Evaluation of Muscle Activity for Loaded and Unloaded Dynamic Squats during Vertical Whole-Body Vibration

Abstract

Hazell, TJ, Kenno, KA, and Jakobi, JM. Evaluation of muscle activity for loaded and unloaded dynamic squats during vertical whole-body vibration. J Strength Cond Res 24(7): 1860-1865, 2010-The purpose of this investigation was to examine if the addition of a light external load would enhance whole-body vibration (WBV)-induced increases in muscle activity during dynamic squatting in 4 leg muscles. Thirteen recreationally active male university students performed a series of dynamic squats (unloaded with no WBV, unloaded with WBV, loaded with no WBV, and loaded with WBV). The load was set to 30% of body mass and WBV included 25-, 35-, and 45-Hz frequencies with 4-mm amplitude. Muscle activity was recorded with surface electromyography (EMG) on the vastus lateralis (VL), biceps femoris (BF), tibialis anterior (TA), and gastrocnemius (GC) and is reported as EMGrms (root mean square) normalized to %maximal voluntary exertion. During unloaded dynamic squats, exposure to WBV (45 Hz) significantly (p < 0.05) increased baseline muscle activity in all muscles, except the TA compared with no WBV. Adding a light external load without WBV increased baseline muscle activity of the squat exercise in all muscles but decreased the TA. This loaded level of muscle activity was further increased with WBV (45 Hz) in all muscles. The WBV-induced increases in muscle activity in the loaded condition (∼3.5%) were of a similar magnitude to the WBV-induced increases during the unloaded condition (∼2.5%) demonstrating the addition of WBV to unloaded or loaded dynamic squatting results in an increase in muscle activity. These results demonstrate the potential effectiveness of using external loads with exposure to WBV.

Introduction

Whole-body vibration (WBV) is a relatively new exercise modality gaining significant interest in the health and fitness realm. This type of exercise, typically involves individuals performing traditional resistance exercise on the platform with their body mass as resistance. Although there is a wide range of exercise protocols used, the positive results of WBV studies report enhanced strength, power, and endurance, and blood flow in working skeletal muscle (^{3,9,13,15,18,21,23,24,27,33,37}); however, there are reports to the contrary suggesting no training benefit of WBV exercise (^12,20,32). Regardless, there are many studies indicating that exposure to WBV during static and dynamic body mass exercises increases exercise intensity (^{1,8,10,17,31,34}).

The mechanical vibrations generated by the WBV platform are thought to induce length changes in extrafusal fibers resulting in the activation of an afferent feedback response through the muscle spindles and 1a afferents, a response akin to the tonic vibration reflex (TVR) (^7,16). The mechanical vibration stimulus may also affect skin and joint receptors that provide sensory input to the gamma motor system increasing the sensitivity and responsiveness of the muscle spindle to further mechanical perturbations (^7,14,28). The WBV perturbations put a high level of stress on the muscular system that requires high levels of neuromuscular activity (^1,8,17,25,34).

Recently, electromyography (EMG) has been used to characterize muscle activation, and previous literature has demonstrated that exposure to WBV results in an increase in leg muscle EMG activity (^1,8,17,25,34). Our group has even shown that exposure to WBV increases EMG in the lower limbs but has little or no effect on the upper limbs (¹⁷). The discrepancy may be the result of the proximity of the muscles to the vibration stimulus (distance from platform), the dampening of the stimulus in the upper body because of body position (³⁸), or the effect that body mass has on loading the lower limbs. If WBV induces a TVR response, then increasing the sensitivity of the system may result in a greater WBV response because it has been reported that the sensitivity of 1a afferents is increased in response to an increased preceding level of muscle activity (^4,6,26,29). Furthermore, from a biomechanical perspective, WBV exposure increases exercise intensity by increasing the accelerations of the body and because force = mass × acceleration, increasing the mass at a given acceleration (WBV stimulus) the resultant should be an increase in force. Although EMG is not a direct measure of force, the 2 are strongly correlated, so any increase in force should be represented by an increase in EMG. Therefore, the addition of a light load should increase baseline muscle activity (preactivation) and potentially increase the sensitivity of the 1a afferents leading to an increase magnitude of the previously demonstrated WBV-induced increase in an unloaded condition. The purpose of this study was to examine the muscle activity in the lower limbs during unloaded and loaded dynamic squatting with WBV. We hypothesize that the addition of a load to dynamic squatting during WBV exposure will augment the WBV response compared with the same condition with no additional load.

Methods

Experimental Approach to the Problem

This study investigated whether the addition of a light external load could augment the WBV induced increases in muscle activity seen in response to dynamic squatting with no load. Electromyography was used to measure changes in muscle activity (dependent variable), in the vastus lateralis (VL), biceps femoris (BF), tibialis anterior (TA), and gastrocnemius (GC). Muscle activity was assessed during loaded and unloaded dynamic squats in the 2 different exercise conditions-with WBV and without vibration. Three vibratory stimuli were used: 25, 35, and 45 Hz at 4-mm amplitude WAVE™. Maximal muscle activation was also recorded to compare muscle activity as a percentage of maximal.

Subjects

Thirteen recreationally active male Kinesiology students (23 ± 2.0 years; 178 ± 6.3 cm; and 84 ± 11.9 kg) volunteered to participate in this study. All were healthy as assessed by the PAR-Q health questionnaire (³⁹). Subjects had no experience with resistance training over the last 4 months. Before any participation, the experimental procedures and potential risks were explained to the subjects, and all subjects provided written informed consent. This study was approved by the University of Windsor Research Ethics.

Electromyographic Electrode Placement

Electromyographic electrodes were placed on the VL, BF, TA, and GC over the midbelly of the muscle parallel to the direction of the fibers (¹⁹). Before electrode placement, the area was shaved, abraded with coarse fabric, and swabbed with alcohol. Interelectrode distance was fixed (10 mm) within the prefabricated electrode bar. High-conductivity electrolyte gel was used with the reference electrode (R200 Biometrics) that was positioned on the lateral malleolus of the fibula. A goniometer was adhered with double-sided tape to the lateral side of the left knee to ensure a consistent dynamic squat (descent to ∼90°; ascent to ∼160°). Also, verbal feedback was provided to ensure consistent performance of the dynamic squat, and a metronome was used to ensure correct cadence (see details below).

Protocol

All subjects performed a familiarization trial, employing both static and dynamic squats, to acclimate subjects to the WBV stimulus. A demonstration of proper technique for the dynamic squat movement occurred, and practice of this movement was undertaken until performance of the squat was consistent and correct. Electromyographic electrodes were not used during the familiarization session.

The vibration stimulus was 4 mm for 3 frequencies (25, 35, 45 Hz) applied with a WAVE™ platform (Whole-body Advanced Vibration Exercise, Windsor, Canada) that oscillates vertically up and down. We have previously demonstrated increases in muscle activity with these WBV stimuli (¹⁷). The load applied in this study was 30% of the subject's body mass (range 20-33 kg) and was applied using a standard 180-cm Olympic bar (∼20.5 kg) with appropriate additional weights held behind the neck and resting atop the shoulders and upper back. This load was selected to provide a reasonable and safe weight that untrained individuals could lift without inducing fatigue and ensuring safety. The 8 conditions examined were as follows: (a) no vibration no load; (b) no vibration load; (c) 25 Hz 4-mm no load; (d) 25 Hz 4-mm load; (e) 35 Hz 4-mm no load; (f) 35 Hz 4-mm load; (g) 45 Hz 4-mm no load; and (h) 45 Hz 4-mm load. All conditions were randomized within and between subjects, and the WBV frequency and amplitude were not verbally conveyed.

Within each condition, subjects were required to complete 7 dynamic squats at a cadence of 1 second down and 1 second up with the use of a metronome with a 5-minute rest period between conditions to prevent fatigue. Platform foot position was slightly wider than shoulder width and was marked on the first trial to be used for all further trials.

The experimental session was held at least 72 hours postfamiliarization protocol, and subjects refrained from exercise or the ingestion of caffeine for 24 hours and did not eat at least 2 hours before any visit to the laboratory. During the actual testing session, subjects performed an EMG noise trial to determine the amount of baseline interference in the spectrum (rested supine on a mat and EMG was measured for 1 minute to determine the inherent noise within the signal), which was then deleted from further EMG signals collected (customized software see EMG Analysis below). Subjects were then asked to perform maximal voluntary exertion (MVE) tests for the muscle groups being evaluated. The MVE trials obtained a maximal EMG profile rather than force output measures; dynamometers were unavailable for all muscles studied (¹⁷). All MVEs were isometric (at 90° joint angles) and were performed 3 times against resistance provided by an immovable object for the VL (knee extension), BF (knee flexion), TA (dorsiflexion), and GC (plantar flexion) muscles. Subjects were provided with approximately 10-minutes rest before beginning the 8 sets of dynamic squats as outlined above.

Electromyographic Analysis

The EMG signal was preamplified by a gain of 1,000 and sampled at 1,000 Hz (DataLOG, Biometrics Ltd., Gwent, United Kingdom), bandpass filtered (20-450 Hz), and stored for offline analysis on a 512-MB MMC flashcard. The EMG was postprocessed using customized software (Labview, National Instruments, Austin, TX, USA); the EMG data were extracted, and the start and end points of the 7 dynamic squats per session were marked. The interference EMG was dual passed sixth-order Butterworth filtered between 100 and 450 Hz, which removed any noise caused by the frequency of the vibration platform (^17,30). The data were then full wave rectified and smoothed with a low-pass filter at 1.5 Hz. The noise was then subtracted, and the data were divided by MVE and multiplied by 100 for normalization. The EMGrms (root mean square) was then calculated.

Statistical Analyses

Analysis was performed using SigmaStat (Version 3.5). A 4 × 2 repeated measures analysis of variance was used to evaluate the independent variables of vibration and load on muscle activity (EMGrms). Post hoc tests were performed using Tukey's Honestly Significant Difference (HSD) tests. Data in the text are values ± SD, whereas figures are reported as values ± SEM, and the level of statistical significance was set at p ≤ 0.05.

Results

Vastus Lateralis

There was no significant vibration × load interaction (p = 0.172). There were main effects for vibration (p = 0.004) and load (p < 0.001). During unloaded dynamic squats without the addition of WBV, VL muscle activity was 42.5 ± 16.7% MVE. Relative to no WBV, exposure to WBV in the unloaded condition increased muscle activity 1.4-5.5% (Figure 1). The 45-Hz vibration condition was significantly greater compared with the no vibration condition (p = 0.009). The addition of a load to the unloaded no WBV condition increased muscle activity 3.2% to 9.7 ± 6.3% (p < 0.001). Subsequent exposure to WBV in this loaded condition further increased muscle activity 0-3.6% (Figure 1). The 45-Hz condition was significantly increased vs. the 25-Hz condition (p = 0.017).

Figure 1: Increases in vastus lateralis muscle activity during unloaded and loaded dynamic squats with and without whole-body vibration. Values are mean ± SEM. All loaded conditions are significantly greater compared with the unloaded condition (p < 0.001). Statistics (A,B) are within condition comparisons. A) Significantly greater than no vibration no load condition (p < 0.09). B) Significantly greater than 25-Hz condition (p < 0.017).

Biceps Femoris

There was no significant interaction between vibration and load (p = 0.368), but main effects were evident for vibration and load (p < 0.001). During unloaded squats without the addition of WBV, BF muscle activity was 6.5 ± 4.2%. Exposure to WBV in the unloaded condition increased muscle activity 0.5-1.7% compared with the no WBV condition (Figure 2), where the 45-Hz condition was significantly increased vs. the no-vibration (p < 0.001) and 25-Hz (p = 0.008) conditions. The addition of a load to the unloaded no WBV condition increased muscle activity from 3.2 to 9.7 ± 6.3% (p < 0.001). Subsequent exposure to WBV, in this loaded condition, further increased muscle activity 0.2-2.0% MVE (Figure 2) compared with the no WBV condition, where 45 Hz was significantly increased vs. the no-vibration (p < 0.001), 25-Hz (p < 0.001), and 35-Hz (p = 0.031) conditions, and the 35-Hz condition was also significantly increased over the no vibration (p = 0.038) condition.

Figure 2: Increases in biceps femoris muscle activity during unloaded and loaded dynamic squats with and without whole-body vibration. Values are mean ± SEM. All loaded conditions are significantly increased over corresponding unloaded condition (p < 0.001). Statistics (A-C) are within condition comparisons. A) Significantly greater than no vibration condition (p < 0.038). B) Significantly greater than the 25-Hz condition (p < 0.008). C) Significantly greater than the 35-Hz condition (p < 0.033).

Tibialis Anterior

There was a significant 2-way interaction between vibration and load (p = 0.030) and a main effect for load (p = 0.008); however, there was no main effect for vibration (p = 0.08). During unloaded dynamic squatting without the addition of WBV, TA muscle activity was 34.6 ± 6.9%. Exposure to WBV in this unloaded condition increased muscle activity 0.1-2.3% (Figure 3), where only 45 Hz was significantly increased over the 35-Hz condition (p = 0.026). The addition of a load to the unloaded no WBV condition resulted in a 7.8% decrease in muscle activity (p < 0.001) to 26.8 ± 6.5%. Subsequent exposure to WBV in the loaded condition increased muscle activity 2.6-3.8% (Figure 3), where the 45-Hz condition was significantly increased compared with the no-vibration condition (p = 0.048).

Figure 3: Increases in tibialis anterior muscle activity during unloaded and loaded dynamic squats with and without whole-body vibration. Values are mean ± SEM. All loaded conditions except 35 Hz are significantly decreased over corresponding unloaded condition (p < 0.015). Statistics (A,C) are within condition comparisons. A) Significantly greater than no vibration condition (p < 0.05). C) Significantly greater than the 35-Hz condition (p < 0.05). D) Significantly greater than no vibration loaded condition (p < 0.01). (*) Significant decrease between no load and load (p < 0.05).

Gastrocnemius

There was no significant interaction between vibration and load (p = 0.15), although main effects were observed for vibration (p < 0.001) and load (p = 0.010). During unloaded dynamic squatting without WBV, GC muscle activity was 7.5 ± 3.2% MVE. Exposure to WBV in the unloaded condition increased muscle activity 2.9-8.9% (Figure 4), where 45 Hz was significantly increased over both the no WBV (p < 0.001) and the 25-Hz condition (p = 0.042). The addition of a load to the unloaded no WBV condition increased GC muscle activity a further 5.9% MVE (p = 0.04) to 13.4 ± 8.6%. Subsequent exposure to WBV in the loaded condition further increased muscle activity 6.3-9.8% MVE (Figure 4), where the 45-, 35-, and 25-HZ conditions were all significantly increased vs. the no WBV condition (p < 0.001; p = 0.03; p = 0.03, respectively).

Figure 4: Increases in gastrocnemius muscle activity during unloaded and loaded dynamic squats with and without whole-body vibration. Values are mean ± SEM. The loaded condition was significantly greater than the corresponding unloaded condition (p < 0.038). All statistics (A-C) are within condition comparisons. A) Significantly greater than no vibration condition (p < 0.05). B) Significantly greater than the 25-Hz condition (p < 0.05).

Discussion

This study examined whether the increase in muscle activity in the lower limbs that occurs with exposure to WBV during dynamic squats (¹⁷) would be further enhanced with the addition of a light external load. Our previous data demonstrated that WBV resulted in a significant increase in skeletal muscle EMG during static and dynamic squats (¹⁷). Our current data confirm this finding as exposure to WBV with a frequency of 45 Hz, resulted in significant increases in muscle activity in all 4 muscles examined. The addition of a light external load increased muscle activity during dynamic squats as expected; however, WBV increased muscle activity during both unloaded and loaded squats to a similar magnitude.

As expected, the addition of a light external load (30% of body mass) increased baseline EMG activity in the VL (∼15%), BF (∼2%), and GC (∼6%) muscles but decreased TA activity (∼8%). The decrease in TA muscle activity was unexpected and may have been caused by a slight and nonvisual change in center of mass. However, monitored squat technique and posture appeared unaltered. The addition of a load was intended to increase the level of muscle activity and potentially enhance the effects of WBV in all muscles studied. The decrease in muscle activity in the TA, highlights the importance of body position on the platform. To date, there has been no biomechanical evaluations of body position and muscle activation during WBV.

This study demonstrates that exposure to WBV (at 45 Hz) results in increases in muscle activity whether performing loaded or unloaded dynamic squats. This result agrees with those of previous studies, all demonstrating a WBV-induced increase in muscle activity (^1,8,17,25,34). However, the average WBV-induced increase in muscle activity of all 4 muscles was 2.5% in the unloaded condition and a similar 3.5% in the loaded condition suggesting that the addition of a light external load did not increase the sensitivity of the 1a afferents. Moreover, although the addition of a load increased muscle activity, exposure to WBV resulted in a further increase in muscle activity. This agrees with the biomechanical perspective that increasing mass at a given acceleration likely results in an increase in force represented by EMG. Thus, the possibility still remains that the use of heavier loads with WBV may increase the sensitivity of the body to the mechanical perturbations further enhancing EMG muscle activity. This idea requires further investigation.

It has been theorized that WBV oscillations increase muscle activity via a reflex response akin to the TVR (⁷), where Ia afferent activity in the muscle spindles alters surface EMG and single motor unit activity (^5,11,35). Therefore, if WBV induces a similar response to the TVR, then muscle activity might be further augmented by increasing the preceding level of muscle activity with the addition of a load (^4,6,26,29). The current data demonstrate that the addition of a light load did not enhance the WBV-induced increase in EMG activity already demonstrated in the unloaded condition. This suggests the increase in baseline muscle activity did not increase the sensitivity of the 1a afferent and that the skeletal muscle response to WBV may not be as analogous to the TVR as originally proposed (⁷). This lends indirect support to the idea that the increase in EMG muscle activity during WBV may occur because of greater demands for postural stability (¹) or may be the muscle's natural response to dampen an imposed vibration stimulus similar to that seen during running (⁴⁰).

In the practical realm, adding a load to dynamic squatting increases muscle activity in the lower body (with the exception of the TA) and has its greatest affect on the primary muscles generating the squat movement (quadriceps and GC). The addition of WBV to the loaded condition increases muscle activity (3.5%) to a similar magnitude because the increase in EMG activity observed in the unloaded condition (2.5%) suggesting the effect of WBV is independent of load. Moreover, although it appears that the addition of a load may cause the greatest stimulus in the thigh (VL), the major effect of WBV appears to be on the muscles most proximal to the generation of the WBV stimulus (TA and GC). Thus, the unique applicability of WBV may be to not only activate the agonist muscles used during a dynamic movement but to also increase the activation of the synergistic muscles closest to the stimulus in all postural conditions. These WBV-induced increases may be the mechanisms resulting in the improved function seen in several WBV training studies (^{2,13,22-24,33}). The current data (while on young, healthy men) may have practical applications, especially in a rehabilitation setting where the addition of a load to dynamic exercises (typical of resistance exercise) is not yet tolerated. This requires further investigation. Furthermore, it remains intriguing to determine whether the use of heavier external loads would enhance the muscle spindle response and augment EMG activity and muscle function as originally hypothesized.

Practical Applications

Our findings demonstrate exposure to WBV results in a similar magnitude increase in muscle activity during both unloaded and loaded dynamic squats. The effect of using a heavier external load during exposure to WBV remains to be examined, as does body positioning on the platform. The results of this study also demonstrate that the cumulative effect of adding a light external load to dynamic squats with WBV increases the intensity of the exercise being performed. This supports early suggestions that the use of loaded resistance training on a WBV platform might be more beneficial than the same training without WBV (³⁶). Moreover, those individuals that are typically discouraged or unable to perform resistance overload training are likely to benefit from this combined light load WBV exercise program. This could include athletes who are recovering from injury or individuals with conditions that preclude heavy weight bearing (bone or joint disorders).

Acknowledgment

Mr. J. Cort and D. Clarke are thanked for assistance with adaptation in the custom designed EMG scripts.