Enhanced Myofiber Recruitment During Exhaustive Squatting Performed as Whole-Body Vibration Exercise : The Journal of Strength & Conditioning Research

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Enhanced Myofiber Recruitment During Exhaustive Squatting Performed as Whole-Body Vibration Exercise

Eckhardt, Holger; Wollny, Rainer; Müller, Helmut; Bärtsch, Peter; Friedmann-Bette, Birgit

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Journal of Strength and Conditioning Research 25(4):p 1120-1125, April 2011. | DOI: 10.1519/JSC.0b013e3181d09e0e
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Eckhardt, H, Wollny, R, Müller, H, Bärtsch, P, and Friedmann-Bette, B. Enhanced myofiber recruitment during exhaustive squatting performed as whole-body vibration exercise. J Strength Cond Res 25(4): 1120-1125, 2011-The purpose of the study was to test the hypothesis that myofiber recruitment is enhanced when whole-body vibration (WBV) is added to squat training. In a randomized cross-over design, 14 recreationally active men were subjected to 2 sessions consisting of 5 sets of 10 squats with external load, performed either on a vibration platform (whole-body vibration squatting [WBVS]) or conventionally without WBV (CON). Electromyographic (EMG) activity of the right vastus lateralis muscle was continuously recorded during WBVS and CON. The integrated EMG values were normalized to the EMG activity recorded during measurement of the maximal voluntary contraction force (MVC) on an isometric leg press at the beginning of each training session. Capillary lactate concentration was determined before and repeatedly after the squatting exercise. Overall mean normalized and integrated EMG (nIEMG) activity during WBVS (62 ± 4% MVC) was significantly (p < 0.001) higher compared with CON (47 ± 2% MVC). There was a tendency for nIEMG to increase during the 5 sets of 10 squats performed as WBVS (p = 0.089), whereas there was a significant (p < 0.001) decrease in nIEMG during CON. Whole-body vibration squatting induced a significantly (p < 0.001) larger increase in capillary lactate than CON (3.03 ± 0.32 vs. 1.60 ± 0.30 mmol·L−1, p < 0.001). The increased myoelectric activity and the enhanced exercise-induced increase in capillary lactate concentration during WBVS provide evidence for augmented recruitment of muscle tissue when WBV is added to exhaustive squatting exercise.


Whole-body vibration (WBV) exercise, where subjects exercise on a vibrating platform with or without additional load, is regarded as an alternative or complement form of strength training (15,17). There is a common agreement that the increase in muscle power, which was observed immediately after WBV exercise without additional load, might be attributed to enhanced motoneuron recruitment (3,7,19,23). Enhanced fiber recruitment during WBV strength training was also suggested as an explanation for the greater increase in strength after periodized squat training with WBV compared with classical squat training in trained athletes (22). Potentiating effects in (explosive) strength gain are thought to occur because of changes in the neuromuscular recruitment pattern during WBV exercise. This hypothesis, however, has only scarcely been investigated by recording electromyographic (EMG) activity during WBV exercise. To our knowledge, one study reported increased EMG activity during unloaded isometric WBV exercise (5) and there are 2 more investigations, where the analysis of EMG activity during unloaded WBV leg extension exercise showed signs of increasing fatigue (9,23). Electromyographic activity during typical quadriceps strength training, performed as WBV exercise, has not been compared with EMG activity during equivalent conventional training to investigate the effects of adding WBV on neuromuscular activity.

The results of scientific studies investigating the effects of WBV training on strength and power are inconsistent, probably because of a wide variety in WBV exercise protocols (6,12,15,17,18). In the present study, we wanted to find out if adding WBV to typical dynamic squatting exercise enhances myofiber recruitment of the quadriceps muscle. Therefore, we studied the EMG activity of the vastus lateralis muscle during 2 sessions of exhaustive squat training performed with and without WBV and measured the exercise-induced increase in capillary lactate concentration. We hypothesized that increased recruitment of muscle tissue during squatting with WBV would enhance EMG activity and also augment capillary lactate concentration because of increased anaerobic glycolysis. The squat training session was designed as a typical session in progressive quadriceps strength training for physically active but not strength-trained subjects according to the recommendations of the American College of Sports Medicine (1). For the application of WBV, we used a platform oscillating around a horizontal axis (Galileo 900; Novotec, Pforzheim, Germany) and chose a frequency of 22 Hz which, to our experience, is best tolerated by subjects without experience in WBV training.


Experimental Approach to the Problem

A prospective, randomized, cross-over design was chosen to test our hypothesis that addition of WBV to exhaustive squatting with external load increases myofiber recruitment. Two sessions of supervised standardized squat training were performed with and without WBV after the subjects had been familiarized with the testing and exercising procedures before the investigation. Electromyographic activity of the right vastus lateralis muscle was continuously recorded on both occasions. The integrated EMG values were normalized to the EMG activity recorded during determination of the maximal voluntary contraction force (MVC) at the beginning of each training session. To address the question of enhanced glyolytic metabolism due to increased recruitment of muscle tissue, the exercise-induced increase in capillary lactate was determined before and after each training session.


Fourteen physically active men (26.0 ± 4.5 years, 179.6 ± 4.5 cm, 75.9 ± 10.2 kg) without previous motor disorders or current injuries volunteered for the study. Subjects were not involved in resistance training or in WBV exercise but in regular recreational sporting activities. None of them had used anabolic steroids or supplemented creatine. The subjects were fully informed about the possible risk and discomfort that might result from the investigations, and the subjects gave their written informed consent. The study was approved by the Ethics Committee of the Medical Faculty of the University of Heidelberg, Germany, and conformed with the standards set by the Declaration of Helsinki. During the investigation, subjects were instructed to refrain from any quadriceps exercise (e. g., cycling or running) at least 24 hours before each experiment.


Subjects participated in 2 familiarization sessions, where they first performed conventional squat training (CON) for determination of the individual external load, applied by a barbell, which was needed to cause exhaustion after 10 repetitions (10 repetition maximum, 10RM). To determine the 10RM, subjects were subjected to 3 series of several repetitions of squatting, starting with an additional load equivalent to 60% of their body weight. Dependent on the number of repetitions that could be performed with the initial load, load was increased or reduced for the next trial that followed after 5 minutes of rest. The evolving 10RM averaged 44.3 ± 2.8 kg (SD) equivalent to 60 ± 4 %(SD) body weight. In the second familiarization session, subjects were accustomed to squat training on the vibrating plate (Galileo 900; Novotec) (whole-body vibration squatting [WBVS]). In both these sessions, subjects were also familiarized with the performance of MVC on an isometric leg press (Motronik-Trainer; Schnell, Peutenhausen, Germany).

After 7-10 days without any knee extension exercise, subjects performed 1 session of CON and 1 session of WBVS in randomized order, one week apart. At the beginning of each session, an 8-minute warm-up took place on a bicycle ergometer (Lode Excalibur Sport; Lode, Groningen, the Netherlands) at a workload of 1.3 W/kg body weight. Then, subjects performed 3 maximal voluntary contractions lasting 5 seconds at 90° knee joint angle on the isometric leg press with a 5-minute interval of rest in between for determination of the maximal EMG activity during development of MVC. Maximal voluntary contraction force was determined accordingly, however without assessment of EMG activity, 10 minutes after CON and WBVS, respectively.

In both sessions, subjects performed 5 sets of 10 squats with additional load equivalent to the subject's 10RM applied by a barbell. The series of 10 repetitions were separated by 3 minutes of rest. After the 3-minute rest, subjects were recovered to perform another 10 repetitions of squats. For consistency in the tests, the subjects' feet were exactly placed at shoulder width and each subject wore his same pair of shoes on both occasions. The knee bending angle was 80°, monitored by an adjustable light barrier. Squatting was performed in a regular rhythm of 3 seconds per squat controlled by a metronome, that is, 10 repetitions were completed within 30 seconds. During WBVS, subjects were standing on the Galileo 900 vibration plate (Figure 1). With this commercially available device, vibration to the entire body is applied by alternating rotation around the central axis of the platform. The amplitude increases with distance from the rotational center to maximally 5.2 mm at the outer edge (mean amplitude in the present study: 4 mm). The frequency can be freely chosen between 1 and 30 Hz and was set at 22 Hz due to the results of the preliminary pilot studies.

Figure 1:
Illustration of squatting performed on a vibration platform (Galileo 900; Novotec, Pforzheim, Germany) with external load. 1, Goniometer; 2, light barrier; 3, vibration platform; 4, EMG box; 5, EMG electrodes. EMG, electromyographic.

Electromyographic Measurements

Electromyographic activity of the right vastus lateralis muscle and goniometric data (Uniaxial Goniometer; Biovision, Wehrheim, Germany) were continuously recorded during the sessions of WBVS and CON. Electromyographic recordings were taken using bipolar Ag/AgCl surface electrodes (Hellige, Freiburg, Germany) with diameter of recording surface of 10 mm applied on vastus lateralis muscle with an interelectrode (center-to-center) distance of 30 mm. Electrodes were placed at 2/3 of a virtual line drawn from the anterior spina iliaca superior to the lateral side of the patella with respect to the direction of the muscle fibers. A reference electrode was applied at the spina iliaca anterior superior. Before applying the surface electrodes, the skin was shaved and cleaned to reduce the skin resistance to less than 5 kOhm, measured with an impedance meter (Digital Multimeter EX-411; Extech, Waltham, MA, USA). Electrode position was set during the first test and retained by using cutaneous ink marks. The same setting was used in all subsequent sessions. Insulated EMG cables were fastened to prevent the cables from swinging and from movement artifact. The raw EMG signal was recorded using a 16-channel EMG system (Biovision). The preamplified signal was bandpass filtered (10-10,000 Hz) before sampling at 1,000 Hz. Spectral analysis of EMG raw data showed no significant peak at 22 Hz. However, to avoid potential contamination of results due to the 22 Hz vibration frequency, we used a 25 Hz high pass Butterworth filter. In the MVC tests and during squatting, root mean squares were calculated during intervals of 0.5 seconds. The maximum value recorded during 3 trials of MVC was chosen for normalization of the integrated EMG values recorded during squatting. Goniometric data were used to facilitate the separation and analysis of each squatting cycle (Figure 1). Because of lacking movement accuracy, the 1st and 10th squat in each set was excluded from the data analysis. The analysis of the EMG data was performed using DASYLab (DasyTec, Amherst, NH, USA). As a measure for the reproducibility of the EMG recordings, the intraclass correlation coefficient (ICC) for the maximal root mean square values in the MVC tests was 0.83.

Blood Lactate Sampling

After warm-up and recording the EMG activity during MVC and 1, 3, and 5 minutes after the last of the 5 sets of 10 squats, 20 μL of capillary blood was obtained from the earlobe for determination of blood lactate concentration with an automated system (EBIO plus; Eppendorf, Hamburg, Germany).

Statistical Analyses

Statistical analysis was performed with the software programs SigmaStat 3.0 and SigmaPlot 8.0 for Windows (Jandel Scientific, San Rafael, CA, USA) and with SPSS for Windows version 16.0.1 (SPSS, Inc., Chicago, IL, USA). Data are presented as mean ± SEM unless stated otherwise. One-way random-effects single measure model (1,1) was used to calculate the ICC from the EMG data recorded during MVC in each training session. For comparison of the mean EMG activities recorded during each out of 8 squats in set 1 of WBVS and CON and of the mean EMG activity determined during each of the 5 sets of WBVS and CON, repeated measures analysis of variance on ranks was used followed by the Tukey test for multiple comparison procedure as most data were not normally distributed. Differences between the values of overall mean EMG activity during WBVS and CON and differences between means of the capillary lactate measured before and after the 2 different training sessions were evaluated with the Wilcoxon-signed rank test.


Electromyographic Measurements

Mean normalized and integrated EMG (nIEMG) increased significantly during each set of squats, by about 11% during WBVS (p < 0.001, χ2 = 75.048 with 7 degrees of freedom) and by about 5% during CON (p < 0.001, χ2 = 44.190 with 7 degrees of freedom) (Figure 2). During WBVS, there was a tendency (p = 0.089) toward an increase in mean nIEMG from 58 ± 6% MVC during set 1 to 64 ± 8% MVC during set 5 (χ2 = 8.072 with 4 degrees of freedom), whereas nIEMG showed a significant decrease during CON from 48 ± 5% MVC during set 1 to 45 ± 5% MVC during set 4 (χ2 = 19,283 with 4 degrees of freedom, p < 0.001) (Figure 3). Overall nIEMG was significantly (p < 0.001) higher during WBVS (62 ± 4% MVC) than during CON (47 ± 2% MVC) (Figure 4).

Figure 2:
Mean normalized and integrated EMG (nIEMG) during one set of squats (squats 2-9) performed with (whole-body vibration squatting [WBVS]) or without (CON) whole-body vibration (whole-body vibration [WBV]). Mean values ± SEM are shown. *p < 0.05 compared with squats 2, 3, 4, and 5. #p < 0.05 compared with squats 2 and 3. +p < 0.05 compared with squat 2.
Figure 3:
Mean normalized and integrated EMG (nIEMG) during 5 sets of squats (squats 2-9) performed with (whole-body vibration squatting [WBVS]) or without (CON) whole-body vibration (WBV). Mean values ± SEM are shown. *p < 0.05 compared with set 1.
Figure 4:
Overall mean normalized and integrated EMG (nIEMG) during the 5 sets of squats (squats 2-9) performed with (whole-body vibration squatting [WBVS]) or without (CON) whole-body vibration (WBV). Mean values ± SEM are shown.

Maximal Voluntary Contraction Force

A significant decrease in MVC was observed after both training sessions by about 9% from 1,814 ± 95 N to 1,653 ± 119 N after WBVS (p = 0.011) and by about 8% from 1,751 ± 74 N to 1,618 ± 108 N after CON (p = 0.034). There were no significant differences in MVC between WBVS and CON.

Blood Lactate

After training, the mean blood lactate concentration was significantly (p < 0.001) increased, from 1.09 ± 0.08 to maximally 4.11 ± 0.33 mmol·L−1 after WBVS and from 1.06 ± 0.07 to maximal values of 2.66 ± 0.29 mmol·L−1 after CON. Whole-body vibration squatting induced a significantly larger increase in lactate than CON (Figure 5).

Figure 5:
Capillary lactate concentration before and after 5 sets of 10 squats performed with (whole-body vibration squatting [WBVS]) or without (CON) whole-body vibration (WBV) and the corresponding maximal increase in capillary lactate (Δ lactate: maximal lactate concentration-resting value). Mean values ± SEM are shown.


We here show for the first time that WBV induces significantly increased EMG activity and capillary lactate when additionally applied during a typical squat training session indicating enhanced myofiber recruitment during WBVS compared with CON. Such increased muscular activation is commonly regarded as the most likely explanation for acute increases in muscle strength and power, which were induced by WBV in several studies (4,12,17). However, there are only few investigations where the effects of superimposed vibrations on muscle activation were studied (5,9,16,23). In none of these investigations, myoelectric activity or measures of muscle metabolism were determined during squatting with external load, one of the most typical and frequently performed types of quadriceps strength training.

In 4 previous studies, EMG analysis during quadriceps exercise with superimposed vibration stimulus either showed increased vastus lateralis muscle activity (5,16) or signs of increasing fatigue (9,23). In one of these investigations, vibrations were directly applied to the quadriceps muscle during heavy leg extension exercise on a leg extension device in the sitting position (16). In the other studies, subjects performed unloaded 60-second isometric exercise (5) or unloaded dynamic exercise with a 4-minute duration (9,23) on a vertically vibrating platform. Thus, the quadriceps exercise performed in these studies was very different from WBV exercise of the present investigation. In our study, 1 set of 10 squats with external load lasted for 30 seconds and the 5 sets were interspersed by 3 minutes of rest. Furthermore, we used a platform that oscillates around a horizontal axis with reciprocal displacements of the right and left side of the platform. In contrast to WBV exercise on a vertically vibrating platform, oscillations are not appreciably distributed above the mid torso. In this setting, differences in muscle activation between WBVS and conventional squatting were obvious, not only because of the enhanced overall nIEMG during the 5 sets of 10 squats performed on a vibration platform. There also was a strong tendency for a gradual increase in mean nIEMG during 5 sets of WBVS, whereas nIEMG significantly decreased when squatting was performed without vibration stimulus. Despite enhanced muscle activity during WBVS, the comparable decrease in MVC after WBVS and CON indicates a similar level of fatigue after dynamic squatting exercise with and without WBV as previously suggested by Rittweger et al. (20). In contrast, WBV seems to increase fatigue during isometric exercise as Erskine et al. (10) observed a significant decrease in MVC after 10 repetitions of WBV isometric half squat exercises while MVC remained unchanged after conventional training.

According to the principle of orderly recruitment, which states that small motoneurons of type I fibers are recruited first, enhanced recruitment of larger type II fibers with high thresholds can be assumed with increasing myoelectric activity during WBVS. Furthermore, the significantly larger exercise-induced increase in capillary lactate after WBVS compared with CON might indicate such enhanced recruitment of the glycolytic type II fibers during WBVS. To our knowledge, effects on muscle metabolism induced by vibrations, applied to the exercising muscles during strength training, have only scarcely been investigated and there is no other study to provide capillary lactate concentrations during or after WBV strength training compared with equivalent conventional strength training. However, our results do not provide valid proof of the type of muscle fiber activated. Further research, for example, biopsy studies, is needed to examine the metabolic activity as altered with WBV.

Enhanced fiber recruitment during WBV strength training compared with conventional strength training would explain the superiority of squat training performed on a vibration platform observed by Ronnestad (22). In his study, strength-trained athletes were subjected to 5 weeks of periodized squat training carrying a barbell on their shoulders for the application of additional load, similar to WBVS in our investigation. The results of one other study also support the assumption that trained muscles might benefit from a vibration stimulus additionally applied during heavy resistance exercise: When vibrations were directly applied to the exercising muscles during 3 weeks of strength training in physical educational students, enhanced training-induced increases in maximal strength were observed (11). In contrast, Kvorning et al. (13) reported no additional effects of WBV squat training with additional load compared with equivalent conventional squat training and unloaded WBV exercise in untrained subjects.

As suggested by Wilcock et al. (25) in their review article, it seems likely that strength-trained athletes and untrained subjects respond differently to WBV exercise. Trained athletes might benefit from the enhanced muscle activity induced by WBV during heavy resistance training but not from unloaded static and dynamic leg exercises on a vibration platform (8,14). In contrast, significant effects of WBV exercise without external load on jump height and leg extension strength were observed in untrained subjects or athletes without strength training background (2,21,24). In sedentary subjects, WBV training could even induce similar gains in knee extensor strength as a conventional strength training program (9). The diverse findings in sedentary and trained subjects might be explained by an overloading of untrained muscles resulting from adding external load plus vibrations to the exercises of untrained subjects, whereas WBV might be a significant stimulus through enhanced fiber recruitment when additionally applied during heavy resistance training in trained athletes.

Practical Applications

The significant increases in EMG activity of the vastus lateralis muscle and in capillary lactate during WBVS compared with CON in the present study provide strong evidence for enhanced muscle fiber recruitment induced by WBV added to conventional heavy resistance exercise. Apparently, WBVS is a means to intensify quadriceps strength training. However, to avoid overuse or overtraining, it may not be advisable to perform all strength training sessions as heavy resistance exercise with additional WBV or to subject untrained individuals to this type of strength training. Further research is needed to find out how to integrate WBVS, that is, training sessions with increased fiber recruitment, in the periodized strength training programs of athletes for optimal performance enhancement.


The authors want to thank Prof Matthias Lochmann for his invaluable help with the EMG analysis. One Galileo 900 platform was provided for the completion of this study by Novotec (Pforzheim, Germany). No other benefits, financial or otherwise, were provided for the completion of this research. The results do not constitute endorsement of the Galileo 900 whole-body vibration platform by the authors or National Strength and Conditioning Association.


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resistance training; squatting exercise; human muscle; fiber recruitment

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