Edited by: David Jiménez-Pavón, University of Cádiz, Spain
Reviewed by: Olaf Prieske, Universität Potsdam, Germany; Steffen Mueller, Trier University of Applied Sciences, Germany
This article was submitted to Exercise Physiology, a section of the journal Frontiers in Physiology
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The aim of this study was to compare the effects of short-term strength training with and without superimposed whole-body electromyostimulation (WB-EMS) on straight sprinting speed (SSS), change of direction speed (CODS), vertical and horizontal jumping, as well as on strength and power in physically active females. Twenty-two active female participants (
The importance of resistance training in order to enhance sprinting and jumping performance is generally accepted. The relevance of maximal strength and power training has been repeatedly underlined, especially in competitive sports for women and men (
Electromyostimulation (EMS) is known as an effective complementary training method to improve athletic performance surrogates (
Up to now, 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 jump-, SSS-, or CODS-specific skill training. Most interestingly, there are no available WB-EMS studies in female athletes. Only two studies dealt with the transfer into sprinting and jumping performance with male athletes (
Therefore, the aim of this study was to compare 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.
This study was designed as a two-armed randomized controlled trial with a parallel-group design comparing effects of submaximal, superimposed dynamic WB-EMS (S+E) with effects of dynamic athletic training without WB-EMS (S) on sprinting and jumping performance as well as on strength and power. S+E and S completed eight training sessions in 4 weeks. To determine training effects, the sprint, jump, strength, and power diagnostics were intra-individually conducted on three occasions at the same time of the day under constant and stable lab conditions: directly before, directly after the training period (pre- and post-tests), and after a 2-week follow-up (retest) (
Participants flow through the study [adapted from
Twenty-eight female strength trained sport students participated in the study. According to
Anthropometric data (mean ± SD).
N | Age (years) | Height (cm) | Weight (kg) | BMI (kg/m2) | Strength Training Experience (years) | |
---|---|---|---|---|---|---|
S+E | 11 | 20.4 ± 2.8 | 172.7 ± 7.3 | 65.5 ± 10.7 | 21.8 ± 2.4 | 6.5 ± 3.9 |
S | 11 | 20.5 ± 1.8 | 170.3 ± 3.8 | 62.0 ± 4.7 | 21.4 ± 1.6 | 3.9 ± 3.2 |
Both groups completed eight training sessions over a 4-week period. The length of the training period and the number of training sessions were derived from numerous EMS studies with an average period of 4–6 weeks with one to seven sessions per week (
The WB-EMS intervention complied with the guidelines for a safe and effective WB-EMS training (
The intensity of WB-EMS was adjusted to 70% of the individual pain threshold (iPT = maximum tolerated amperage, 0–120 mA). The iPT was verified separately for each pair of electrodes before each session and lasted 2 min for each S+E participant. The participants stood with an interior knee angle of 170° while tensing their lower limbs muscles. The verification of iPT began with the electrodes at the buttock, followed by the thigh, the lower leg, the abdominal, and the lower back electrodes. Then, the intensity was subsequently downregulated with the main controller at the WB-EMS device to an intensity of 70% to enable dynamic movements. The impulse frequency was set at 85 Hz, the impulse width at 350 μs, the impulse type as bipolar and rectangle (
Strength training exercises with characteristics about repetitions, sets, rest between the sets, contraction mode (ecc = eccentric, iso = isometric, con = concentric) per repetition, range of motion per repetition and time under tension (TUT) per exercise for both groups as well as on/off ratio of WB-EMS impulse (70% of the individual pain threshold) for the strength training group with superimposed WB-EMS (S+E) and additional load (individual 8–10 repetition maximum) for the strength training group (S).
S+E Group and S Group |
S+E Group |
S Group |
||||||
---|---|---|---|---|---|---|---|---|
Strength Training Exercises | Repetition (n) | Set (n) | Rest (s) | ECC: ISO: CON (s) | ROM# (°) | TUT (s) | On/Off Ratio (s) | Additional Load |
(1) Bulgarian Split Squat | 10 | 3 per leg∗ | 60 | 2: 1: 2 | 170–90 | 300 | 50/5 | ≤20 kg Barbells |
(2) Nordic Curl | 8 | 3 | 60 | 2: 0: 2 | 90–135 | 96 | 32/0 | Softer Rubber Band |
(3) Knee Tuck | 8 | 3 | 60 | 2: 0: 2 | 180–70 | 96 | 32/0 | – |
(4) Side Abs | 8 per side | 3 | 60 | 0.5: 0: 0.5 | – | 48 | 16/0 | ≤4 kg Medicine Ball |
Jumping and sprinting training exercises with characteristics about repetitions, sets, rest between the sets, range of motion per repetition and time under tension (TUT) per exercise for both groups as well as on/off ratio of WB-EMS impulse (70% of the individual pain threshold) for the strength training group with superimposed WB-EMS (S+E) and additional load (individual 8–10 repetition maximum) for the strength training group (S).
S+E Group and S Group |
S+E Group |
S Group |
|||||
---|---|---|---|---|---|---|---|
Jumping and Sprinting Training Exercises | Repetition or Duration (n or s) | Set (n) | Rest (s) | ROM# (°) | TUT (s) | On/Off-Ratio (s) | Additional Load |
(1) Skipping | 8 s | 3 | 30 | 180–90 | 24 | 8/0 | – |
(2) Heeling | 10 | 3 per leg∗ | 30 | 180–90 | 60 | 10/5 | – |
(3) Side Jump | 5 per side | 3 | 30 | – | 36 | 12/0 | – |
(4) Box Jump | 5 | 3 per leg∗ | 30 | – | 90 | 3/5 | – |
(4) Drop Jump | 5 | 3 | 30 | – | 45 | 3/5 | – |
The strength sessions involved four exercises for both groups: (1) Bulgarian split squat: single leg split squat with heel raise, elevated rear foot, and hands remaining in the akimbo position, (2) Nordic curl: two-legged hamstring curl with supporting rubber bands at chest height, (3) knee tuck: knee pull to the chest while feet hang in loops and upper body remain in push-up position, as well as (4) side abs: side-to-side medicine ball crunch with raised legs (exercise characteristics are presented in
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) (
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
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.
Sprint testing involved a T-run for CODS (
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
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.
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) (
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.
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 (
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. Previously, measures of CODS, SSS, and jump performance have been shown to be highly reliable (CV 1–9%; ICC 0.80–0.99) (
Sprint values for both groups are provided in
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.
Parameter | Group | Pretest | Posttest | Pre–Post |
Retest | Pre–Re |
ANOVA p ( |
|||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
% Delta | SMD | % Delta | SMD | Time | Group | Time∗ Group | ||||||
Straight Sprinting Speed | LS TT | S+E | 4.80 0.23 | 4.75 0.19 | –1.0 | 0.24 | 4.69 0.24 | −2.3∗ | 0.47 | 0.336 (0.046) | 0.954 (0.002) | |
(s) | S | 4.73 0.20 | 4.67 0.21 | −1.3 | 0.29 | 4.60 0.21 | −2.8∗ | |||||
LS 5-m ST | S+E | 1.11 0.05 | 1.09 0.04 | −1.8 | 0.44 | 1.04 0.05 | −6.3∗ | 0.666 (0.009) | 0.620 (0.024) | |||
(s) | S | 1.12 0.05 | 1.07 0.06 | −4.5 | 1.03 0.03 | −8.0∗ | ||||||
LS 10-m ST | S+E | 1.92 0.10 | 1.92 0.07 | 0.0 | 0.0 | 1.85 0.09 | −3.7∗ | 0.527 (0.020) | 0.705 (0.013) | |||
(s) | S | 1.92 0.07 | 1.90 0.09 | −1.0 | 0.25 | 1.82 0.07 | −5.2∗ | |||||
LS 20-m ST | S+E | 3.40 0.16 | 3.37 0.12 | −0.9 | 0.21 | 3.30 0.17 | −2.9∗ | 0.392 (0.037) | 0.863 (0.007) | |||
(s) | S | 3.36 0.13 | 3.32 0.15 | −1.2 | 0.29 | 3.24 0.13 | −3.6∗ | |||||
Tappings | Total Steps in 5 s | S+E | 45.00 6.05 | 46.82 5.31 | +4.0 | 0.32 | 49.27 5.29 | +9.5∗ | 0.419 (0.033) | 0.506 (0.025) | ||
(n) | S | 45.09 8.41 | 49.18 4.45 | +9.1 | 51.64 3.85 | +14.5∗ | ||||||
Change of Direction Speed | T-run TT | S+E | 8.72 0.36 | 8.67 0.25 | −0.6 | 0.16 | 8.56 0.33 | −1.8∗ | 0.46 | 0.426 (0.032) | 0.816 (0.010) | |
(s) | S | 8.62 0.33 | 8.62 0.23 | 0.0 | 0.00 | 8.45 0.19 | −2.0∗ | |||||
T-run ST | S+E | 2.40 0.11 | 2.35 0.09 | −2.1 | 2.39 0.12 | −0.4 | 0.09 | 0.400 (0.036) | 0.002 (0.247) | |||
(s) | S | 2.38 0.10 | 2.39 0.06 | +0.4 | 0.12 | 2.28 0.06° | −4.2 |
Significant main effects of time were found for SSS for total time (
Jump values for both groups are provided in
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.
Parameter | Group | Pretest | Posttest | Pre–Post |
Retest | Pre–Re |
ANOVA p ( |
|||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
% Delta | SDM | % Delta | SDM | Time | Group | Time∗ Group | ||||||
SLJ | Length | S+E | 148.82 17.13 | 159.30 14.67 | +7.0∗ | 158.64 13.24 | +6.6∗ | 0.333 (0.047) | 0.145 (0.092) | |||
(cm) | S | 155.55 8.78 | 161.55 14.71 | +3.9∗ | 166.64 13.91 | +7.1∗ | ||||||
SJ | Height | S+E | 25.25 3.79 | 27.17 3.59 | +7.6 | 27.58 4.66 | +9.2∗ | 0.140 (0.106) | 0.245 (0.068) | |||
(cm) | S | 28.59 4.03 | 28.98 4.72 | +1.4 | 0.09 | 30.46 4.65 | +6.5∗ | 0.43 | ||||
CMJ | Height | S+E | 27.36 3.83 | 28.89 3.14 | +5.6∗ | 0.44 | 30.97 4.70 | +13.2∗ | 0.099 (0.130) | 0.746 (0.015) | ||
(cm) | S | 30.54 3.89 | 32.08 4.72 | +5.0∗ | 0.36 | 33.36 5.05 | +9.2∗ | |||||
DJ | Height | S+E | 25.21 2.66 | 27.04 3.03 | +7.3 | 27.53 3.35 | +9.2 | 0.486 (0.035) | 0.241 (0.068) | 0.163 (0.087) | ||
(cm) | S | 24.85 4.69 | 24.60 6.53 | −1.0 | 0.01 | 24.21 5.34 | −1.5 | 0.04 | ||||
Contact Time | S+E | 0.177 0.02 | 0.167 0.01 | −5.7 | 0.177 0.02 | +0.0 | 0.0 | 0.218 (0.075) | ||||
(s) | S | 0.199 0.03 | 0.176 0.02° | −11.6 | 0.178 0.03° | −10.6 | ||||||
RSI | S+E | 1.46 0.31 | 1.63 0.24 | +11.6∗ | 1.59 0.32 | +8.9 | 0.41 | 0.149 (0.101) | 0.969 (0.002) | |||
S | 1.28 0.32 | 1.43 0.43∗ | +11.7∗ | 0.40 | 1.39 0.35 | +8.6 | 0.33 |
Significant main effects of time were observed for SLJ (
Strength and power values for both groups are provided in
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) during pre-, post-, and retests.
Parameter | Group | Pretest | Posttest | Pre-Post |
Retest | Pre-Re |
ANOVA p ( |
|||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
% Delta | SDM | % Delta | SDM | Time | Group | Time∗ Group | ||||||
LC | Fmax | S+E | 725 116 | 843 177 | +16.3∗ | 857 168 | +18.2∗ | 0.886 (0.001) | 0.560 (0.029) | |||
(N) | S | 722 243 | 859 161 | +19.0∗ | 813 195 | +12.6∗ | 0.41 | |||||
Pmax 40% | S+E | 339 81 | 400 100 | +18.0∗ | 392 94 | +15.6∗ | 0.910 (0.001) | 0.139 (0.098) | ||||
(W) | S | 333 107 | 390 102 | +17.1∗ | 422 103 | +26.7∗ | ||||||
Pmax 60% | S+E | 405 81 | 445 94 | +9.9∗ | 0.46 | 449 95 | +10.9∗ | 0.701 (0.008) | 0.769 (0.013) | |||
(W) | S | 382 98 | 433 106 | +13.4∗ | 441 88 | +15.5∗ | ||||||
LE | Fmax | S+E | 1507 202 | 1657 330 | +10.0∗ | 1697 337 | +12.6∗ | 0.509 (0.022) | 0.899 (0.005) | |||
(N) | S | 1445 255 | 1566 277 | +8.4∗ | 0.45 | 1622 258 | +12.2∗ | |||||
Pmax 40% | S+E | 691 150 | 768 171 | +11.1∗ | 0.48 | 784 204 | +13.5∗ | 0.716 (0.007) | 0.711 (0.017) | |||
(W) | S | 717 193 | 812 199 | +13.3∗ | 0.49 | 795 173 | +10.9∗ | 0.43 | ||||
Pmax 60% | S+E | 663 143 | 707 154 | +6.6 | 0.30 | 684 145 | +3.2 | 0.15 | 0.091 (0.113) | 0.391 (0.037) | 0.105 (0.106) | |
(W) | S | 714 185 | 720 131 | +0.8 | 0.04 | 777 128 | +8.8 | 0.40 |
Changes in maximal strength (Fmax) and power (Pmax) with 60% additional load for leg press (LP) in group S (strength training) and S+E (strength training with superimposed WB-EMS) during pre-, post-, and retests.
Parameter | Group | Pretest | Posttest | Pre–Post |
Retest | Pre–Re |
ANOVA p ( |
|||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
% Delta | SDM | % Delta | SDM | Time | Group | Time∗ Group | ||||||
LP | Fmax | S+E | 2905 565 | 3087 706 | +6.3 | 0.29 | 2719 781 | −6.4 | 0.27 | 0.127 (0.098) | 0.315 (0.050) | 0.546 (0.030) |
(N) | S | 2616 656 | 2737 667 | +4.6 | 0.18 | 2619 448 | +0.1 | 0.01 | ||||
Pmax 60% | S+E | 742 161 | 837 140 | +12.8 | 804 158 | +8.4∗ | 0.39 | 0.480 (0.025) | 0.113 (0.103) | |||
(W) | S | 820 250 | 837 231 | +2.1 | 0.07 | 897 205 | +9.4∗ | 0.34 |
Significant main effects of time were found for LC for Fmax (
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 (
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
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 (
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
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 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.
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.
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.
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 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.
The Supplementary Material for this article can be found online at: