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Original Research

Relationship Between Lower Limb Muscle Activity and Platform Acceleration During Whole-Body Vibration Exercise

Lienhard, Karin; Vienneau, Jordyn; Nigg, Sandro; Meste, Olivier; Colson, Serge S.; Nigg, Benno M.

Author Information
The Journal of Strength & Conditioning Research: October 2015 - Volume 29 - Issue 10 - p 2844-2853
doi: 10.1519/JSC.0000000000000927
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Abstract

Introduction

Whole-body vibration (WBV) has been gaining significant interest as an exercise modality in athletic settings (6). The benefits of WBV exercises include improved performance immediately after the exposure and after several weeks of training. More specifically, rate of force development (22) and peak force (27) of the lower limbs were increased to a higher extent after a WBV exercise than after the same exercise without vibration. Positive long-term effects of WBV training have also been found with increased jump height (21) and knee extensor strength (10). However, the effects of WBV training on muscle performance remain inconclusive, as other research groups have reported contradictory findings (9,39,13). This inconsistency may be related to the different vibration parameters and protocols used in these studies such as the frequency, amplitude, and platform type.

Improvement in neuromuscular performance after WBV could be attributed to several mechanisms. Whole-body vibration is believed to elicit the tonic vibration reflex (34,38), that is, stretch reflex responses induced by changes in length of the muscle spindles because of rapid changes in the length of the muscle tendon units (7). In this context, WBV has been shown to increase corticospinal pathway excitability not only in upper limb muscles at frequencies above 80 Hz (44) but also more recently in lower limb muscles at 30 Hz (29). Another mechanism occurring during WBV could be muscle damping; increased muscle activity during the vibration changes the natural frequency of the tissue to reduce potential adverse effects (47). As a matter of fact, numerous studies have found that the muscle activity of the lower limbs is greater during WBV than without vibration using surface electromyography (sEMG) recordings (3,7,16,23,25,37,40).

The WBV platforms used in these studies operated by a side-alternating or a vertical synchronous mode, thus inducing vibrations mainly in the vertical direction. Whole-body vibration platforms that vibrate in the horizontal plane are available on the market (31) and seem to have a similar effect on jump performance as the previously mentioned platform types (2). This leads to the assumption that horizontal vibrations increase the muscle activity during the exposure in equal measures as vertical vibrations. Yet, this remains to be determined. Additionally, there is a new model available (i.e., dual mode) that induces vibrations in all 3 planes. Dual-mode WBV platforms are unique in that 2 single modes, individually adjustable in frequency and amplitude, are combined. Unlike the commonly used single-mode platforms that allow vibrations in either the vertical direction (4,6,31,37) or the anterior-posterior and mediolateral directions (2,30,31), dual-mode platforms permit vibrations in all 3 planes. It needs to be verified if dual-mode WBV platforms are more beneficial than single-mode platforms that induce vibrations in either the vertical or horizontal directions.

Considering that WBV exercises are still increasing in popularity, more platform types are produced that differ in their operating mode, frequency, and amplitude. The diverse use of these parameters in WBV studies is partly responsible for the inconsistent findings in the literature. Thus, finding a factor that entails all 3 parameters would allow uniform description of the induced vibrations.

The acceleration load imposed on the neuromuscular system during WBV exercises is characterized by the interaction between the vibration frequency and the amplitude of the platform displacement (28). Thus, the magnitude and direction of the platform acceleration fully describe the vibrations induced by the WBV platform. Previous studies have found that increasing the vibration frequency (15,25,35,37) and the amplitude (15,20,23,25,35) results in augmentation of the sEMG activity, indicating a direct relationship. It remains to be determined whether such a relation is present using the platform acceleration load instead of the frequency and amplitude separately and whether this is the case for vertical, horizontal, and dual-mode vibrations.

Marín et al. (26) tested the effect of frequency and amplitude vs. the effect of the resulting vertical platform acceleration. Two WBV conditions consisting of different frequencies and amplitudes that added up to identical acceleration magnitudes were used. In that study, similar increases in sEMG activity between the 2 conditions were found, indicating that the main load parameter is rather the vertical acceleration than the frequency and amplitude, per se. Besides the changes in muscle activity during the exposure to WBV, the vertical acceleration load also seems to be the key factor for immediate and chronic benefits. Using different frequencies and amplitudes adding up to the same vertical acceleration load showed identical adaptations after WBV exercises (5,45). Although many studies have been searching for the frequency-amplitude combination and the platform type that maximize the muscle activity of the lower limbs, it would be more convenient to determine the magnitude and direction of the platform acceleration that is required for significant increases in sEMG activity.

Therefore, the goal of this study was to verify if vertical, horizontal, and dual-mode vibrations enhance the muscle activity of selected lower limb muscles during the WBV exercise. A further goal was to test whether there is a relationship between the platform acceleration and the sEMG magnitude. Based on previous results, it was hypothesized that (1) vertical, horizontal, and dual-mode vibrations would lead to significant increases in sEMG activity compared with no vibration and that (2) there would be a strong relationship between the platform acceleration and the lower limb sEMG activity.

Methods

Experimental Approach to the Problem

To test the first hypothesis, a WBV platform was used that delivers vibrations in the vertical direction by a side-alternating mode, in the horizontal direction by a circular mode, and in all 3 planes by a dual mode. To test the second hypothesis, different acceleration thresholds were induced using the lowest and the highest possible frequency-amplitude settings for each vibration mode. All measurements were performed during 1 visit, and the participants were familiarized with the WBV stimulus before the assessment. The sEMG activity of 6 lower limb muscles was measured while the participants performed static squats at a knee angle of 60° (0° corresponding to fully extended). For comparison, control trials without vibration were assessed. The sEMG magnitude measured during the WBV and control trials was normalized to the sEMG magnitude measured during isometric maximal voluntary contractions (MVCs).

Subjects

Thirty physically active adults (15 males and 15 females, age: 25.9 ± 4.3 years, height: 171.4 ± 9.5 cm, body mass: 69.5 ± 11.2 kg; mean ± SD) participated in this study. All participants were new to WBV platforms and free from injuries to the lower extremities. The study was approved by the University of Calgary's Conjoint Health Research Ethics Board, and all participants provided informed consent before their participation.

Surface Electromyography Recordings

Surface electromyography electrodes were placed on the muscle bellies of the tibialis anterior (TA), gastrocnemius medialis (GM), soleus (SOL), vastus lateralis (VL), vastus medialis (VM), and biceps femoris (BF) of the right lower limb according to the SENIAM guidelines (17) using bipolar surface electrodes (Ag-AgCl) with preamplifiers (Biovision, Wehrheim, Germany). A ground electrode was placed on the right tibial tuberosity. The electrodes had a diameter of 10 mm and interelectrode spacing of 22 mm. Before the placement of the electrodes, the skin was shaved, dead skin cells were removed using abrasive tape, and the area was cleaned with an isopropyl wipe. The electrodes were secured to the skin with Cover-Roll stretch tape (Beiersdorf AG, Hamburg, Germany). Voltage signals were preamplified at the source (gain = 1,000) and sampled at 2,400 Hz.

Assessment of Maximal Voluntary Contractions

After completion of a standardized 5-minute warm-up on a stationary bike, 2 isometric MVCs were completed for each contraction in the order of plantarflexion, dorsiflexion, knee extension, and knee flexion on an isokinetic dynamometer (Biodex System 3, Medical Systems Inc., New York, NY, USA). Maximal voluntary contraction measurements of the ankle (48) and the knee (46) have previously been shown to provide reliable data. Surface electromyography activity of the SOL and GM was recorded during plantarflexion, TA during dorsiflexion, VL and VM during knee extension, and BF during knee flexion. Plantarflexion and dorsiflexion were performed at an ankle joint angle of 90° (neutral position), and the rotational axis of the dynamometer was visually aligned to the lateral malleolus. Knee extension and flexion were performed at a knee angle of 60° (0° corresponding to fully extended), and the rotational axis of the dynamometer was aligned to the lateral femoral condyle. All participants completed 3 familiarization trials at submaximal strength followed by two 5-second MVC trials for each setting for a total of 8 MVCs. For the MVC trials, the main instruction was to “contract as hard as you can” and to “maintain this force at a constant level.” During the contractions, the participants were verbally encouraged. After each trial, the participants were given a 1-minute break. In the event of an increase or decrease in strength of more than 10% between the 2 MVCs, a third trial was conducted. During each 5-second contraction, the peak MVC torque over 500 milliseconds was calculated using a sliding window function. As there were 2 trials for each setting, the higher torque was retained to calculate the maximal sEMG. The root mean square from the sEMG data (sEMGRMS) was computed during the 500-millisecond period of the selected torque output from the muscle corresponding to the contraction.

Whole-Body Vibration Platform

The platform used in this study (TBS 100A; Total Image Fitness, Calgary, Alberta, Canada) delivers oscillations by 2 single modes (i.e., side-alternating and circular mode) and a dual mode that is the combination of the 2 single modes. The side-alternating mode generates vibrations mainly in the vertical plane by rotation along the sagittal axis, with a frequency range of 6–16 Hz. The amplitude of the vibration is dictated by the positioning of the feet in relation to the axis of rotation and was determined by double integration of the platform acceleration signal using a 3-dimensional accelerometer (ADXL 78, encapsulated in a 20 × 12 × 5 mm plastic shell, measuring range ±70 g, frequency response of 0–400 Hz, mass: 2 g; Analog Devices USA) that was placed on the platform in line with the third toe (24). A foot position of 23.3 cm from the central axis corresponded to an amplitude (displacement from baseline to peak (36)) of 4 mm, and a foot position of 14.3 cm corresponded to an amplitude of 2.5 mm. The circular mode generates vibrations mainly in the horizontal plane by circular rotation with a fixed amplitude (radius of the circular movement) of 0.8 mm. The frequency for this mode ranges from 14 to 43 Hz. The dual mode of this platform consists of the combination of the 2 single modes, whereas the intensity of each mode is individually adjustable. Figure 1 illustrates the acceleration signals measured on the platform level during side-alternating, circular, and dual-mode WBV in the anterior-posterior plane (Acc a-p), the mediolateral plane (Acc m-l), and the vertical plane (Acc v).

Figure 1
Figure 1:
Surface electromyography and platform acceleration data of 1 participant during side-alternating WBV at 16 Hz and 4 mm amplitude (left panel), during circular WBV at 43 Hz and 0.8 mm amplitude (middle), and during dual-mode WBV consisting of the combination of the side-alternating (16 Hz, 4 mm) and the circular (43 Hz, 0.8 mm) mode (right panel). The top 3 rows show sEMG signals of the gastrocnemius medialis, vastus lateralis, and biceps femoris. The bottom 3 rows illustrate the platform acceleration signals in the anterior-posterior plane (Acc a-p), mediolateral plane (Acc m-l), and the vertical plane (Acc v). sEMG = surface electromyography; WBV = whole-body vibration.

Whole-Body Vibration Protocol

The WBV assessment was conducted after the MVC measurements, with a 10-minute break in between. The WBV protocol consisted for each mode of the lowest and the highest possible intensity settings to achieve a wide range of the acceleration magnitudes. For the side-alternating mode, the low-intensity setting consisted of the lowest platform setting, which was 6 Hz and an amplitude of 2.5 mm (Side-L) corresponding to the narrow foot position. The high-intensity setting consisted of the highest platform setting, which was 16 Hz and an amplitude (displacement from baseline to peak) of 4 mm (Side-H) corresponding to the wide foot position. For the circular mode, the low- and high-intensity settings were 14 Hz (Circ-L) and 43 Hz (Circ-H), respectively, with a fixed amplitude of 0.8 mm. Both circular mode trials were performed using the narrow foot position. To create dual-mode WBV, both side-alternating settings and both circular settings were combined. This resulted in 4 dual-mode settings (Side-L/Circ-L, Side-L/Circ-H, Side-H/Circ-L, and Side-H/Circ-H). Additionally, control trials without vibration were assessed for the wide and the narrow foot positions, which resulted in a total of 10 trials (2 control trials + 2 side-alternating trials + 2 circular trials + 4 dual-mode trials), presented in a randomized order (Table 1). Each trial consisted of 20 seconds of vibration with a 1-minute break between trials. During the exercise, the participants were advised to perform a static squat at a knee angle of 60°. The knee flexion angle was monitored using a digital goniometer (SG150 Twin Axis; Biometrics Ltd., Newport, United Kingdom) that was taped to the participant's left knee. During the exercise on the WBV platform, the participants were provided with a visual real-time feedback of their knee angle, illustrated on a laptop screen using MATLAB software (version 7.13; The MathWorks, Inc., Natick, MA, USA). The trial was repeated if the measured knee angle varied more than ±3° at any point during the exercise. The participants stood barefoot on the WBV platform to avoid damping effects because of different footwear, and they were instructed not to hold onto the handle bar of the platform.

Table 1
Table 1:
Whole-body vibration trials assessed in this study and their mean acceleration amplitudes (displacement from baseline to peak, mean ±SD).*

Surface Electromyography Analysis

Surface electromyography analysis was performed using MATLAB software (version 7.13; The MathWorks). The first 7 and last 3 seconds were clipped from the sEMG signals to delete potential error signals originating from, for example, the onset of the platform or the reflex activity. Then, the sEMG signals were filtered using a band-pass wavelet filter with a low cutoff frequency of 5 Hz and a high cutoff frequency of 300 Hz; potential noise occurring from surrounding electrical equipment was removed analogically with the help of a band-stop filter. Surface electromyography signals recorded during WBV contain motion artifacts, visible in the sEMG spectrum at the vibration frequency and its multiple harmonics (14,42). These artifacts were removed by spectral linear interpolation in the sEMG spectrum (23). First, the sEMG signals were transformed into the power spectral density (PSD) with the help of the Welch method, using Hamming windows with the length of L = 1,024. Then, the spikes in the sEMG spectrum were located at the vibration frequency, and its multiple harmonics and the peaks were replaced by a straight line (Figure 2). As this method does not allow a time-reversal transformation, the sEMGRMS was calculated directly within the PSD using the formula:

with

being the interpolated sEMG signal in the PSD and Fe the sampling frequency. Then, sEMGRMS during WBV and the control trials were calculated and expressed as a percentage value of the maximum sEMGRMS of the respective muscle obtained during the MVC trials using the formula:

Figure 2
Figure 2:
Example of an sEMG signal during side-alternating WBV at 16 Hz in the power spectral density. The black signal represents the unfiltered sEMG and the gray signal the linear interpolated sEMG. sEMG = surface electromyography; WBV = whole-body vibration.

Statistical Analyses

All statistical procedures were performed using IBM SPSS software (Version 20; Chicago, IL, USA). The Kolmogorov-Smirnov test confirmed the normality of the data. One-way repeated analysis of variance was used to compare sEMGRMS recorded during WBV with the sEMGRMS during no vibration. The WBV trials with the wide foot position (Side-H, Side-H/Circ-L, Side-H/Circ-H) were compared with the no-vibration trial with the wide foot position, and the WBV trials with the narrow foot position (Side-L, Circ-L, Circ-H, Side-L/Circ-L, Side-L/Circ-H) were compared with the no-vibration trial with the narrow foot position. The alpha level was adjusted using the Bonferroni's correction, which resulted in a level of significance of α = 0.017 for the wide foot position and α = 0.01 for the narrow foot position. Pearson's correlation coefficients were computed between the sEMGRMS averaged for all muscles and the platform acceleration of each plane. Furthermore, a linear regression analysis was performed on the sEMGRMS averaged for all muscles, including the side-alternating, circular, and dual-mode trials. Only the directions of the platform acceleration that showed a high and significant correlation with the lower-body sEMGRMS were included as predictors. The level of significance was set at α = 0.05, and the results are presented as mean ± SD.

Results

Whole-Body Vibration Compared With No Vibration

Figure 3 illustrates the sEMGRMS during side-alternating, circular, and dual-mode WBV compared with the no-vibration trial. Comparing side-alternating WBV with the corresponding no-vibration trial, significant increases in sEMGRMS were found for Side-H in the TA (percentage change between scores: +35.3 ± 8.2%, F(1,29) = 8.38, p = 0.007), the GM (+134.5 ± 9.1%, F(1,29) = 46.23, p < 0.001), and the SOL (+138.2 ± 17.6%, F(1,29) = 17.42, p < 0.001).

Figure 3
Figure 3:
Root mean square of the surface electromyography (sEMGRMS) during side-alternating (L: 6 Hz, 2.5 mm; H: 16 Hz, 4 mm), circular (L: 14 Hz, 0.8 mm; H: 43 Hz, 0.8 mm), and dual-mode (L–L: 6 Hz, 2.5 mm–14 Hz, 0.8 mm; L–H: 6 Hz, 2.5 mm–43 Hz, 0.8 mm; H–L: 16 Hz, 4 mm–14 Hz, 0.8 mm; H–H: 16 Hz, 4 mm–43 Hz, 0.8 mm) WBV for all the assessed conditions. The asterisks indicate significant increases in sEMGRMS during WBV (in gray, mean ± SD) compared with the corresponding sEMGRMS measured during the no-vibration trial (in black, *p < 0.017 for wide foot position [side-alternating: H; dual mode: H-L, H-H] and *p < 0.01 for narrow foot position [side-alternating: L; circular: L, H; dual mode: L-L, L-H]). sEMG = surface electromyography; WBV = whole-body vibration.

No significant increases in sEMGRMS were found comparing circular WBV with the corresponding no-vibration trial (p > 0.05).

Dual-mode WBV Side-L/Circ-L showed significantly higher sEMGRMS compared with no vibration in the GM (+58.9 ± 6.7%, F(1,29) = 12.57, p = 0.001). The dual-mode trial Side-L/Circ-H significantly enhanced muscle activity in the GM (+62.9 ± 7.5%, F(1,29) = 17.32, p < 0.001), SOL (+53.8 ± 8.7%, F(1,29) = 9.63, p = 0.004), VL (+17.3 ± 2.5%, F(1,29) = 7.91, p = 0.009), VM (+20.5 ± 2.7%, F(1,29) = 11.44, p = 0.002), and BF (+54.7 ± 5.8%, F(1,29) = 28.05, p < 0.001). sEMGRMS during dual-mode WBV Side-H/Circ-L was significantly higher than the no-vibration trial in the TA (+54.5 ± 11.8%, F(1,29) = 13.84, p = 0.001), GM (+226.5 ± 19.9%, F(1,29) = 12.51, p = 0.001), SOL (+120.8 ± 10.2%, F(1,29) = 32.90, p < 0.001), VL (+11.5 ± 2.5%, F(1,29) = 8.35, p = 0.007), and VM (+7.4 ± 2.2%, F(1,29) = 7.45, p = 0.01). Finally, the dual-mode trial Side-H/Circ-H significantly enhanced sEMGRMS compared with no vibration in all the measured muscles (TA: +48.0 ± 5.5%, F(1,29) = 30.39, p < 0.001; GM: +207.0 ± 13.9%, F(1,29) = 21.23, p < 0.001; SOL: +134.1 ± 10.1%, F(1,29) = 13.81, p = 0.001; VL: +21.9 ± 2.9%, F(1,29) = 16.37, p < 0.001; VM: +23.9 ± 3.2%, F(1,29) = 15.15, p < 0.001; BF: +61.9 ± 5.9%, F(1,29) = 19.64, p < 0.001).

Relation Between sEMGRMS and Platform Acceleration

The Pearson's correlation coefficient between the averaged lower limb sEMGRMS and the platform acceleration was high and significant for the vertical component (r = 0.970, p < 0.001) and nonsignificant for the anterior-posterior (r = 0.543, p = 0.164) and mediolateral (r = 0.462, p = 0.249, Figure 4) directions. Therefore, only the vertical platform acceleration was included in the regression analysis. This analysis showed that the vertical direction of the platform acceleration added significantly to the prediction (p < 0.001) with the equation sEMGRMS (%MVC) = 19.39 + (0.10 × Acc v) and with an r value of 0.970.

Figure 4
Figure 4:
Scatter plots of the averaged sEMGRMS for all the measured lower limb muscles and the platform acceleration in the anterior-posterior plane (left), the mediolateral plane (middle), and the vertical plane (right). Each point represents the averaged sEMGRMS of all muscles and participants for a given WBV trial. The vertical platform acceleration showed a high and significant Pearson's correlation coefficient (r = 0.970, p < 0.001) with the lower-body sEMGRMS. MVC = maximal voluntary contraction; WBV = whole-body vibration; sEMGRMS = Root mean square of the surface electromyography.

Discussion

Dual-mode WBV consisting of the high-intensity side-alternating mode or the high-intensity circular mode (Side-H/Circ-L, Side-L/Circ-H, Side-H/Circ-H) showed significant increases in sEMG activity of most muscles compared with the no-vibration trial. Additionally, side-alternating WBV at high intensity (Side-H) significantly increased the sEMG activity of the lower leg muscles. Furthermore, the magnitude of the sEMG activity of the lower limb muscles was only positively correlated to the magnitude of the vertical platform acceleration. Additionally, the vertical component of the platform acceleration was a significant predictor of the muscle activity.

To the best of our knowledge, this is the first study quantifying muscle activity during circular WBV in the horizontal plane and during dual-mode WBV in all 3 planes. Numerous studies have investigated the effects of side-alternating WBV on muscle activity and, similar to this study, found significant increases in sEMG activity compared with baseline values (1,7,37). Furthermore, the measured sEMGRMS values expressed as a percentage of the maximum are comparable with the ones found in previous studies (32,35) using the same platform type and acceleration magnitudes measured at the platform level.

Against the first hypothesis, the circular mode did not result in significant increases in sEMG activity in any of the measured muscles. The lacking effect of the circular mode could be linked to the finding that only the magnitude of the vertical platform acceleration was positively correlated to the magnitude of the sEMG activity. Thus, it can be concluded that horizontal vibrations have little effect on the sEMG activity of the lower limbs, which could explain why the circular mode in the horizontal plane did not elevate the muscle activity. However, the second hypothesis was confirmed only for the vertical component of the platform acceleration, as a linear relationship was found with the sEMG activity of the lower limb muscles. This finding seems to be applicable to various platform types, as a previous study performed on a synchronous WBV platform reported that the vertical acceleration of the platform was correlated with the lower-body sEMG activity, although the acceleration was not a predictor of the muscle activity (25).

The mechanism often proposed for the increases in sEMG activity during WBV compared with no vibration is reflex responses induced by activation of muscle spindles because of changes in length in the muscle tendon units (7). Indeed, recent studies have shown evidence that synchronous motor unit activity (34) and a tonic vibration reflex-like responses (49) occur during WBV and might account for the augmented sEMG activity. It is possible that reflex responses were only provoked during vertical vibrations because of the anatomical function of the muscles that were measured in this study. The selected lower limb muscles induced movements mainly in the sagittal plane, thus reflex responses would be expected during a stretch in the vertical plane, that is, during vertical accelerations. This could explain why the vertical, and not the horizontal vibrations, increased the sEMG activity of the selected lower limb muscles. Furthermore, increased muscle activity as a response to vibration has been described as a mechanism to damp the oscillations to protect the soft tissue from damage (47). This mechanism might occur primarily during vertical than horizontal vibrations. It is important to mention that no studies have investigated whether these responses could also occur during horizontal WBV. Also, no measurements were made in this study to ascertain the occurrence of these mechanisms.

The linear relationship between the vertical platform acceleration and the lower-body sEMG indicates that WBV exercises should be performed using a side-alternating or a vertical synchronous WBV platform. Dual-mode platforms are another option, although it needs to be considered that the additional costs and resources required for the accelerations in the horizontal plane do not seem to be necessary. Another result following from this study, which is in line with previous investigations (32,35,37), is that the highest increases in muscle activity can be achieved by the highest frequency-amplitude combination available in the vertical direction. Although no acceleration limit has been determined specifically for WBV exercises (18), using the highest frequency-amplitude combination might be hazardous (31,41). For this reason, it would be advised to induce vibrations that are relatively low but that are still providing a significant stimulus to the muscles. From the current data, it can be concluded that vertical accelerations below 18 m·s−2 (∼1.8 g) have little to no effect on muscle activity levels. The only condition that induced significant increases in sEMG activity with vertical accelerations below 18 m·s−2 was the dual-mode Side-L/Circ-L condition (4 m·s−2) in the GM. Conversely, vertical accelerations of 18 m·s−2 and higher resulted in a significant enhancement in muscle activity in almost all the measured muscles. Thus, it can be concluded that the vertical acceleration threshold that is required to significantly increase the sEMG activity of the lower limb muscles is around 18 m·s−2. However, this conclusion is only applicable to side-alternating WBV platforms. Synchronous WBV platforms have been shown to induce lower responses in sEMG activity compared with side-alternating WBV platforms (1,37), despite using the same acceleration load. Hence, the marginal threshold of a synchronous WBV platform is likely higher than 18 m·s−2 and needs to be quantified in future studies.

Enhanced performance immediately or several weeks after WBV training has been strongly associated with vibration-induced increases in sEMG activity during the exposure and not with the resistance training, per se (10,21,22,27). In fact, in these WBV studies, vertical accelerations above the here-established acceleration threshold of 18 m·s−2 were induced, which resulted in immediate improvement in the rate of force development (22) and peak force (27) of the lower limbs and in long-term enhancement of jump height (21) and knee extensor strength (10). It has also been shown that using a WBV training protocol with a high–frequency-amplitude setting leads to improved knee extensor strength and jump performance as compared with a low–frequency-amplitude setting (33). In particular, the greatest responses from the neuromuscular system are produced by maximizing the sEMG responses during the vibration (11,12). Together with the findings of this study, this indicates that WBV training protocols performed on side-alternating platforms should be conducted with a minimal vertical acceleration load of 18 m·s−2. Additionally, greater performance-related improvements can be expected the greater the increases in sEMG activity during the WBV exercise. However, additional research with WBV training interventions is required to confirm these assumptions, and the potential effects of horizontal and dual-mode vibrations on performance-related measures need to be studied.

Future studies should also focus on the effect of horizontal vibrations, as it is possible that vibrations in the horizontal plane affect the sEMG activity of muscles that induce movements not mainly in the sagittal plane (e.g., invertors and evertors of the ankle). Omission of such muscle groups in this study may explain the lacking effect of horizontal vibrations on muscle activity levels. Another possible reason why no effect of horizontal vibrations was found could be the characteristics of the used platform. Even if designed to induce mainly horizontal vibrations in the circular mode, substantial vibrations were induced in the vertical plane. Thus, it was impossible to verify the isolated effect of horizontal vibrations, which poses a limitation to this study. Future studies are required to clarify the neuromuscular changes during the exposure to horizontal WBV that seems to lead to improved balance (43) and jump performance (2).

Another limitation of this study was the relatively low-frequency range of the side-alternating mode. Commonly, side-alternating platform vibration frequencies reach up to 30 Hz (1,32,35,37), whereas the maximal frequency of the current platform was 16 Hz. Nevertheless, combined with the amplitude, this frequency setting induced a vertical acceleration platform of 50 m·s−2, which was sufficient to significantly increase the sEMG activity of the calf muscles.

There are also limitations specific to the determination of the acceleration threshold. First, when resonance occurs, increased muscle activity is expected to reduce resonance by changing the natural frequency or the damping characteristics of the tissue (47). Although it was shown that the vertical platform acceleration determined by the frequency and amplitude was linearly related to the lower-body sEMG, this relationship might deviate for resonance frequencies. Second, the acceleration threshold was evaluated using a sample of 8 acceleration magnitudes (Table 1), which were unequally distributed. Therefore, it is possible that the true threshold was not exactly at 18 m·s−2. Third, the acceleration thresholds were assessed for the averaged lower-body sEMG and not for each muscle separately for simplicity reasons. Also, the accelerations were assessed on the platform and not on the respective muscle tissue. It is likely that the acceleration threshold would be different for certain muscle groups, especially considering that the vibrations are damped throughout the body (8,19). Fourth, it needs to be considered that the thresholds were determined in a young, healthy, physically active study population. Lower thresholds might be sufficient in older or health-compromised individuals, whereas higher thresholds might be required in professional athletes.

Practical Applications

The results of this study suggest that trainers and therapists should advise their clients to exercise on WBV platforms that induce vibrations mainly in the vertical plane such as side-alternating or vertical synchronous platforms. The linear relationship that was found between the vertical platform acceleration and the lower-body sEMG activity suggests that the stimulus can be increased by increasing the vertical acceleration load. If there is a concern with regard to high accelerations, frequencies and amplitudes resulting in a minimal vertical platform acceleration of 18 m·s−2 (∼1.8 g) can be used. Vertical accelerations of this magnitude and higher resulted in significant increases in sEMG activity of most lower limb muscles. However, it is recommended to measure the vertical acceleration effectively delivered by the platform rather than using the theoretical frequency and amplitude provided by the platform manufacturers.

Acknowledgments

The vibration platform was provided by Total Image Fitness, Calgary, Alberta, Canada. The study was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Total Image Fitness, Calgary, Alberta, Canada. Financial support for travel-related expenses was obtained from the Fondation Partenariale DreamIT, France. The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.

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Keywords:

surface electromyography; muscle strength; frequency; amplitude; dual-mode platform

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