EVALUATION OF MUSCLE ACTIVITY FOR LOADED AND UNLOADED DYNAMIC SQUATS DURING VERTICAL WHOLE-BODY VIBRATION
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EVALUATION OF MUSCLE ACTIVITY FOR LOADED AND
UNLOADED DYNAMIC SQUATS DURING VERTICAL
WHOLE-BODY VIBRATION
TOM J. HAZELL,1 KENJI A. KENNO,2 AND JENNIFER M. JAKOBI3
1
Exercise Nutrition Research Laboratory, School of Kinesiology, Faculty of Health Sciences, The University of
Western Ontario, London, Ontario, Canada; 2Department of Kinesiology, Faculty of Human Kinetics, University of Windsor,
Windsor, Ontario, Canada; and 3Department of Human Kinetics, Faculty of Health and Social Development, University of
British Columbia, Okanagan, Kelowna, British Columbia, Canada
ABSTRACT INTRODUCTION
W
Hazell, TJ, Kenno, KA, and Jakobi, JM. Evaluation of muscle hole-body vibration (WBV) is a relatively new
activity for loaded and unloaded dynamic squats during vertical exercise modality gaining significant interest
whole-body vibration. J Strength Cond Res 24(7): 1860–1865, in the health and fitness realm. This type of
2010—The purpose of this investigation was to examine if the exercise, typically involves individuals per-
addition of a light external load would enhance whole-body forming traditional resistance exercise on the platform with
their body mass as resistance. Although there is a wide range
vibration (WBV)–induced increases in muscle activity during
of exercise protocols used, the positive results of WBV studies
dynamic squatting in 4 leg muscles. Thirteen recreationally
report enhanced strength, power, and endurance, and blood
active male university students performed a series of dynamic
flow in working skeletal muscle (3,9,13,15,18,21,23,24,27,
squats (unloaded with no WBV, unloaded with WBV, loaded
33,37); however, there are reports to the contrary suggesting
with no WBV, and loaded with WBV). The load was set to 30% no training benefit of WBV exercise (12,20,32). Regardless,
of body mass and WBV included 25-, 35-, and 45-Hz frequencies there are many studies indicating that exposure to WBV
with 4-mm amplitude. Muscle activity was recorded with surface during static and dynamic body mass exercises increases
electromyography (EMG) on the vastus lateralis (VL), biceps exercise intensity (1,8,10,17,31,34).
femoris (BF), tibialis anterior (TA), and gastrocnemius (GC) and is The mechanical vibrations generated by the WBV platform
reported as EMGrms (root mean square) normalized to %maximal are thought to induce length changes in extrafusal fibers
voluntary exertion. During unloaded dynamic squats, exposure to resulting in the activation of an afferent feedback response
WBV (45 Hz) significantly (p , 0.05) increased baseline muscle through the muscle spindles and 1a afferents, a response akin
activity in all muscles, except the TA compared with no WBV. to the tonic vibration reflex (TVR) (7,16). The mechanical
Adding a light external load without WBV increased baseline vibration stimulus may also affect skin and joint receptors
that provide sensory input to the gamma motor system
muscle activity of the squat exercise in all muscles but decreased
increasing the sensitivity and responsiveness of the muscle
the TA. This loaded level of muscle activity was further increased
spindle to further mechanical perturbations (7,14,28). The
with WBV (45 Hz) in all muscles. The WBV-induced increases in
WBV perturbations put a high level of stress on the muscular
muscle activity in the loaded condition (~3.5%) were of a similar system that requires high levels of neuromuscular activity
magnitude to the WBV-induced increases during the unloaded (1,8,17,25,34).
condition (~2.5%) demonstrating the addition of WBV to Recently, electromyography (EMG) has been used to
unloaded or loaded dynamic squatting results in an increase in characterize muscle activation, and previous literature has
muscle activity. These results demonstrate the potential effec- demonstrated that exposure to WBV results in an increase in
tiveness of using external loads with exposure to WBV. leg muscle EMG activity (1,8,17,25,34). Our group has even
shown that exposure to WBV increases EMG in the lower
KEY WORDS vibration exercise, reflex, electromyography,
limbs but has little or no effect on the upper limbs (17). The
strength exercise discrepancy may be the result of the proximity of the muscles
to the vibration stimulus (distance from platform), the
Address correspondence to Jennifer Jakobi, Jennifer.Jakobi@ubc.ca. dampening of the stimulus in the upper body because of body
24(7)/1860–1865 position (38), or the effect that body mass has on loading the
Journal of Strength and Conditioning Research lower limbs. If WBV induces a TVR response, then increasing
Ó 2010 National Strength and Conditioning Association the sensitivity of the system may result in a greater WBV
the TM
1860 Journal of Strength and Conditioning Researchthe TM
Journal of Strength and Conditioning Research | www.nsca-jscr.org
response because it has been reported that the sensitivity of the fibula. A goniometer was adhered with double-sided tape
1a afferents is increased in response to an increased preceding to the lateral side of the left knee to ensure a consistent
level of muscle activity (4,6,26,29). Furthermore, from a dynamic squat (descent to ;90°; ascent to ;160°). Also,
biomechanical perspective, WBV exposure increases exercise verbal feedback was provided to ensure consistent perfor-
intensity by increasing the accelerations of the body and mance of the dynamic squat, and a metronome was used to
because force = mass 3 acceleration, increasing the mass at ensure correct cadence (see details below).
a given acceleration (WBV stimulus) the resultant should be
an increase in force. Although EMG is not a direct measure Protocol
of force, the 2 are strongly correlated, so any increase in force All subjects performed a familiarization trial, employing both
should be represented by an increase in EMG. Therefore, static and dynamic squats, to acclimate subjects to the WBV
the addition of a light load should increase baseline muscle stimulus. A demonstration of proper technique for the
activity (preactivation) and potentially increase the sensitivity dynamic squat movement occurred, and practice of this
of the 1a afferents leading to an increase magnitude of movement was undertaken until performance of the squat
the previously demonstrated WBV-induced increase in an was consistent and correct. Electromyographic electrodes
unloaded condition. The purpose of this study was to exam- were not used during the familiarization session.
ine the muscle activity in the lower limbs during unloaded The vibration stimulus was 4 mm for 3 frequencies (25, 35, 45
and loaded dynamic squatting with WBV. We hypothesize Hz) applied with a WAVETM platform (Whole-body
that the addition of a load to dynamic squatting during WBV Advanced Vibration Exercise, Windsor, Canada) that oscillates
exposure will augment the WBV response compared with vertically up and down. We have previously demonstrated
the same condition with no additional load. increases in muscle activity with these WBV stimuli (17).
The load applied in this study was 30% of the subject’s body
METHODS mass (range 20–33 kg) and was applied using a standard 180-
Experimental Approach to the Problem cm Olympic bar (;20.5 kg) with appropriate additional
This study investigated whether the addition of a light weights held behind the neck and resting atop the shoulders
external load could augment the WBV induced increases in and upper back. This load was selected to provide a reason-
muscle activity seen in response to dynamic squatting with able and safe weight that untrained individuals could lift
no load. Electromyography was used to measure changes in without inducing fatigue and ensuring safety. The 8 condi-
muscle activity (dependent variable), in the vastus lateralis (VL), tions examined were as follows: (a) no vibration no load; (b) no
biceps femoris (BF), tibialis anterior (TA), and gastrocnemius vibration load; (c) 25 Hz 4-mm no load; (d) 25 Hz 4-mm load;
(GC). Muscle activity was assessed during loaded and unloaded (e) 35 Hz 4-mm no load; (f) 35 Hz 4-mm load; (g) 45 Hz 4-mm
dynamic squats in the 2 different exercise conditions—with no load; and (h) 45 Hz 4-mm load. All conditions were
WBV and without vibration. Three vibratory stimuli were randomized within and between subjects, and the WBV
used: 25, 35, and 45 Hz at 4-mm amplitude WAVETM. frequency and amplitude were not verbally conveyed.
Maximal muscle activation was also recorded to compare Within each condition, subjects were required to complete
muscle activity as a percentage of maximal. 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
Subjects
between conditions to prevent fatigue. Platform foot position
Thirteen recreationally active male Kinesiology students
was slightly wider than shoulder width and was marked on
(23 6 2.0 years; 178 6 6.3 cm; and 84 6 11.9 kg) volunteered
the first trial to be used for all further trials.
to participate in this study. All were healthy as assessed by the
The experimental session was held at least 72 hours post-
PAR-Q health questionnaire (39). Subjects had no experience
familiarization protocol, and subjects refrained from exercise
with resistance training over the last 4 months. Before any
or the ingestion of caffeine for 24 hours and did not eat at least
participation, the experimental procedures and potential risks
2 hours before any visit to the laboratory. During the actual
were explained to the subjects, and all subjects provided
testing session, subjects performed an EMG noise trial to
written informed consent. This study was approved by the
determine the amount of baseline interference in the spec-
University of Windsor Research Ethics.
trum (rested supine on a mat and EMG was measured for
Electromyographic Electrode Placement 1 minute to determine the inherent noise within the signal),
Electromyographic electrodes were placed on the VL, BF, which was then deleted from further EMG signals collected
TA, and GC over the midbelly of the muscle parallel to the (customized software see EMG Analysis below). Subjects
direction of the fibers (19). Before electrode placement, the were then asked to perform maximal voluntary exertion
area was shaved, abraded with coarse fabric, and swabbed (MVE) tests for the muscle groups being evaluated. The MVE
with alcohol. Interelectrode distance was fixed (10 mm) trials obtained a maximal EMG profile rather than force out-
within the prefabricated electrode bar. High-conductivity put measures; dynamometers were unavailable for all muscles
electrolyte gel was used with the reference electrode (R200 studied (17). All MVEs were isometric (at 90° joint angles)
Biometrics) that was positioned on the lateral malleolus of and were performed 3 times against resistance provided by
VOLUME 24 | NUMBER 7 | JULY 2010 | 1861Vibration and Muscle Activity
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
Figure 1. Increases in vastus lateralis muscle activity during unloaded
Hz, which removed any noise caused by the frequency of the and loaded dynamic squats with and without whole-body vibration. Values
vibration platform (17,30). The data were then full wave are mean 6 SEM. All loaded conditions are significantly greater
compared with the unloaded condition (p , 0.001). Statistics (A,B) are
rectified and smoothed with a low-pass filter at 1.5 Hz. The
within condition comparisons. A) Significantly greater than no vibration no
noise was then subtracted, and the data were divided by load condition (p , 0.09). B) Significantly greater than 25-Hz condition
MVE and multiplied by 100 for normalization. The EMGrms (p , 0.017).
(root mean square) was then calculated.
Statistical Analyses
Analysis was performed using SigmaStat (Version 3.5). A 4 3 muscle activity 0.5–1.7% compared with the no WBV con-
2 repeated measures analysis of variance was used to evaluate dition (Figure 2), where the 45-Hz condition was signifi-
the independent variables of vibration and load on muscle cantly increased vs. the no-vibration (p , 0.001) and 25-Hz
activity (EMGrms). Post hoc tests were performed using (p = 0.008) conditions. The addition of a load to the unloaded
Tukey’s Honestly Significant Difference (HSD) tests. Data in no WBV condition increased muscle activity from 3.2 to
the text are values 6 SD, whereas figures are reported as 9.7 6 6.3% (p , 0.001). Subsequent exposure to WBV, in this
values 6 SEM, and the level of statistical significance was loaded condition, further increased muscle activity 0.2–2.0%
set at p # 0.05. MVE (Figure 2) compared with the no WBV condition,
where 45 Hz was significantly increased vs. the no-vibration
RESULTS (p , 0.001), 25-Hz (p , 0.001), and 35-Hz (p = 0.031)
Vastus Lateralis conditions, and the 35-Hz condition was also significantly
There was no significant vibration 3 load interaction (p = increased over the no vibration (p = 0.038) condition.
0.172). There were main effects for vibration (p = 0.004) and
load (p , 0.001). During unloaded dynamic squats without Tibialis Anterior
the addition of WBV, VL muscle activity was 42.5 6 16.7% There was a significant 2-way interaction between vibration
MVE. Relative to no WBV, exposure to WBV in the and load (p = 0.030) and a main effect for load (p = 0.008);
unloaded condition increased muscle activity 1.4–5.5% however, there was no main effect for vibration (p = 0.08).
(Figure 1). The 45-Hz vibration condition was significantly During unloaded dynamic squatting without the addition
greater compared with the no vibration condition (p = 0.009). of WBV, TA muscle activity was 34.6 6 6.9%. Exposure to
The addition of a load to the unloaded no WBV condition WBV in this unloaded condition increased muscle activity
increased muscle activity 3.2% to 9.7 6 6.3% (p , 0.001). 0.1–2.3% (Figure 3), where only 45 Hz was significantly
Subsequent exposure to WBV in this loaded condition increased over the 35-Hz condition (p = 0.026). The addition
further increased muscle activity 0–3.6% (Figure 1). The of a load to the unloaded no WBV condition resulted in
45-Hz condition was significantly increased vs. the a 7.8% decrease in muscle activity (p , 0.001) to 26.8 6 6.5%.
25-Hz condition (p = 0.017). Subsequent exposure to WBV in the loaded condition
increased muscle activity 2.6–3.8% (Figure 3), where the
Biceps Femoris 45-Hz condition was significantly increased compared with
There was no significant interaction between vibration and the no-vibration condition (p = 0.048).
load (p = 0.368), but main effects were evident for vibration
and load (p , 0.001). During unloaded squats without Gastrocnemius
the addition of WBV, BF muscle activity was 6.5 6 4.2%. There was no significant interaction between vibration and load
Exposure to WBV in the unloaded condition increased (p = 0.15), although main effects were observed for vibration
the TM
1862 Journal of Strength and Conditioning Researchthe TM
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Figure 2. Increases in biceps femoris muscle activity during unloaded Figure 4. Increases in gastrocnemius muscle activity during unloaded
and loaded dynamic squats with and without whole-body vibration. Values and loaded dynamic squats with and without whole-body vibration. Values
are mean 6 SEM. All loaded conditions are significantly increased over are mean 6 SEM. The loaded condition was significantly greater than the
corresponding unloaded condition (p , 0.001). Statistics (A–C) are corresponding unloaded condition (p , 0.038). All statistics (A–C) are
within condition comparisons. A) Significantly greater than no vibration within condition comparisons. A) Significantly greater than no vibration
condition (p , 0.038). B) Significantly greater than the 25-Hz condition condition (p , 0.05). B) Significantly greater than the 25-Hz condition
(p , 0.008). C) Significantly greater than the 35-Hz condition (p , 0.033). (p , 0.05).
(p , 0.001) and load (p = 0.010). During unloaded dynamic activity a further 5.9% MVE (p = 0.04) to 13.4 6 8.6%.
squatting without WBV, GC muscle activity was 7.5 6 3.2% Subsequent exposure to WBV in the loaded condition further
MVE. Exposure to WBV in the unloaded condition increased increased muscle activity 6.3–9.8% MVE (Figure 4), where
muscle activity 2.9–8.9% (Figure 4), where 45 Hz was the 45-, 35-, and 25-HZ conditions were all significantly
significantly increased over both the no WBV (p , 0.001) increased vs. the no WBV condition (p , 0.001; p = 0.03;
and the 25-Hz condition (p = 0.042). The addition of a load p = 0.03, respectively).
to the unloaded no WBV condition increased GC muscle
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 demon-
strated 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 ex-
ternal 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
Figure 3. Increases in tibialis anterior muscle activity during unloaded TA activity (;8%). The decrease in TA muscle activity was
and loaded dynamic squats with and without whole-body vibration. Values unexpected and may have been caused by a slight and
are mean 6 SEM. All loaded conditions except 35 Hz are significantly
decreased over corresponding unloaded condition (p , 0.015). nonvisual change in center of mass. However, monitored
Statistics (A,C) are within condition comparisons. A) Significantly greater squat technique and posture appeared unaltered. The
than no vibration condition (p , 0.05). C) Significantly greater than the addition of a load was intended to increase the level of
35-Hz condition (p , 0.05). D) Significantly greater than no vibration
loaded condition (p , 0.01). (*) Significant decrease between no load muscle activity and potentially enhance the effects of WBV in
and load (p , 0.05). all muscles studied. The decrease in muscle activity in the TA,
highlights the importance of body position on the platform.
VOLUME 24 | NUMBER 7 | JULY 2010 | 1863Vibration and Muscle Activity
To date, there has been no biomechanical evaluations of body resulting in the improved function seen in several WBV
position and muscle activation during WBV. training studies (2,13,22–24,33). The current data (while
This study demonstrates that exposure to WBV (at 45 Hz) on young, healthy men) may have practical applications,
results in increases in muscle activity whether performing especially in a rehabilitation setting where the addition of
loaded or unloaded dynamic squats. This result agrees with a load to dynamic exercises (typical of resistance exercise)
those of previous studies, all demonstrating a WBV-induced is not yet tolerated. This requires further investigation.
increase in muscle activity (1,8,17,25,34). However, the Furthermore, it remains intriguing to determine whether the
average WBV-induced increase in muscle activity of all use of heavier external loads would enhance the muscle
4 muscles was 2.5% in the unloaded condition and a similar spindle response and augment EMG activity and muscle
3.5% in the loaded condition suggesting that the addition of function as originally hypothesized.
a light external load did not increase the sensitivity of the
1a afferents. Moreover, although the addition of a load
PRACTICAL APPLICATIONS
increased muscle activity, exposure to WBV resulted in Our findings demonstrate exposure to WBV results in a similar
a further increase in muscle activity. This agrees with the magnitude increase in muscle activity during both unloaded
biomechanical perspective that increasing mass at a given and loaded dynamic squats. The effect of using a heavier
acceleration likely results in an increase in force represented external load during exposure to WBVremains to be examined,
by EMG. Thus, the possibility still remains that the use of as does body positioning on the platform. The results of this
heavier loads with WBV may increase the sensitivity of the study also demonstrate that the cumulative effect of adding
body to the mechanical perturbations further enhancing a light external load to dynamic squats with WBV increases the
EMG muscle activity. This idea requires further investigation. intensity of the exercise being performed. This supports early
It has been theorized that WBVoscillations increase muscle suggestions that the use of loaded resistance training on a WBV
activity via a reflex response akin to the TVR (7), where Ia platform might be more beneficial than the same training
afferent activity in the muscle spindles alters surface EMG without WBV (36). Moreover, those individuals that are
and single motor unit activity (5,11,35). Therefore, if WBV typically discouraged or unable to perform resistance overload
induces a similar response to the TVR, then muscle activity training are likely to benefit from this combined light load
might be further augmented by increasing the preceding level WBV exercise program. This could include athletes who are
of muscle activity with the addition of a load (4,6,26,29). The recovering from injury or individuals with conditions that
current data demonstrate that the addition of a light load did preclude heavy weight bearing (bone or joint disorders).
not enhance the WBV-induced increase in EMG activity
already demonstrated in the unloaded condition. This sug- ACKNOWLEDGMENT
gests the increase in baseline muscle activity did not increase
Mr. J. Cort and D. Clarke are thanked for assistance with
the sensitivity of the 1a afferent and that the skeletal muscle
adaptation in the custom designed EMG scripts.
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
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