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Articoli piattaforma vibrante

La Piattaforma Vibrante

Physical Fitness and Performance
Effect of four-month vertical whole body
vibration on performance and balance
SAILA TORVINEN1,2, PEKKA KANNUS1,2, HARRI SIEVA¨ NEN1, TERO A. H. JA¨ RVINEN2, MATTI PASANEN1,
SAIJA KONTULAINEN
1, TEPPO L. N. JA¨ RVINEN2, MARKKU JA¨ RVINEN2, PEKKA OJA1, and ILKKA VUORI1
1Bone Research Group, UKK Institute, Tampere, FINLAND, and 2Department of Surgery, Medical School and Institute of
Medical Technology, University of Tampere, and Tampere University Hospital, Tampere, FINLAND
ABSTRACT
TORVINEN, S., P. KANNUS, H. SIEVA¨ NEN, T. A. JA¨ RVINEN, M. PASANEN, S. KONTULAINEN, T. L. JA¨ RVINEN, M.
JA¨ RVINEN, P. OJA, and I. VUORI. Effect of four-month vertical whole body vibration on performance and balance.
Med. Sci. Sports
Exerc.
, Vol. 34, No. 9, pp. 1523–1528, 2002. Purpose: This randomized controlled study was designed to investigate the effects of
a 4-month whole body vibration-intervention on muscle performance and body balance in young, healthy, nonathletic adults.
Methods:
Fifty-six volunteers (21 men and 35 women, aged 19–38 yr) were randomized to either the vibration group or control group. The
vibration-intervention consisted of a 4-month whole body vibration training (4 min·d
_1, 3–5 times a week) employed by standing on
a vertically vibrating platform. Five performance tests (vertical jump, isometric extension strength of the lower extremities, grip
strength, shuttle run, and postural sway on a stability platform) were performed initially and at 2 and 4 months.
Results: Four-month
vibration intervention induced an 8.5% (95% CI, 3.7–13.5%,
P _ 0.001) net improvement in the jump height. Lower-limb extension
strength increased after the 2-month vibration-intervention resulting in a 3.7% (95% CI, 0.3–7.2%,
P _ 0.034) net benefit for the
vibration. This benefit, however, diminished by the end of the 4-month intervention. In the grip strength, shuttle run, or balance tests,
the vibration-intervention showed no effect.
Conclusion: The 4-month whole body vibration-intervention enhanced jumping power in
young adults, suggesting neuromuscular adaptation to the vibration stimulus. On the other hand, the vibration-intervention showed no
effect on dynamic or static balance of the subjects. Future studies should focus on comparing the performance-enhancing effects of a
whole body vibration to those of conventional resistance training and, as a broader objective, on investigating the possible effects of
vibration on structure and strength of bones, and perhaps, incidence of falls of elderly people.
Key Words: VERTICAL JUMP,
LOWER LIMB EXTENSION STRENGTH, SHUTTLE RUN, POSTURAL SWAY, YOUNG ADULTS
Mechanical vibration has recently aroused large interest
because it has been hypothesized that a
low-amplitude, high-frequency stimulation of the
whole body could positively influence many risk factors of
falling and related fractures by simultaneously improving
muscle strength, body balance, and mechanical competence
of bones (3–5,9,10,21,23–28,35). There is, however, very
little scientific evidence about the effects of whole body
vibration on these parameters. Bosco et al. showed that a
single vibration bout resulted in a significant temporary
increase in muscle strength of the lower extremities (5) and
arm flexors (4). They also studied the effects of 10-d vibration
on muscular performance of physically active subjects
and showed that whole body vibration applied for 10
min·d
_1 induced an enhancement in explosive power (3).
Runge et al., in turn, showed that whole body vibration
could enhance muscle performance in elderly people (2-
month training program three times a week at the frequency
of 27 Hz) (29). There is also some preliminary evidence that
vibration-loading could stimulate trabecular bone formation
and prevent postmenopausal and ovariectomy-induced bone
loss (10,24,26).
Despite the above noted preliminarily positive findings
and wide use of different vibration devices among athletes,
conclusive evidence on the efficacy and safety of vibration
training is lacking. This lack of data is especially clear
concerning the long-term effects. The purpose of this study
was, therefore, to investigate the effects of a 4-month whole
body vibration intervention on muscle performance and
body balance of young, healthy volunteers, using a randomized
controlled study design.
MATERIALS AND METHODS
Subjects and Study Design
Fifty-six young, healthy, nonathletic volunteers (21 men
and 35 women aged 19–38 yr) participated in the study.
Half of the subjects were randomized to the vibration group
0195-9131/02/3409-1523/$3.00/0
MEDICINE & SCIENCE IN SPORTS & EXERCISE
®
Copyright © 2002 by the American College of Sports Medicine
Submitted for publication November 2001.
Accepted for publication April 2002.
DOI: 10.1249/01.MSS.0000027713.51345.AE
1523
and half to the control group. The men and women were
randomized separately into the groups so that number of
men and women would be approximately equal in both
groups. The vibration protocol consisted of a 4-month whole
body vibration training (see below). The performance tests
were done at baseline (before randomization) and at 2 and 4
months.
The exclusion criteria from the study were: any cardiovascular,
respiratory, abdominal, urinary, gynecological,
neurological, musculoskeletal, or other chronic disease;
pregnancy; prosthesis; medication that could affect the musculoskeletal
system; menstrual irregularities; and regular
participation in any exercise-inducing impact-type loading
on the skeleton more than three times a week.
The subjects completed a questionnaire detailing their
physical activity and calcium intake (from a 7-d calcium
intake diary) (36) at the beginning of the study and at 2 and
4 months. All participants gave their informed written consent
before enrollment, and the study protocol was approved
by both the Institutional Review Board and the Ethics Committee
of the UKK Institute.
Vibration Loading
Vibration loading was carried out in a standing position
on a whole body vibration platform (Kuntota¨ry, Erka Oy,
Kerava, Finland) and the subjects were asked to train with
it 3 to 5 times a week. The duration of daily stimulus was 4
min. While standing on the platform, the subjects repeated
four times a 60-s light exercise program according to instructions
prepared earlier. The rationale of the exercise
program was to provide a multidirectional vibration exposure
on the body and make the standing on the platform less
monotonous in a way that would be feasible for a long-term
intervention trial. The program comprised of light squatting
(0–10 s), standing in the erect position (10–20 s), standing
in a relaxed position with slightly flexed knees (20–30 s),
light jumping (30–40 s), alternating the body weight from
one leg to another (40–50 s), and standing on the heels
(50–60 s).
During the 4-min vibration exposure, the vibration frequency
increased in one min intervals. During the first two
weeks, the duration of the loading was 2 min, and the
frequency of vibration was 25 Hz for the first minute and 30
Hz for the second minute (the practice period). During next
1.5 months, the duration of the vibration loading was 3 min
and frequency 25 Hz/60 s
_ 30 Hz/60 s _ 35 Hz/60 s.
During the remaining 2 months, the length of exposure was
4 min, and the frequency was 25 Hz/60 s
_ 30 Hz/60 s _
35 Hz/60 s _ 40/60 s. The peak-to-peak amplitude of the
vertical vibration was 2 mm. Considering the amplitude and
the sinusoidal nature of the loading, the theoretical maximal
acceleration was some 2.5 g (where g is the Earth’s gravitational
field, or 9.81 ms
2) with 25 Hz loading, 3.6 g with 30
Hz loading, 4.9 g with 35 Hz loading, and 6.4 g with 40 Hz
loading.
Performance Tests
At the beginning of each test session, a 4-min warm-up
was performed on a bicycle ergometer (workload 40 W for
women and 50 W for men). The subjects wore the same
shoes during all three performance test sessions, and the
order of the performance tests was the same in every test
session (see below). Use of alcohol or strenuous physical
activity was not allowed during the test-day nor the preceding
day. Before each test, the subjects had one to two
unintensive familiarization trials.
A vertical countermovement jump test was used to assess
the lower-limb explosive performance capacity (2). The
subject kept hands on the pelvis. The tests were performed
on a contact platform (Newtest, Oulu, Finland), which measures
the flying time. The obtained flight time (t) was used
to estimate the height of the rise of body center of gravity (h)
during the vertical jump (i.e., h
_ gt2/8, where g _ 9.81
m·s
_1) (2). The median value of three measurements was
used as the test score.
A postural sway platform (Biodex Stability System, New
York, NY) was used to assess static body balance (32). The
subjects stood on a labile platform on both legs, with eyes
opened and arms beside the trunk. The platform provides
eight different stability-levels (level 8 is virtually stable and
level 1 is the most labile). As a test, we employed a 40-s
protocol in successive 10-s intervals [level 5 (0–10 s), level
4 (10–20 s), level 3 (20–30 s), and level 2 (30–40 s). This
system provides a numerical stability index that reflects the
body sway variation around the body’s center of gravity so
that the lower the index, the higher the level of stability (32).
Each subject’s feet position coordinates on the platform
were recorded after the first stability measurement, and the
same coordinates were used throughout the study to obtain
consistency between the tests. The mean value of two stability
indices was used as the test score.
Grip strength was measured using a standard grip strength
meter (Digitest, Muurame, Finland). The median value of
three readings was used as the test score.
Maximal isometric strength of the leg extensors was
measured with a standard leg press dynamometer (12). The
subjects sat on the dynamometer chair with their knees and
ankles at an angle of 90° of flexion while pressing maximally
against strain gauges (Tamtron, Tampere, Finland)
under their feet. The isometric strength was recorded for
three maximal efforts, and the median value of three readings
was used as the test score.
A shuttle run test over a 30-m course was used to assess
the dynamic balance or agility (1). The subjects were asked
to run as fast as possible six times between markers placed
four meters apart, to touch the floor after each 4-m run, and
finally to run a 6-m course over the finish line. A single
performance was done and the running time was recorded
with photoelectric cells.
Safety
Possible side effects or adverse reactions were asked from
the subjects of the vibration group monthly and from the
1524
Official Journal of the American College of Sports Medicine http://www.acsm-msse.org
control group in 2-month intervals. The subjects also had the
liberty of consulting the responsible study physician whenever
needed.
Statistical Analysis
Means and standard deviations are given as descriptive
statistics.
The 2-month and 4-month effects of
the whole body vibration on physical performance and balance
were defined as absolute and relative mean differences
[with 95% confidence intervals (CI)] between the vibration
and control groups, respectively. The relative differences
were achieved through log-transformation of the variables.
The time-effect at 2 and 4 months was determined by
one-way ANCOVA, using the baseline values as the covariate.
In all tests,
P _ 0.05 was considered significant.
RESULTS
Twenty-six subjects in the vibration group and 26 controls
completed the study without side effects or adverse
reactions. Two participants in the control group withdrew
from the study because of loss of interest, and two participants
in the vibration group withdrew because of musculoskeletal
problems that were independent from the vibrationloading
(the first one for rib fracture; the second one for an
orthopedic operation). The basic characteristics of the 52
subjects are given in Table 1.
The reported mean vibration-training frequency was 3.1
(
_0.9) times per week. Because there were no gender differences
in the time-effect at the 2-month and 4-month tests,
the data of women and men were pooled and analyzed
together.
Muscle Performance and Body Balance
Power and strength tests.
The vertical jump height
increased an average of 2.0 cm after 2 months vibration as
compared with a mean decrease of 0.6 cm in the control
group, resulting in a significant 10.2% net benefit (95% CI,
5.6–15.1%,
P _ 0.000) in the vibration group. At the
4-month test, jump height had increased 2.5 cm (from the
baseline) in the vibration group and 0.3 cm (from the baseline)
in the control group, resulting in a significant 8.5% net
benefit (95% CI, 3.7–13.5%,
P _ 0.001) in the vibration
group (Table 2 and Figure 1A).
Isometric lower limb extension strength improved an
average 11.2 kg after the 2-month vibration-intervention,
while in the control group a mean increase was 4.8 kg,
resulting in a statistically significant 3.7% net benefit (95%
CI, 0.3–7.2%,
P _ 0.034) for the vibration group. At the
4-month test, this net benefit had diminished to 2.5% (
P _
0.25) (Table 2 and Fig. 1B). In this context it must be noted
that the lower limb extension strength of one control subject
was clearly higher than that of the other control subjects,
thus increasing the standard deviations in the control group
(Table 2). This had, however, no effect on the absolute or
relative mean between-groups differences. As expected, in
neither group were changes observed in the grip strength at
the 2- and 4-month tests (Table 2 and Fig. 1 C).
Performance and balance tests. There were no differences
at the 2- and 4-month shuttle run tests between the
vibration and control groups (the mean between-groups net
difference
_0.5% at both time points, P _ 0.52 and P _
0.57, respectively) (Table 2 and Fig. 1D). Neither effect was
observed in the postural sway at the 2-month or 4-month
tests (Table 2 and Fig. 1E).
DISCUSSION
This randomized controlled study showed that a 4-month
whole body vibration-loading was safe to use and induced a
significant 8.5% mean increase in the jump height of young
healthy adults. This improvement was already seen after 2
months of the vibration. Lower limb extension strength was
also enhanced by the 2-month vibration-period. This increase,
however, slowed down by the end of the intervention,
and at 4 months the difference between the groups was
no more statistically significant, mostly due to increased
extension strength in the control group (learning effect).
Concerning the dynamic and static body balance, the
4-month whole body vibration-intervention showed no
effect.
Effects of resistance training on neuromuscular properties
of skeletal muscle are well known (6,13–18), and their
knowledge may help to interpret and understand the above
noted vibration findings. First, structural changes within a
skeletal muscle are of great importance when adapting to
strength training. However, voluntary strength performance
is determined not only by intramuscular factors but also by
the extent of neural activation, since training-induced
changes in the nervous system (neural adaptation) allow
more complete activation of the prime movers of a specific
movement and better coordination of the activation of the
relevant muscles, both of which result in a greater force in
the intended direction of movement (6,30).
The first adaptation mechanism of a skeletal muscle to
resistance training is neural (6,18,30). Changes in the neural
factors in response to training occur within a few months,
whereas changes in the morphological structure of the muscle
take longer (from several months to years). Specific
adaptations to training depend much on the training program
employed (6,30,31). In addition to pure maximal strength,
explosive power is an important factor in several sport
activities, and various stretch-shortening cycle (SSC) exercises
(e.g. jumping or plyometric exercises) have been used
TABLE 1. Basic characteristics of the vibration and control groups.
Vibration Group
N _ 26
Control Group
N _ 26
Women/Men 17/9 16/10
Age (yr) 23.2 (4.4) 25.5 (5.8)
Height (cm) 174.4 (8.0) 174.0 (7.7)
Weight (kg)
Baseline 71.6 (13.3) 71.1 (12.8)
At 2 months 71.5 (13.6) 71.7 (12.4)
At 4 months 70.8 (13.0) 70.7 (12.4)
Values are mean (SD).
EFFECT OF VIBRATION ON PERFORMANCE Medicine & Science in Sports & Exercise_ 1525
to improve this performance trait. The exact mechanism by
which the explosive power training can enhance neuromuscular
activation is not known, but there are several possible
explanations which could cause this enhancement, e.g., adaptation
of certain reflex responses, increase in motor unit
synchronization, co-contraction of the synergist muscles, or
increased inhibition of the antagonist muscles. Strength and
power training may also increase the ability of motor units
to fire briefly at very high rates, which may induce an
increase in the rate of force development even if the peak
force does not necessarily increase (6,18,30).
Whole body vibration-induced improvements in muscle
performance (3) have been suggested to be similar (and
occur via similar pathways) to those after several weeks of
resistance training (4,7,15). During a whole body vibration
loading, skeletal muscles undergo small changes in muscle
length, most likely since mechanical vibration is able to
induce a tonic excitatory influence on the muscles exposed
to it (33). In other words, vibration elicits a response called
“tonic vibration reflex,” including activation of muscle spindles,
mediation of the neural signals by 1a afferents (11),
and finally, activation of muscle fibers via large
_-motoneurons.
The tonic vibration reflex is also able to cause an
increase in recruitment of the motor units through activation
of muscle spindles and polysynaptic pathways (8).
In this study, neurogenic enhancement or changes in the
morphological structure of the muscles could not be assessed
directly because the study protocol included neither
EMG recordings nor muscle biopsies. However, on the basis
of the evidence mentioned above, it is likely that the given
FIGURE 1—The percentage changes in
the power, strength, performance, and
balance tests after the 2-month and
4-month vibrations. Mean and 95% confidence
interval. * Indicates
P < 0.05.
TABLE 2. The performance and balance test parameters at baseline and after 2-month and 4-month whole body vibration intervention.
Between Groups Difference for the Relative
Vibration Group Change by Time
(
N _ 26)
Control Group
(
N _ 26)
Mean Difference
Between Groups
a Mean 95 % CIa P value
Vertical jump* (cm)
Baseline 27.7 (7.9) 28.9 (8.2)
2-month 29.7 (7.2) 28.3 (8.1) 2.5 10.2 5.6 to 15.1
_0.001
4-month 30.2 (7.6) 29.2 (8.5) 2.1 8.5 3.7 to 13.5 0.001
Lower limb extension strength (kg)
Baseline 194.8 (64.5) 216.5 (103.4)
2-month 206.0 (69.8) 221.3 (110.7) 7.8 3.7 0.3 to 7.2 0.034
4-month 207.8 (65.8) 227.7 (116.9) 3.6 2.5
_1.8 to 7.1 0.250
Grip strength (kg)*
Baseline 30.8 (7.7) 32.4 (9.8)
2-month 31.2 (8.1) 32.3 (9.8) 0.6 1.6
_1.0 to 4.4 0.228
4-month 30.5 (7.9) 32.5 (9.9)
_0.4 _1.3 _3.1 to 0.5 0.143
Shuttle run (s)*
Baseline 11.0 (1.3) 11.2 (1.4)
2-month 10.8 (1.4) 11.0 (1.4)
_0.05 _0.5 _2.0 to 1.0 0.516
4-month 10.7 (1.2) 10.9 (1.4)
_0.07 _0.5 _2.2 to 1.3 0.570
Postural sway (stability index)
Baseline 3.1 (1,7) 3.5 (1,2)
2-month 3.1 (1,7) 3.7 (1,5)
_0.3 _7.1 _21.7 to 10.0 0.385
4-month 3.0 (1,3) 3.4 (1,3)
_0.2 _1.7 _14.9 to 13.7 0.817
Mean (SD), mean difference between the vibration and control groups, and mean between-groups difference (95 % CI and
P-value) for the relative change by time (%).
*
N _ 25 in the vibration group.
aOne-way analysis of covariance.
1526 Official Journal of the American College of Sports Medicine http://www.acsm-msse.org
whole body vibration training elicited neural adaptation.
This was also supported by the results of the study; i.e., the
quickly and clearly enhanced jump height suggested that
neural adaptation did occur in response to the vibrationintervention.
In addition, the lower-limb extension strength
increased only after 2 months of vibration, thus also referring
to neural potentiation. The rate of increase in the lower
limb extension strength and difference between the intervention
groups, however, diminished by the end of the
4-month intervention. This could be explained by general
muscular adaptation to the vibration program. Further improvement
in the extension strength might have required a
greater change in the training stimulus.
When interpreting the results of the current study (the rise
in vertical jump height), one has to remember that the
training group subjects also did a light exercise program
during the 4-min vibration exposure (see Materials and
Methods), and thus, one could suspect that the improvement
in the jump height was because of this exercise. However, it
was very unlikely that these exercises were behind the clear
rise in the jump height in that the exercises were very light.
Considering the effects of whole body vibration on falls
and related fractures in elderly people, our study showed
that the vibration-intervention had no direct influence on
body balance. However, muscle power and strength are also
important and independent predictors of functional performance
and falls of older people (19,20,22,34); therefore,
whole body vibration exercise may be efficient training
stimulus for these people, too. Future studies should focus
on comparing the performance-enhancing effects of whole
body vibration to those of a conventional resistance training,
and as a broader objective, on investigating the possible
effects of vibration on structure and strength of bones, and
perhaps on the incidence of falls of the elderly.
This study was supported by the grants from Medical Research
Fund of Tampere University Hospital, Tampere, Finland, and the
Research Foundation of the Institute of Sports, Helsinki, Finland.
The authors thank all the participants for excellent cooperation,
and they thank their research assistant Seppo Niemi for help in
preparing the figure of this report.
Address for correspondence: Dr. Saila Torvinen, UKK Institute,
Kaupinpuistonkatu 1, FIN-33500 Tampere, Finland; E-mail:
losato@uta.fi.
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1528 Official Journal of the American College of Sports Medicine http://www.acsm-msse.org

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