methods to stop or attenuate the loss of LBM during energy restriction, the current state of research suggests resistance exercise training. In their systematic review and meta-analysis, Sardeli et al. (2018) observed that RT, particularly when applied in multiple sets over 12–24 weeks, stops the decline of LBM in obese elderly people during a phase of caloric restriction (472 and 800 kcal/day; Sardeli et al., 2018). Hunter et al. (2008) investigated the effect of RT on fat-free mass in premenopausal overweight women during a diet-induced (800 kcal/day, 21 weeks) weight loss of approximately 12 kg. Compared with participants without exercise training (47.9 ± 4.7 to 46.4 ± 5.1 kg) or aerobic training (45.4 ± 4.2 to 44.4 ± 4.1 kg), RT maintained fat-free mass (46.9 ± 5.2 to 47.2 ± 5.0 kg) to a similar degree as in the present study (Hunter et al., 2008). Monitoring 40 obese premenopausal females in an eight-week weight-loss study, Ballor et al. (1988) draw a similar conclusion. Following a daily deficit of 1,000 kcal and supplementing protein to ensure a protein intake of ≥1.0 g/kg body mass/day, the exercise group conducting RT 3 days per week preserved and even significantly increased LBM compared with the "diet only" group (0.43 ± 0.26 vs. −0.91 ± 0.28 kg; *p* < 0.05).

Another possibility to protect LBM during a phase of negative energy balance might be a higher daily protein intake. In their meta-analysis of 27 RCTs, Stonehouse et al. (2016) compared the effects of dairy products (including whey protein) on body composition during energy restriction in 18–50-year-old predominately overweight to obese adults (*n* = 1,278) (Stonehouse et al., 2016). The daily energy restriction of the trials was usually 500 kcal or more for the average of 16 weeks. In summary, intervention groups with increased protein intake lost (non-significantly) less LBM than control groups (−0.12 ± 1.57 vs. −0.56 ± 1.54 kg). Of importance, the authors observed the most favorable effects on LBM maintenance using a protein intake of >1.2 g/kg body mass/day and additional RT. Similar findings were provided by another meta-analysis or meta-regression (Krieger et al., 2006; Wycherley et al., 2012). The comparison of 24 RCTs by Wycherley et al. (2012) with adults (mean study duration 12.1 ± 9.3 weeks) under energy restriction (exact amount not specified) demonstrated that diets with high protein intake (1.25 ± 0.17 g/kg body mass/day) resulted in significantly lower reductions in fat-free mass than diets with standard protein consumption (0.72 ± 0.09 g/kg/day). However, differences were significant for interventions longer than 12 weeks only. Krieger et al. (2006) reported that the degree of FFM retention during energy-restricted weight loss tended to increase with each successive quartile of dietary protein intake and that protein intake of 1.05 g/kg body mass/ day or more might improve FFM retention. A 12-week RCT with 46, 28–80-year-old overweight and obese women on energy deficit (750 kcal/day) compared the effect of normal versus higher (0.92 vs. 1.52 g/kg body mass/day) protein intake (Leidy et al., 2007). In summary, the high protein group lost less LBM than the lower protein group (−1.5 ± 0.3 vs. −2.8 ± 0.5 kg; *p* < 0.05). This finding confirmed the results of an earlier trial (Farnsworth et al., 2003) with 43 overweight and obese women, 20–65 years old, on a 12-week energy restrictive diet (≈−500 kcal/day). Again, participants with higher protein intake (1.4 g/kg body mass/day) lost less LBM (−0.1 ± 0.3 kg vs. −1.5 ± 0.3 kg, *p* = 0.02) than their peers' standard protein consumption (0.8 g/kg body mass/day). Summing up the results in his systematic review, Pasiakos et al. (2013) recommend a daily protein intake twice (1.6 g/kg body mass) or three times (2.4 g/kg body mass) as high as the usual standard recommendation (0.8 g/kg body mass) to preserve lean body mass.

Although a recent meta-analysis (Stonehouse et al., 2016) did not fully confirm this strategy, a combination of strength training and increased protein intake during energy restriction might promise additive effects on LBM retention. Closest to our study, Layman et al. (2005) provided a high protein (1.6 g/kg body mass/day) reduced carbohydrate diet (400–500 kcal/day) combined with RT twice a week for 16 weeks for 48 overweighed women, 40–56 years old. In summary, this protocol resulted in more favorable changes in LBM (−0.9%) and fat mass (−22%) than a low protein (0.8 g/kg body mass/day) high carbohydrate diet (−5.4 and −12%) or an isolated high protein diet (−4.0 and 18%). The authors (Layman et al., 2005) applied a single set RT to fatigue of 30 min, a protocol with similar effects on body composition compared with WB-EMS (Kemmler et al., 2016).

Apart from effectiveness, adherence to the exercise protocol is a crucial aspect of weight loss (Acharya et al., 2009). In summary, a recent systematic review on WB-EMS reported low dropout (<10%) and high attendance (>90%) rates without any injuries (Kemmler et al., 2018). Even excluding studies with a high dropout (i.e., >30%), in their systematic review addressing early postmenopausal women, Asikainen et al. (2004) reported less favorable findings for RT, aerobic, or mixed protocols (Asikainen et al., 2004).

Summing up, we conclude that WB-EMS combined with high protein intake is an effective and feasible option for maintaining LBM under negative net energy balance and thus preventing a decrease of RMR with negative consequences on further weight management. However, some features and study limitations might decrease the evidence of the present trial or at least aggravate its proper interpretation. (1) One may argue that the study design with three groups and a mix of energy restriction, protein supplementation with different dose, physical activity, and WB-EMS might be too sophisticated and complex for a single study. Retrospectively, we partially agree with this criticism; however, the aim of the study was to determine the specific effect of WB-EMS on LBM retention applying a "state of the art" weight loss protocol1 . As discussed in detail above, this includes adequate protein supplementation, which was provided in all the groups, albeit in different doses (1.7 vs. 1.2 g/kg/body mass/day). The reason for the latter strategy was the increased demand for protein due to RT- (or WB-EMS-) induced negative muscle protein balance (Phillips et al., 1997). (2) Considering the rather modest decreases in fat mass (−6.3 ± 7.8 to −10.7 ± 8.7%) in this cohort of overweight to obese women, it is unlikely that net energy deficiency truly fell within the range of about 650–700 kcal/day (i.e., ≈75.000 kcal/16 weeks). While physical activity was tracked by calibrated, valid devices and can be thus considered as a reliable outcome (Bunn et al., 2019), results on energy restriction as reported by the participants were dubious. Although we strictly emphasized a practicable protocol that focused on carbohydrate reduction only, extensively discussed the dietary protocol with the participants and contacted participants every second week to check compliance with the dietary recommendations, the majority of participants obviously "overreported" their caloric restriction. (3) We applied multi-segmental multi-frequency bio-impedance analysis (BIA) to determine body composition of our cohort. However, the more common way of assessing body composition in research is dual-energy X-ray absorptiometry (DXA) – considered as the gold standard in body composition assessment. Recent comparisons, however, report that BIA appears to be an adequate alternative - not only because of its easier handling in the clinical setting and the lack of x-rays (Fosbol and Zerahn, 2015; Gomez-Arbelaez et al., 2017). Fosbol and Zerahn (2015) and Gomez-Arbelaez et al. (2017) compared different methods of measurement for body composition and concluded that there is no general recommendation for any type of measurement, since every single method has its advantages and disadvantages. Our own research has confirmed this conclusion. In studies with different cohorts (Von Stengel et al., 2013; Kemmler et al., 2017), we observed a high agreement for lean and fat mass between the Inbody 770 used in this trial and a Hologic 4500a DXA Scanner with a narrow limit of agreement. Even more important, the reliability of the BIA device to determine muscle mass determined by a test-retest approach with 25 participants resulted in high intra class correlation (ICC) of 0.86–91 (95% CI: 84–94) for LBM (Kemmler et al., 2017). (4) Although physical activity is frequently listed as a component of weight reductions programs (PA & WB-EMS), its transfer to everyday practice is problematical. With respect to our aim of 250/225 kcal energy consumption by physical activity in everyday life, the increase of daily walking ranged from 6,000 to 10,000 additional steps. Since all the women are employed and the predominant majority of them has to manage their families including younger children, it is understandable that most of them failed to realize their daily step specification. The latter aspect might further hinder the generality of our result. Additionally, the wide range of overweight and obesity (28–55% body fat rate) within our study might limit the proper transfer to dedicated

premenopausal cohorts. Thus, more dedicated eligibility criteria for the present study might have generated more meaningful and transferable results.

In summary, the combination of WB-EMS and higher protein intake is an effective tool for favorably affecting body composition in overweight premenopausal women following a moderate energy deficit. Considering the time efficiency, joint friendliness and high degree of customization of this novel training technology, WB-EMS might be a feasible alternative to RT at least in people unmotivated or unable to join demanding resistance exercise protocols during their weight loss programs. However, further WB-EMS studies are needed to overcome the limitations of the present study and to check transferability on other cohorts.

#### DATA AVAILABILITY

The raw data supporting the conclusions of this manuscript will be made available by the authors, without undue reservation, to any qualified researcher.

#### ETHICS STATEMENT

This study was carried out in accordance with the recommendations of "Declaration of Helsinki", with written informed consent from all subjects. The protocol was approved by the "Ethics Commission of the Friedrich-Alexander University Erlangen-Nürnberg" (number: 19\_16b).

#### AUTHOR CONTRIBUTIONS

SW, AW, SS, DS, MT, and WK designed the study, completed the data analysis on each location and/or interpretation, and drafted the manuscript. MF and HK contributed to study conception and design and contributed to revise the manuscript. MK performed the statistical analysis of the data. SW accepts responsibility for the integrity of the data sampling, analysis, and interpretation.

#### FUNDING

The study was exclusively funded by own resources of the Institute of Medical Physics (IMP), University of Erlangen-Nuremberg (FAU). We acknowledge support by Deutsche Forschungsgemeinschaft and Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) within the funding program Open Access Publishing.

#### ACKNOWLEDGMENTS

The present work was performed in (partial) fulfillment of the requirements for obtaining the degree "Dr. rer. biol. hum." We would like to take this opportunity to thank Hansepharm (Roth, Germany), which provided the protein powder for the study participants.

1 Considering that LBM reduction in all of the study groups averaged below 10% of total weight loss we obviously realized this aim….

#### REFERENCES

electromyostimulation (wb-ems) to improve health-related outcomes in nonathletic adults. A systematic review. *Front. Physiol.* 9:573. doi: 10.3389/ fphys.2018.00573

Wycherley, T. P., Noakes, M., Clifton, P. M., Cleanthous, X., Keogh, J. B., and Brinkworth, G. D. (2010). A high-protein diet with resistance exercise training improves weight loss and body composition in overweight and obese patients with type 2 diabetes. *Diabetes Care* 33, 969–976. doi: 10.2337/dc09-1974

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The handling editor declared a past co-authorship with several of the authors WK and MK.

*Copyright © 2019 Willert, Weissenfels, Kohl, von Stengel, Fröhlich, Kleinöder, Schöne, Teschler and Kemmler. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

# The Impact of Whole-Body Electromyostimulation on Body Posture and Trunk Muscle Strength in Untrained Persons

Oliver Ludwig1 \*, Joshua Berger1 , Stephan Becker1 , Wolfgang Kemmler2 and Michael Fröhlich1

1 Department of Sports Science, Faculty of Social Sciences, University of Kaiserslautern, Kaiserslautern, Germany, 2 Institute of Medical Physics, Friedrich-Alexander University Erlangen-Nürnberg, Erlangen, Germany

#### Edited by:

Kohei Watanabe, Chukyo University, Japan

#### Reviewed by:

Toshiaki Miyamoto, Hyogo University of Health Sciences, Japan Moritz Schumann, German Sport University Cologne, Germany

> \*Correspondence: Oliver Ludwig<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Exercise Physiology, a section of the journal Frontiers in Physiology

Received: 08 February 2019 Accepted: 24 July 2019 Published: 20 August 2019

#### Citation:

Ludwig O, Berger J, Becker S, Kemmler W and Fröhlich M (2019) The Impact of Whole-Body Electromyostimulation on Body Posture and Trunk Muscle Strength in Untrained Persons. Front. Physiol. 10:1020. doi: 10.3389/fphys.2019.01020 Muscular imbalances of the trunk muscles are held responsible for changes in body posture. At the same time, whole-body electromyostimulation (WB-EMS) has been established as a new training method that enables simultaneous stimulation of many muscle groups. This study was aiming to analyze if a 10 weeks WB-EMS training changes posture-relevant parameters and/or improves isometric strength of the trunk extensors and flexors, and if there are differences based on stimulation at 20 Hz and 85 Hz. Fifty eight untrained adult test persons were divided into three groups (control, CON; training with 20 Hz stimulation, TR20; training with 85 Hz, TR85). Anthropometric parameters, trunk extension and flexion forces and torques, and posture parameters were determined before (n = 58) and after (n = 53: CON: n = 15, TR20: n = 19, TR85: n = 19) a 10 weeks WB-EMS training program (15 applications, 9 exercises). Differences between the groups were calculated for pre- and post-tests using univariate ANOVA and between the test times using repeated (2 × 3) ANOVA. Comparisons of pairs were calculated post hoc based on Fisher (LSD). No differences between the groups were found for the posture parameters. The post hoc analysis of both trunk flexion and trunk extension forces and torques showed a significant difference between the groups TR85 and CON but no difference between the other group pairs. A 10 weeks whole-body electrostimulation training with a stimulation frequency of 85 Hz in contrast to training with a stimulation frequency of 20 Hz improves the trunk muscle strength of an untrained group but does not significantly change posture parameters.

Keywords: WB-EMS, muscle training, trunk flexion, trunk extension, posture training

# INTRODUCTION

Basically, body posture is based on the interaction of muscles, tendons, and bones. Changes of the habitual posture develop as the body adapts to routine daily postures (Harrison et al., 1999; Prins et al., 2008; Claus et al., 2009; Roussouly and Pinheiro-Franco, 2011; Drzal-Grabiec and Snela, 2012; Jung et al., 2016). It is assumed that permanent poor posture in

daily routines leads to muscular and articular overload, which in turn results in physical problems (Bruno et al., 2012; Araujo et al., 2017; Jentzsch et al., 2017). Particularly posture issues such as hyperlordosis and a hunched back are considered to be the reasons for back problems (Jentzsch et al., 2017; Murray et al., 2017). Current studies show an interrelationship between individual posture parameters and the occurrence of lower back pain (Kim et al., 2006; Dolphens et al., 2012; Aggarwal et al., 2013).

Poor posture is currently assumed to be caused by muscular imbalances and dysfunctional body perception (Kim et al., 2006; Buchtelová et al., 2013). Muscular imbalances are understood to be an imbalance of strength between agonist and antagonist, which moves a joint's resting position from its neutral position (Nadler et al., 2001; Frank et al., 2009; Buchtelová et al., 2013). The position of the pelvis in the sagittal plane is particularly important in this case (Schwab et al., 2006; Sorensen et al., 2015). Functionally, the pelvis is considered a seesaw that is kept in equilibrium by muscular activity (Buchtelová et al., 2013). The abdominal muscles (Musculus rectus abdominis, Musculus transversus), the gluteal muscles (Musculus gluteus maximus), and the hamstrings (Musculus biceps femoris, Musculus semitendinosus, Musculus semimembranosus) all seem to influence the position of the pelvis by lifting the anterior pelvic rim and thus reduce anterior pelvic tilt (Bridger et al., 1992; Nourbakhsh and Arab, 2002; López-Miñarro et al., 2012; Jeong et al., 2015). Preventive recommendations include strengthening the pelvis-straightening muscles (Kim et al., 2006; Ludwig et al., 2016). Physical therapy to correct poor posture starts with practicing targeted muscle activation and improving muscular strength, joint flexibility, as well as body perception (Pope et al., 1985; Calvo-Muñoz et al., 2012; Laird et al., 2014; Barczyk-Pawelec et al., 2015; Kim et al., 2015; Szczygiel et al., 2018).

A proven therapeutic option to strengthen muscles is neuromuscular electrical stimulation (NMES) (Doucet et al., 2012). This method is based on the application of electrodes to the skin, which generate an electric field that changes membrane potentials and thus results in muscle fiber contractions (Filipovic et al., 2012). Depending on the frequency of the voltage pulse applied, the efferent nerve is usually not involved (Gondin et al., 2005; Maffiuletti, 2010). Particularly in physical therapy, NMES can help to rebuild muscles atrophied due to an injury or after immobilization (Adams, 2018). In contrast to NMES, functional electromyostimulation (FEMS) means the combination of electrical stimulation with functional movements such as walking or lifting objects.

Whole-body electromyostimulation (WB-EMS) has been established as a new training method that enables simultaneous stimulation of many muscle groups, for example by means of electrode vests (Kemmler and von Stengel, 2013). In contrast to the electromyostimulation known from physical therapy, newer WB-EMS concepts promote the idea to perform active movements during stimulation, i.e., to add an active centralnervous muscle activation to the passive electric stimulation (Herrero et al., 2010; Amaro-Gahete et al., 2018). Therefore, the application of WB-EMS has to be grouped under FEMS, because movements are carried out at the same time as voltage is applied. Studies have shown that WB-EMS improves strength in elite soccer players (Filipovic et al., 2016) and in untrained middleaged men (Kemmler et al., 2016b) by increasing the number of muscle fibers brought to contraction during an exercise movement (Kemmler et al., 2010).

A number of stimulation parameters, such as impulse amplitude, impulse type, and impulse frequency can be modified to control EMS training. Research is not in agreement in this context, though. Most working groups describe a frequency range between 20 and 110 Hz to stimulate as many muscle fibers as possible (Moreno-Aranda and Seireg, 1981; Binder-Macleod and Barrish, 1992; Jones, 1996; Dreibati et al., 2010; Weissenfels et al., 2018). Certain frequencies are said to elicit a stronger stimulation of specific muscle fibers (Dreibati et al., 2010). Stimulation frequencies of up to 50 Hz appear to activate mainly the slower type-I muscle fibers, while frequencies between 50 and 120 Hz seem to stimulate the faster type-II fibers. However, there is no scientific consensus (Kemmler and von Stengel, 2013; Weissenfels et al., 2018), and this means that studies examining the effect of WB-EMS on specific muscle groups must observe the presumable effects of different stimulation frequencies.

Studies on the influence of WB-EMS training on body posture do not exist to date. Considering the high prevalence of poor posture (Bansal et al., 2014) and the time-consuming physical therapy treatments required, the research demand concerning new intervention methods is substantial. Therefore, the question arises whether an unspecific WB-EMS training might contribute to an improvement of posture parameters. This could be the case if the neuromuscular balance of the trunk muscles connected to the pelvis could be changed by WB-EMS training. Strengthening the lumbar parts of the back extensor also appears to be useful from a therapeutic point of view because this is already a proven form of therapy to treat lower back pain (Kahanovitz et al., 1987; Mannion et al., 2001; França et al., 2010). It has already been shown that conventional EMS training is able to achieve an improvement of muscular strength of Musculus erector spinae to prevent lower back pain (Kahanovitz et al., 1987; Kemmler et al., 2017; Weissenfels et al., 2018). Based on the seesaw model of the pelvis WB-EMS training could, however, also have a negative effect on the pelvis position through unspecific strengthening of the lumbar back muscles by increasing the pelvic tilt and thus leading to a more pronounced lumbar lordosis. On the other hand, strengthening the thoracic sections of M. erector spinae, which acts as a trunk extensor, WB-EMS training could result in a reduced forward body tilt and reduced spinal column curvatures, thus having a posture-improving effect.

In the light of the above, any potential influence of WB-EMS training on posture-constituting muscle groups and thus on body posture in general is unclear. In gyms, WB-EMS training is usually performed unspecifically, i.e., many superficial muscle groups in trunk and gluteus are activated during a training session. General strengthening of the trunk muscles does not necessarily have to have an effect on body posture, because muscular relationships often seem to be more important than

absolute muscle force (Buchtelová et al., 2013). For this reason and since this training method spreads very quickly, research has a very strong interest in delimiting potential preventive and curative effects from potentially posture-damaging effects. As WB-EMS training is less time-consuming compared to conventional strength training (usually, a complete whole-body workout only takes 20 min), it could become interesting for large target groups in both prevention and therapy.

This study is therefore aiming to analyze the following questions:

#### MATERIALS AND METHODS

#### Test Persons

The sample size was calculated using G∗Power 3.1 (University of Kiel, Germany). For a repeated (2 × 3) ANOVA (within and between interactions, f = 0.3, α = 0.05) a minimum group size of 48 persons was calculated (power 0.958), which we increased due to the expected drop outs. Finally, 58 test persons participated in the study. They were recruited by means of flyers distributed on the university campus. Five test persons did not complete the 10 weeks training and were removed from the study. In the end, the data from 53 test persons was included in the evaluation (see **Table 1** and flowchart **Figure 1**). All participating test persons did not have any previous experience or knowledge concerning EMS training. The test persons had to be between 18 and 40 years old, were not to perform any regular athletic activity, and needed to be free of internal and orthopedic limitations. Muscle stretchability and joint mobility were not explicitly examined before the start of the study. However, since all participants were able to perform all movements in the required range of motion (ROM) during the dynamic exercises, we assume that there was no movement restriction relevant for our study. All test persons were informed about the process and objectives of the study and gave their written consent before the study started. In order to exclude any risks associated with EMS training, a comprehensive anamnesis questionnaire had to be completed (Kemmler et al., 2016a). The study was approved by the ethics

TABLE 1 | Anthropometric data of the three groups (means ± standard deviation).

commission of the Technical University Kaiserslautern (ref. no. 02/17) and was conducted based on the Declaration of Helsinki (World Medical Association [WMA],, 2013).

# Measurements

In both the initial and the final examination, the following anthropometric parameters were determined: body weight, height, and body fat percentage. The static trunk extension and flexion forces were measured by means of the isometric force testing device Back Check 607 (Dr. Wolff GmbH, Arnsberg, Germany). This required the test persons to stand with their arms hanging loosely and with slightly bent knee joints, fixated in the sagittal plane at the iliac crest area by one dorsal and one ventral pad. The flexion in the knee joints served to reduce the influence of the iliopsoas, since we only wanted to measure the strength of the trunk muscles. Two pads with force transducers were placed without pressure at the sternum and between the shoulder blades at an individual height (**Figure 2**). Alternatingly, three maximum strength measurements of the trunk flexion and the trunk extension were performed for 5 s each and 30 s breaks in between the measurements. The test persons were instructed to press against the pads as strongly as they could. If the isometric strength value of the last measurement was the largest, the sequence was continued after 30 s breaks until the value decreased. The highest values for extension and flexion were included in the evaluation. The flexion and extension torques were calculated as M = F∗ (vertical distance between force transducer pad and pelvis fixation pad).

The reliability of this measurement system was analyzed in other studies and found to be in a good range [intraclass correlation coefficient 0.76–0.89 (Scheuer and Friedrich, 2010)].

To evaluate posture, surface scans of the entire body were performed using the Paromed 4D Sanner (Paromed GmbH, Neubeuern, Germany). For those scans, the test persons stood barefoot and shirtless or wearing a sports bra about 2.5 m away from the scanner. They were instructed to stand at ease (habitual posture), let their arms hang loosely, look straight ahead, and breathe normally. Anatomic landmarks such as C7, S1, and the PSIS (posterior superior iliac spines) were marked by means of adhesive stickers on the skin (Patias et al., 2010). The PSIS markers were not used for further calculations but are required by the software used. By projecting a coded light stripe grid on to the body, the system reconstructed the three-dimensional contours of the body's back. Three scans were averaged. The total measurement itself took about 10 s. The following parameters were calculated based on the 3D representation (**Figure 3**):

measurement device. FT ext, force transducer extension; FT flex, force transducer flexion; Fix, fixation pads.

Flèche cervicale and flèche lombaire are orthopedic measurements that show the depth of the curvature of the spine by using the distance of the extreme points of the lordosis and the thoracic kyphosis. Increased values suggest posture deficits (Stagnara et al., 1982; Vrtovec et al., 2009; Allier and Monnet, 2013). Both parameters were additionally calculated as percentage values by having been relativized to the vertical distance between C7 and S1.

# Training

The participating test persons were randomly assigned to two training groups (TR20, TR85) and the control group (CON) by lots. At no time during the study were the members of the training groups or the investigators informed about this assignment (training groups were double-blinded).

The CON did not perform any athletic activity during the 10 weeks training phase. In all, the training groups performed a total of 15 WB-EMS training sessions. They alternatingly exercised once or twice a week, so that an average of 1.5 training sessions was performed per week (Kemmler et al., 2016b, 2018). Before the training phase started, all test persons took part in a familiarization session in order to get used to the imminent WB-EMS application (Jee, 2018). The familiarization session took 12 min and was characterized by reduced voltage intensity to avoid muscular overload (Kemmler et al., 2016a). At the same time, the test persons were taught the proper exercise techniques and learned about the RPE (rating of perceived

exertion) scale [0 = no exertion, 10 = maximum exertion (Borg and Kaijser, 2006)].

Stimulation was applied in line with common parameters: impulse width of 350 µs, bipolar impulse without impulse increase, 4 s load and 4 s break intervals, overall training duration 20 min (Kemmler et al., 2016a). Both training groups differed only in the stimulation frequency applied, i.e., 20 Hz (TR20) and 85 Hz (TR85), respectively. The test persons were instructed to exercise with an intensity of 6 ("hard") on the modified Borg scale, while maintaining the entire ROM during the exercise. In order to ensure adequate exertion, the intensity of the electric impulses both during the sessions and between the sessions was adapted by the investigator. Every 2–3 min, the participants provided feedback and the intensity for each muscle group was changed in a way that the participant felt the exertion to be "hard". The devices produce a maximal peak output voltage of 75 V at 1 kOhm (corresponding to a current of 75 mA), but do not display the actual intensity in absolute voltage. The output is only displayed in device-specific units from 0–100, in proportion to the initial voltage. The intensity data was noted at the end of each training session for each muscle group.

The training times were kept at a constant level for each test person in order to avoid influences caused by the time of day. Before each training session, a new anamnesis questionnaire on the current condition was completed. This was to exclude potential contraindications (e.g., dizziness, nausea, and pain) and to ensure safe training for the test persons. The test persons were instructed to drink sufficient liquids before the training started (at least 500 ml). During the course of the study, the test persons were instructed not to perform any additional physical training. This was asked repeatedly at the beginning of each training session. They were also requested not to physically exert themselves in the preceding 24 h by everyday activities. Furthermore, the questionnaire was used to make sure that medication or pain killers had not been taken, so that overexertion due to a missing ability to assess the extent of exertion was avoided.

#### Devices and Exercises

A Miha Bodytec 2 WB-EMS (Miha Bodytec, Augsburg, Germany) was used. This system consists of an impulse transmitter with control panel and optical feedback. A 2 m cable connects the training vests including the integrated electrodes to the system. The training vests were available in various sizes, which were tightly attached to the upper body. The cranial tip of the sternum served as an anatomical reference for the correct placement of the vest. Caudal, the vest reached up to the iliac crests. The electrode surfaces and arrangement of all vests were proportional in all sizes (**Figure 4**). Upper arms and thighs were also equipped with additional circular electrodes that were attached with velcro at the level of the muscle bellies. The gluteus was stimulated by flat electrodes integrated into a belt that was closed at the abdomen. All electrodes and accessories were provided by the same manufacturer (Miha Bodytec, Augsburg, Germany). The system allows free programming of the stimulation parameters, whereas the stimulation parameters, except the applied voltage, are the same for all electrodes. The device settings were saved on a chip card for each test person.

Exercises were selected based on the exercises integrated in the WB-EMS device used. They were therefore representative of a typical exercise program performed in a gym, targeting as many stimulated muscle groups as possible. These exercises were performed:

FIGURE 4 | Positioning of the electrodes in a typical training vest. Only the largest target muscles below the electrodes are mentioned. Pect. – M. pectoralis, Trapezius – M. trapezius, Latiss. – M. latissimus dorsi, Rect. abdom. – M. rectus abdominis, Erect. spinae lumb. – M. erector spinae pars lumbalis.

Active movements were initiated just before the electrical impulse was applied and continued to be performed during the entire impulse duration. Between impulses, the participants adopted a resting position.

The training was conducted exclusively under supervision of trained investigators with a support ratio of 1:2. Both safety and accurate exercising were ensured by the permanent supervision by the investigator (Moreno-Aranda and Seireg, 1981).

#### Statistics

Differences between the groups (TR20, training at 20 Hz stimulation; TR85, identical training at 85 Hz stimulation; CON, untrained control group) were calculated for pre- and posttests using univariate ANOVA (XLSTAT 2018.4, Addinsoft, Paris, France). Group differences between the test times were calculated using repeated (2 × 3) ANOVA. Comparisons of pairs were calculated post hoc based on Fisher (LSD). A possible influence of sex on the percentual increase of flexion and extension torques was calculated using (2 × 3) ANOVA. The significance level was set at 5%.

#### RESULTS

A significant difference between the three groups was not found for any of the posture parameters measured, neither at the beginning nor at the end of the study. Only the upper body anteversion decreased over time, but for all groups including the control group with no significant differences between the groups (TR85 vs. CON: p = 0.14, TR85 vs. TR20: p = 0.96; TR20 vs. CON: p = 0.16). All other posture parameters did not change. **Table 2** shows the results of the statistic tests.

The **trunk flexion force** values were not significantly different between the groups at the beginning of the study (df = 2, F = 1.44, p = 0.25). At the end of the study, significant differences were identified for the factor time (df = 1, F = 36.97, p < 0.0001) and for the interaction group∗ time (df = 2, F = 3.92, p = 0.02). The post hoc analysis showed a significant difference between the groups TR85 and CON (p = 0.02). There was no difference between the other group pairs (TR85 vs. TR20: p = 0.31; TR20 vs. CON: p = 0.16).

The **trunk flexion torque** values showed comparable results: they were not significantly different between the groups at the beginning of the study (df = 2, F = 1.61, p = 0.21). At the end of the study, we found significant differences for the factor time (df = 1, F = 31.90, p < 0.0001) and for the interaction group∗ time (df = 2, F = 3.45, p = 0.04). The post hoc analysis showed a significant difference between the

groups TR85 and CON (p = 0.03), but no difference between the other group pairs (TR85 vs. TR20: p = 0.42; TR20 vs. CON: p = 0.14).

There were also no group differences at the beginning of the study for the **trunk extension force** (df = 2, F = 1.38, p = 0.26). After the treatment, significant effects were identified for the factor time (df = 1, F = 56.59, p < 0.0001) and for the interaction group∗ time (df = 2, F = 4.27, p = 0.02). The post hoc analysis showed a significant difference between the groups TR85 and CON (p = 0.04). There was no difference between the other group pairs (TR85 vs. TR20: p = 0.49; TR20 vs. CON: p = 0.15).

For the **trunk extension torque** we could not find a significant difference between the groups at the beginning of the study (df = 2, F = 1.46, p = 0.24). At the end of the study, significant differences for the factor time (df = 1, F = 44.84, p < 0.0001) and for the interaction group∗ time (df = 2, F = 4.37, p = 0.02) could be found. The Fisher post hoc analysis showed a significant difference between the groups TR85 and CON (p = 0.04), and no difference between the other group pairs (TR85 vs. TR20: p = 0.55; TR20 vs. CON: p = 0.13).

The percentage improvement in the training groups did not differ between men and women (extension torque increase in %: men: 20.18% ± 16.64 vs. women 19.2% ± 17.5; flexion torque increase in %: men: 18.1% ± 12.03 vs. women 16.2% ± 15.9). A 2 × 3 ANOVA (sex, group) did not provide significant results; based on the Type III sum of squares, the variable sex did not bring any additional significant information (flexion: df = 1, F = 0.20, p = 0.66, extension: df = 1, F = 0.06, p = 0.80).

Body mass index did not change significantly during the study (group∗ time: F(2) = 0.68, p = 0.52). This applied to all groups.

# DISCUSSION

The objective of this study was to find out the extent to which unspecific WB-EMS training over a period of 10 weeks would achieve an improved body posture and increased trunk muscle strength in untrained persons, and whether any differences exist in terms of stimulation frequency.

## Trunk Muscle Strength

In the training group stimulated with 85 Hz, we were able to prove a significant improvement of the isometric strength and torque of trunk extension and flexion compared to the control group. The strength increases identified were on average between 15.0 and 21.4% (force) and between 15.9 and 26.6% (torque) for the groups in training (**Table 2**) and were of a similar size as increases found in other studies (Kemmler et al., 2016b, 2018). In the WB-EMS training, large electrodes in the areas of M. rectus abdominis (flexion), the lumbar and thoracic M. erector spinae, and the Mm. multifidii (flexion) induced contractions, so that an adequate training stimulation and corresponding muscle strengthening over the 10 weeks of the study can be assumed (Ng and Richardson, 1994). However, we also found force increases between 4.8 and 7.4% and torque increases between 4.4 and 9.2% in the control group. We interpret this finding as test habituation, i.e., that familiarity with the task of isometric maximal strength exercises was higher in the post-test and had a positive effect on the initially unfamiliar isometric strength development.

No significant difference was identified in the group stimulated at 20 Hz (**Table 3**). This difference between the groups was unexpected because the back musculature (M. erector spinae), which is important for posture control, consists of mainly slow type-I fibers [men 62.0 ± 9.3%, women 67.8 ± 10.5% (Mannion et al., 2001)].  The straight abdominal muscles, on the other hand, consist of a slightly higher percentage of type-II fibers [type I 46.1%, type II 53.9% (Johnson et al., 1973)] and should therefore be more susceptible to an 85 Hz stimulation. However, Gregory and Bickel found that an EMS-induced recruitment of motor units does not always proceed selectively. Type I and type II fibers seem to be recruited by EMS without adhering to any specific sequence (Gregory and Bickel, 2005). We therefore assume that muscle fibers were activated independently of the stimulation frequency, and that this fiber activation did not primarily depend on the fiber type, but rather on the physical location of the electrodes. Since the tetanic contraction of muscle fibers increases from a stimulation frequency of 20 Hz and more (Smith et al., 1985), we assume that the applied training stimulus was too low in the group training at 20 Hz. In addition, the duty cycle (on-off ratio) of the muscle fibers was more than four times lower with the lower stimulation frequency than with the 85 HZ stimulation. However, the duty cycle seems to play an important role in generating a training stimulus (Lloyd et al., 1986). In a comprehensive review, Filipovic et al. (2012) recommend a stimulation frequency greater than 50 Hz in order to generate a stimulation intensity that is sufficient to activate strength adaptation. Collins et al. (2007) were additionally able to prove that higher EMS stimulation frequencies (50– 100 Hz) lead to electrically evoked sensory potentials, which induce the spinal motor neurons via reflex circuits to activate additional motor units. This contribution to muscle contraction, which is generated by the central nervous system, does not develop with frequencies < = 20 Hz and seems to max out at frequencies > = 80 Hz (Dean et al., 2007). Dean et al. (2007) were able to achieve an additional strength increase of 10.2% of the maximum voluntary contraction by applying four two-second stimuli. For this study, the stimulation was applied in 4 s stimuli, i.e., they were similar in intensity. We therefore assume that the lack of additional central-nervous contractions at an applied frequency of 20 Hz resulted in a lower training stimulus and thus in lower strength increases.

#### Posture

A classification of the test persons based on the average flèche lombaire percentage of 8.06% (see **Table 2**) shows that all test persons together constitute a group with poor posture. Proprietary, unpublished data of 724 test persons showed a flèche lombaire percentage of 6.92% ± 3.06 (95% confidence interval 6.63–7.21%) for a normally pronounced lumbar lordosis and

 (Delta), and the ratio

"extension/flexion"

 (Ext/Flex).