15

Bull Yamaguchi Med Sch 59（1-2）:15-24, 2012

Neuromuscular Responses to Acute Whole Body Vibration in Healthy Young Men
Keiichi Hiroshige,1,2 MH Mahbub1 and Noriaki Harada1
1

Department of Hygiene, Yamaguchi University Graduate School of Medicine,
1-1-1 Minami-Kogushi, Ube, Yamaguchi 755-8505, Japan
2
Kyushu Rosai Hospital Center for Preventive Medicine, Japan Labour Health andWelfare Organization, 1-1 Sone-Kitamachi, Kokura-Minamiku, Kitakyushu,
Fukuoka 800-0296, Japan(Received March 21, 2012, accepted June 4, 2012)
Abstract The purpose of this study was to investigate the acute effects of wholebody vibration stimulus on neuromuscular performances in healthy male students.
Twenty healthy students (mean age 19.7 ± 0.5 years) were recruited for this study.
They were randomly divided into two groups of 10 subjects: whole body vibration(WBV) group and control group. The intervention consisted of standing on a vibrating (for the WBV group) or non-vibrating (for the control group) platform in a semisquat posture for 3 minutes. Vibration was produced at a frequency of 20 Hz, peak
amplitude of 3 mm and frequency-weighted acceleration of 21.3 m/s2 rms. Statisticalanalysis with two-way repeated measures ANOVA revealed that the reaction timedecreased significantly in both groups (P < 0.05) and the absolute angular error value
at 30 degree knee flexion decreased significantly only in the WBV group (P < 0.05).
The findings indicate that the acute WBV stimulus used in this study is not adequate
to induce any significant improvement in neuromuscular responses among young
healthy men. To reveal the training effects of WBV, future research works should be
conducted among both young and elderly subjects with long-term or chronic exposure to WBV within a safe exposure limit.
Key words: whole-body vibration, neuromuscular response, cardiovascular response,
acute effect, healthy young men
Introduction
Generally, vibration stimulus has beensuggested to induce various effects on humanbody. Local mechanical vibration has beenused by physiotherapists as treatment options for relief of pain, to relax tense muscles
and to increase blood flow. The treatment
or training intervention using whole body
vibration (WBV) device has recently become
increasingly popular.
Numerous previous studies with application of WBV training have shown positive
health effects; for example, improvements
in walking ability or postural balance af-

ter WBV intervention in elderly people, 1,2increase in vertical jump height and counter movement jump height in young female
subjects,3,4 and improvements in proprioception and postural stability of patients withanterior cruciate ligament reconstruction. 5
Rehn et al. 6 demonstrated that long-term
WBV exercise can have positive effects on theleg muscular performance among untrained
people and elderly women.
In the literature, both short-term andlong-term of effects WBV training on humanhealth have been reported. Several previous
studies suggested that regular long-term
exposure to WBV may represent a health

16

Keiichi Hiroshige et al.

risk and can cause low back pain, sciaticpain, and degenerative changes in the spinal
system including lumbar intervertebral discdisorders.7-9 According to the International
standard ISO 2631-1, 10 subjects should not
be exposed to high levels of vibration during experiments involving human exposure
to vibration and shock. To control harmful
WBV exposure in the industry, the EU Directive 2002/44/EU11 mandates that people
regularly exposed to whole-body vibration,an A(8) value of 1.15 m/s2 rms should not be
exceeded. A(8) is calculated as: A(8)= a× (t/8)
m/s2 rms, where a is the frequency-weighted
acceleration and t is the exposure duration
in hours. However, this issue has not been
mentioned by the previous researchers investigating health effects from WBV exercise.
Furthermore, though previous studies demonstrated beneficial effects of WBV training
on human health, there is a lack of clear evidence for the effects of acute WBV exercise
on neuromuscular performances in healthy
young men.
Therefore, the purpose of the current study
was to investigate the acute effects of exposure to WBV on neuromuscular performances in young healthy adult male subjects, taking into consideration the recommendationsof the international standard. Additionally,
cardiovascular responses were also measured
to investigate the physiological stability ofthe subjects from such exposure.

before enrollment in the study, the subjects
answered to a questionnaire about their dominant leg (side of kicking a ball) and healthstatus. The baseline data for both groups areshown in Table 1 with statistical comparisons between the groups.
All experimental procedures used in thisstudy were approved by the ethics committeeof the Kyushu Rosai Hospital.

Measurement procedure and condition
Participants were exposed to intervention on 3 different sessions each separated
by an equal period of 1 week. The 3 sessions
were attended by the WBV group and control group on different days. An interval of
1 week between interventions was used tominimize any effects of previous exposure on
the intervention or the measurement.
The measurements and/or tests were performed before (pre-intervention) and after
intervention (post-intervention) in all participants. Blood pressure, heart rate and reaction time were measured on the 1st session.
On the 2nd session, knee joint reposition test
was performed. And on the 3rd session, rectus femoris muscle activity was measured
by surface EMG during maximum isometricknee extension; also maximum isometricknee extensor torque was measured during
the same session. The measurements/tests
were performed on different sessions to remove any potential effects of previous measurements/tests on the subsequent measureMethods
ments/tests (Fig. 1).
All experiments were performed in a temSubjects
perature-controlled experiment room with an
A matched case-control intervention study ambient temperature of 21 ∓ 2 ℃.
was designed. Twenty young healthy malestudents (mean age ∓ SD, 19.7 ∓ 0.5 years; Measurement of blood pressure and heart ratemean height ∓ SD, 172.7 ∓ 4.3 cm; mean
Systolic blood pressure (SBP), diastolicweight ∓ SD, 65.0 ∓ 5.8 kg) from a school of blood pressure (DBP) and heart rate (HR)physical therapy and occupational therapy were measured with a sphygmomanometer
were recruited for this study and assigned to BP-8800 (COLIN co., JAPAN). The manchette
2 groups of 10 subjects each (WBV group and of sphygmomanometer was wrapped aroundcontrol group). They did not have any mus- the left upper arm of a participant. Before
culoskeletal or neurological disorders, car- and after intervention, the measurements
diovascular disease, kidney stones or diabetes were performed automatically in standing
mellitus.
posture of the participant.
After explanation of the study procedures,the participants provided written informed Measurement of static postural balance
consent to participate in this study. Also,
The static postural balance was measured

Acute Effects of Whole-body Vibration

Fig. 1

17

Flow diagram of the study.

by the stabilometer UM-BAR2 (UMC co.,Ltd., JAPAN). The standardized procedure
for stabilometry test recommended by theJapan Society for Equilibrium Research12 wasfollowed.
The subjects stood on a triangular strain
gauge platform, with their shoes off and upper limbs hanging loosely by the side. Thenthey looked forward pointing at a marker
positioned at eye level at a distance of 2 m.
Under this condition, the subjects were asked
to keep a static posture for 60 sec with theireyes open, and subsequently, next 60 sec with
their eyes closed. The sampling frequency forrecording was 20 Hz. Two parameters were
calculated in this study: locus length (L) andperiphery area (A) of center of foot pressure.

ing mat T.K.K.5408 (TAKEI co., Ltd., JAPAN).
The subject stood on the mat with theirknees slightly bent without shoes and was
asked to jump upright as quickly as possible
in response to the flash of a light set at a
distance of 2 m in front of the subject. The
lapse of time between the flash signal and thewithdrawal of foot pressure from the sensor
mat was recorded. After practices twice, continuous 5 measurements were taken. Then,the average for 5 measurements was calculated for each subject. Pre and post-intervention
measurements of reaction time were recorded
after the above-mentioned measurements
performed on the same session.

The knee joint reposition test
Measurement of reaction time
The knee joint reposition test was perThe measurement of reaction time as an formed by Biodex 3 Isokinetic Dynamometer
agility test was performed using flash light (Biodex Corp, NY, USA), according to theequipment and the connected pressure sens- method mentioned by Moezy et al.5 The elec-

18

Keiichi Hiroshige et al.

trogoniometer of this dynamometer is sensitive to 1 degree increments. By this test, theability of a seated subject to reproduce actively an angle at which the joint was placed
before is evaluated.
In this experiment, the subject sat on theseat with the popliteal fossa positioned outside of the Biodex chair. The subjects wereblindfolded. An air splint was kept aroundthe dominant leg of the subject to minimize
any cutaneous sensory input during the measurement. Alignment of the axis of the kneeof test leg with the axis of the dynamometer
was performed; the tibial pad was secured tothe shank 3 cm superior to the lateral malleolus.
Then the subject actively moved the kneefrom the starting angle of 90 degree flexion
to the target angle of 60 degree flexion. After reaching the target angle, the leg position was held for 10 sec and then returnedto the starting position of 90 degree knee
flexion. After an interval of 5 sec, the subject was asked to move the leg and reproduce
the target angle actively and press the handheld stop button after achieving the target
angle. Test for the target angle of 30 degreeof knee flexion was conducted using the sameprocedure. Each target angle was measured
3 times at the dominant leg. The average absolute angular error in the test knee of eachsubject at 60 degree (60AE) and 30 degree
flexion (30AE) was calculated.
Measurement of isometric torque and surface
EMG
The maximum isometric knee extensor
torque of dominant leg was measured on the
Biodex system 3 Isokinetic Dynamometer(Biodex Medical, Inc., Shirley, NY, USA).
The subject sat on the seat upright withthe axis of the knee aligned with the axisof the dynamometer. To prevent any additional body movement, subjectʼs trunk andthigh were fixed to the chair with seat belts;
and the tibial pad was secured to the shank
3 cm superior to the lateral malleolus. Thenthe subject was asked to perform the maximum isometric voluntary knee extension at
60 degree knee flexion angle held for 15 sec.
For normalization, the maximum isometricvoluntary knee extension torque value wasdivided by the body weight of the subject
and the resulting value was used in statistical analysis. To obtain subjectʼs maximum
effort, the same encouraging recorded voice
instructions were played for all subjects.
The EMG activity of the rectus femorismuscle was recorded during the measurementof the knee extensor muscles torque, for 15
sec, by the MyoSystem 1200 and analyzed by
the MyoResearch Software (Noraxon U.S.A.
Inc., USA). In accordance with the European
recommendations for non-invasive assessment of muscle activity by surface EMG (SENIAM project),13 surface Ag-AgCl disposable
electrodes of 20 mm diameter were placed ina bipolar lead, at the mid-point of the line
between the anterior superior iliac spine andthe superior part of the patella. To improvethe electrode-skin contact, the contact areaof the skin with electrodes was shaved andrubbed with sandpaper carefully.
Electrodes were fixed parallel to muscle-fiber direction, with an inter-electrode (centerto-center) distance of 25 mm. The referenceelectrode was attached to the right patella.
The EMG cables were fastened to prevent
movement artifact from swinging cables. A
sampling frequency of 1000 Hz was used forthe analysis of EMG data.
Vibration intervention
The participants stood on the vibration
platform wearing socks, and placed their feetwith the 2nd toe and the heel located overline 3 on either side of the platform (distance
between feet, 325 mm). They were instructed
to maintain a static half squat posture withknee angles at approximately 135 degrees offlexion and with hands holding the handlebarof the equipment.
The vibration frequency was set to 20 Hzwith a peak-to-peak displacement of 6 mm.
The root mean squared acceleration and frequency-weighted root mean squared acceleration in our study, calculated according to ISO
2631-110 were 33.5 m/s2 rms and 21.3 m/s2
rms, respectively. The WBV group subjects
were exposed to vibration for a period of 3
min. Hence, A(8) in our study was 1.68 m/s2
rms. The control group followed the same experimental procedure except that they were
not exposed to vibration (0 Hz).

Acute Effects of Whole-body Vibration

Statistical analysis
All analyses were carried out using IBM
SPSS statistics version 19 (IBM, Japan).
The Shapiro-Wilk test was used to check forNormal distribution of data. If any variables
did not follow the normal distribution, thesewere used after natural logarithmic transformation of data. Mean and standard deviation (SD) were calculated for all measurement
variables. A two-way repeated measures
ANOVA was used using time (pre and postintervention) and group (WBV and control)as the independent variables. The level of significance was set at P < 0.05.
Results
The subject characteristics are shown in
Table 1. There were no significant differences between the two groups with respect toheight or weight.

Table 1

19

The post-intervention values of SBP and
DBP or HR did not differ significantly fromthe corresponding pre-intervention values inboth WBV and control groups except that
DBP decreased significantly in WBV group(Table 2). Also, those values for WBV groupwere not significantly different from the corresponding control values.
Similarly, static postural balance parameters did not show any difference between preand post-intervention measurements of twogroups, or between the two groups at preintervention or post-intervention (Table 3). In
both groups, the post-intervention reaction
time was significantly shorter than the corresponding pre-intervention reaction time (P< 0.05); but pre or post-intervention values
did not differ significantly between WBV andcontrol groups (Table 3).
The results of knee joint reposition test are
shown in Table 4. The absolute angular error
at 60 degree flexion was not significant dif-

Characteristics of the study participants

Body height (cm)Body weight (kg)

WBV group (N=10)
Mean (SD)
172.6 (3.75)
64.0 (5.23)

Control group (N=10)
Mean (SD)172.7 (4.99)
65.9 (6.40)

WBV vs Control
P value
0.960
0.477

Values represent mean ±SD.
Table 2Comparison of cardiovascular parameters between WBV and control groups
WBV (N=10)
Mean (SD)Control (N=10)
Mean (SD)

Pre
Post

126.3 (6.5)
129.7 (7.7)

125.3 (7.3)
128.1 (10.2)

Pre
Post

78.1 (6.0)
74.1 (8.5)*

73.7 (6.1)
71.9 (5.1)

Pre
Post

89.6 (18.6)
94.0 (13.0)

92.3 (15.7)
95.4 (17.1)

SBP (mmHg)

DBP (mmHg)

HR (beats/min)Values represent mean ± SD.
SBP: systolic blood pressure, DBP: diastolic blood pressure, HR: heart rate
Pre: measurements before intervention; Post: measurements after intervention*Significant difference between pre- and post-intervention measurements (P < 0.05).
For statistical analysis, natural logarithmic transformation of data was performed.

20

Keiichi Hiroshige et al.

Table 3 Comparison of postural balance and reaction time between WBV and control groups

Locus length (mm)eye openeye close
Periphery area (mm2)eye openeye close

WBV (N=10)
Mean (SD)Control (N=10)
Mean (SD)

Pre
Post
Pre
Post

498.5 (75.4)
530.2 (120.4)
757.4 (156.9)
709.5 (153.9)

444.2 (123.9)
456.9 (103.8)
742.5 (287.8)
666.9 (188.6)

Pre
Post
Pre
Post

188.0 (44.7)
173.2 (74.8)
243.1 (86.3)
249.3 (179.4)

155.5 (92.5)
144.1 (59.3)
267.2 (153.5)
222.1 (106.1)

Pre
Post

0.34 (0.05)
0.32 (0.04)*

0.37 (0.03)
0.36 (0.03)*

Reaction time (sec)Values represent mean ± SD.
Pre: measurements before intervention; Post: measurements after intervention*Significant difference between pre- and post-intervention measurements (P < 0.05).

Table 4 Comparison of absolute angular error, muscle activity and muscle torque between
WBV and control groups
WBV (N=10)
Mean (SD)Control (N=10)
Mean (SD)

Pre
Post

3.03 (1.75)
4.73 (2.74)

2.80 (1.44)
3.77 (3.46)

Pre
Post

5.53 (2.31)
3.83 (2.99)*

3.80 (1.31)
3.30 (1.31)

Pre
Post

270.8 (169.1)
263.0 (141.0)

265.3 (150.3)
284.0 (162.0)

Pre
Post

1.70 (0.34)
1.65 (0.33)

1.82 (0.46)
1.89 (0.44)

60AE (° )30AE (° )EMG amplitude (μV)

Torque/Body weight (Nm/kg)

Values represent mean ± SD.
Pre: measurements before intervention; Post: measurements after intervention
Values represent mean ± standard deviation (SD).
60AE: average of absolute angular error at a target angle of 60 degrees in the dominant sideknee; 30AE: average of absolute angular error at a target angle of 30 degrees in the dominant side knee. EMG amplitude: average of rectus femoris muscle activity during the maximum isometric contraction for 15 second. Torque / Body weight: average torque per bodyweight of maximum isometric knee extension for 15 seconds.
*Significant difference between pre- and post-intervention measurements (P < 0.05). For statistical analysis, natural logarithmic transformation of data was performed for all the variables except muscle torque.

Acute Effects of Whole-body Vibration

ferent in any of the groups between pre- andpost-intervention measurements or between
the groups (at pre- or post-intervention).
However, the absolute angular error value at
30 degree flexion for the WBV group at postintervention decreased significantly from its
pre-intervention value; in contrast, the control group did not show such a difference inabsolute angular error at 30 degree flexion.
In general, the post-intervention values of
surface EMG and isometric torque showeda decreasing pattern in the WBV group andan increasing pattern in the control group.
However, the post-intervention values of surface EMG and isometric torque did not differ
significantly from the corresponding preintervention values in both WBV and control
groups. Also, at pre- or post-intervention,
the values for WBV group did not differ significantly from the corresponding control
values (Table 4).
Discussion
To our knowledge, no previous research
work involving human exposure to wholebody vibration exercise followed the recommendations of the international standard or
the European standard for such exposure.
Experimental subjects in some previous studies were exposed to a higher level of WBVthan the A(8) value of 1.15 m/s2 rms recommended in the standards.14-16 For example, thecalculated A(8) values in the study of Rittweger et al.14 was 6.43 m/s2 rms, and of deRuiter et al.15 or Pollock et al.16 was 6.52 m/
s2 rms. A(8) in our study was 1.68 m/s2 rms,
which slightly exceeds the suggested value of
1.15 m/s2 rms. However, the A(8) value usedin our study seems to be within a safe exposure limit as our study involved exposure ofhuman subjects on an occasional, rather thana long-term daily basis (ISO 13090-1).17 Overall, the vibration level used in this study waswell tolerable by the participants except that
2 participants in the WBV group reported an
itching sensation in the leg immediately after
WBV stimulation.
As the blood pressure and HR did notchange significantly before and after intervention, it indicated the physiological stability of the subjects with the current level

21of WBV stimulation. Rittweger et al.14 also
demonstrated that, even if performed to exhaustion (peak acceleration of 147 m/s2), cardiovascular effects of whole-body vibration
exercise are mild.
Various balance tests are used to test for
postural control during standing such as
Timed Up & Go test, test with Tinetti Balance Scale, or test with Berg Balance Scale,etc. However, the test systems using force
platform or strain gauge are particularly
useful because of their ability to test balance
control more accurately.18 For Japanese males
20-25 years of age, the average values of L
and A were shown to be 710.2 mm (length,
open eye) and 970.6 mm (length, closed eye),and 286 mm 2 (area, open eye) and 364 mm2(area, closed eye), respectively.19 The valuesof L and A obtained in this study are comparable with the standard values, indicating the
subjects had comparably stable posture balance.
Torvinen et al.20 and Mahieu et al.21 reported no effects on postural balance from WBVtraining in healthy young people. On theother hand, some studies demonstrated improved postural stability after WBV training
among patients with knee ligament injuries5
or chronic stroke22 etc. Like the former studies with healthy subjects, in this study also,
no significant effect of WBV on static postural balance of young healthy subjects could
be revealed.
The reaction time test is an agility test bywhich a subjectʼs ability to jump quickly inresponse to a flashing light is evaluated. It
depends on several factors including the accuracy of its operation, the processing time of
the central nervous system and the thresholdof sensory stimulation, nerve conduction velocity, muscle contraction speed etc. The reaction time of the current study participants
ranged between 0.338 sec and 0.374 sec whichcorresponds to average level (0.323～0.377 sec)of the reference standard for Japanese males
20-24 years of age (Japan Industrial Safety& Health Association, 1996).23 Enhancementin neuromuscular performance should result in a decrease in reaction time. However,Cochrane et al.24 did not observe any significant difference for agility between WBV andcontrol groups. In this study, a significant

22

Keiichi Hiroshige et al.

decrease in reaction time in both groups indicates that it may have been caused by the improvement in neuromuscular performance orby learning effects from repeated measurements of participants.
In the literature, only a few studies verified the acute effects of WBV stimulation on
joint position sense in healthy young people.
However, the results of those studies demonstrate that in young healthy subjects, WBV
did not affect joint position sense.16,25 In thecurrent study, joint position sense was evaluated using the absolute angular error value
at 30 degree or 60 degree knee flexion; WBVintervention caused a significant decreasein the absolute angular error value at 30 degree knee flexion, but it was not sufficient to
induce any change at the target angle of 60
degree. WBV treatment probably influenced
proprioceptive feedback mechanisms andled to an improvement in the joint position
sense.26
Muscle activity induced by the applicationof WBV was monitored by recording the
EMG signal from the muscles. Although ourstudy did not show any significant change inmuscle activity or peak isometric torque after exposure to WBV, a decreasing pattern inthose values was revealed.
Cardinale and Lim27 found an increase in
EMG activity of vastus lateralis muscle during exposure of subjects to WBV transmitted
through a vibrating platform. Krol et al.28
also reported an increase in vastus lateralis
and vastus medialis muscle activities duringexposure of subjects to WBV stimulation. In
those studies, the researchers measured EMG
activity during exposure to WBV, whereas
we measured it after exposure to WBV.
But, our study results are consistent withthe findings of Jordan et al. 29 The authors
reported a decrease in voluntary muscle activation of knee extensors and peak isometrictorque following WBV. They explained thisas a compensatory effect of vibration in afatigued state. In contrast, enhanced muscle
activation and increase in peak isometric observed in some studies may be due the training effect of long-term exposure to WBV.
de Ruiter et al.15 also mentioned that in the
short-term, WBV training does not improvemuscle activation during maximal isometricknee extensor force production and maximal
rate of force rise in healthy untrained students.
Conclusions
In this study, the neuromuscular responses
to acute WBV were examined using a vibration acceleration value considered to be within a safe exposure limit. The findings indicatethat the exposure to acute WBV within therecommended dose limits by international
standards appears to be not adequate to induce any significant improvement in neuromuscular responses among healthy youngmen. These data may have implications forthe researchers in selecting the WBV dosethat has potential in enhancing neuromuscular performances in human subjects. Basedon the current findings, it seems necessary toinvestigate the health and safety aspects of ahigher dose of WBV than that recommended
by the international standards. To investigate the effects of WBV on human health,future research works should be conductedamong both young and elderly subjects withlong-term or chronic exposure to WBV within a safe exposure limit.
Acknowledgment
We wish to thank all the students of Higashi-Chikushi Gakuen Kyushu Rehabilitation College who participated in this study.
Conflict of Interest
The authors state no conflict of interest.
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