Vibration as an exercise modality: how it may work, and what its potential might be.

TL;DR: Evidence suggests that acute vibration exercise seems to elicit a specific warm-up effect, and that vibration training seems to improve muscle power, although the potential benefits over traditional forms of resistive exercise are still unclear.

Abstract: Whilst exposure to vibration is traditionally regarded as perilous, recent research has focussed on potential benefits. Here, the physical principles of forced oscillations are discussed in relation to vibration as an exercise modality. Acute physiological responses to isolated tendon and muscle vibration and to whole body vibration exercise are reviewed, as well as the training effects upon the musculature, bone mineral density and posture. Possible applications in sports and medicine are discussed. Evidence suggests that acute vibration exercise seems to elicit a specific warm-up effect, and that vibration training seems to improve muscle power, although the potential benefits over traditional forms of resistive exercise are still unclear. Vibration training also seems to improve balance in sub-populations prone to fall, such as frail elderly people. Moreover, literature suggests that vibration is beneficial to reduce chronic lower back pain and other types of pain. Other future indications are perceivable.

Summary

Introduction

• A visit at the local gym will demonstrate how popular vibration exercise currently is, with numerous devices available for exercise and physical therapy.
• That view originated from occupational exposure to vibration.
• Two fascinating fields, however, will go more or less unmentioned, namely central nervous information processing of vibration, as it is too complex to be covered here, and the potential effects upon adipogenesis, for which the available evidence is still preliminary.
• Vibration applied to the tendons during isometric contractions leads to an over-estimation of the force generated by 30% (Cafarelli and Kostka 1981), and, conversely, a 25% smaller than intended force is generated.
• The important question therefore is whether the current interest in vibration as an exercise modality is only due to unduly perceived exertion, or whether it can really constitute a physiological training stimulus.

Physical principles

• Vibration is a mechanical oscillation, i.e. a periodic alteration of force, acceleration and displacement over time.
• During vibration exercise, the human body is accelerated, which causes a reactive force by and within the human body.
• When controlling for the effective stimulatory parameters (force, velocity, energy), the physiological responses to these devices should resemble the response to platform devices.
• As a consequence, the rigid body will lose contact and become air-bound when the acceleration of the platform is smaller than -1g (see Fig. 3).
• Moreover, not all amplitude amplification causes resonance catastrophe, as this will happen only if the generated forces exceed the resonator’s structural strength.

Acute physiological effects

• They indicate that amplitude amplification may occur under certain circumstances, and it is therefore important to assess vibration transmission by physical measurements (see also ‘‘Vibration transmission’’).
• On the other hand, and in apparent contrast to the reflex contraction mediated by the Ia-loop, the stretch reflex and the H-reflex are suppressed while vibration is applied to the muscle (Arcangel et al.
• During whole body vibration exercise (f = 15 Hz, A = 2.5 mm, side-alternating), combined with squatting exercise, an increase in tissue oxygenation was found when the leg muscles were vibrated passively (Yamada et al. 2005).
• Other studies have challenged this view and suggested that vibration affects muscle perfusion only at certain frequencies (Calvisi et al. 2006; Cardinale et al. 2007).

Single muscle vibration

• Earlier studies with an occupational background have reported accelerated muscle fatigue by vibration (Farkkila et al. 1980; Samuelson et al. 1989).
• Moreover, brief bouts of vibration can reinstate EMG amplitude, and to a lesser degree force generation applied in a muscle fatigued by isometric contraction (Bongiovanni and Hagbarth 1990; Gabriel et al. 2002).
• This enhancement, however, lasts only 10 s and reverts to its contrary after this time and to accelerated fatigue progression (Bongiovanni and Hagbarth 1990).
• Finally, it should be remembered that some motor units are turned on by vibration more easily than others (Bongiovanni and Hagbarth 1990; Bongiovanni et al. 1990).

Vibration as an exercise modality

• Bosco et al. (1999b) were the first to study the acute effects of vibration as an exercise modality on muscular power.
• Taken together, the literature therefore is quite unanimous in stating that muscular power output can be acutely increased by vibration exercise whilst force generation capacity seems to be depressed post vibration exercise.
• It is also important here to consider that afferents from cutaneous mechanoreceptors can affect motor reflexes and stance (Magnusson et al. 1990).
• Vibratory foot stimulation causes a shift of posture towards the front when applied under the heel, and to the rear when applied under the forefoot (Kavounoudias et al. 1999).
• Provided that vibration exercise involves mechanical stretching (see ‘‘Muscle and tendon mechanics’’), there is good justification for the idea to increase flexibility by vibration exercise (Atha and Wheatley 1976).

Testosterone

• The initial report of small but significant increases of testosterone levels in response to whole body vibration exercise (Bosco et al. 2000) could not be confirmed by subsequent studies (Cardinale et al.
• Moreover, Kvorning et al. (2006) demonstrated increase in plasma testosterone levels in response to conventional resistive exercise with or without vibration, but not in response to vibration alone, suggesting that there is probably no vibration-specific effect upon testosterone deliberation.

Growth hormone (GH) and IGF-1

• An originally reported acute increase of GH levels after vibration exercise (Bosco et al. 2000) could not be replicated by subsequent studies (Cardinale et al.
• One should consider that GH is released in a pulsatile fashion (Corpas et al. 1993), and that it is therefore inherently difficult to monitor its acute responses to exercise.
• Quite interestingly, another study found vibrationspecific GH responses that were potentiated when combining vibration with conventional resistive exercise (Kvorning et al. 2006).
• Moreover, whole body vibration fosters IGF-1 levels in elderly people (Cardinale et al. 2008), thus suggesting that there may indeed be a vibration-specific effect on the GH-IGF axis.

Cortisol

• Whilst exercise often leads to increased serum cortisol levels, there is some evidence to suggest that vibration may have the contrary effect (Bosco et al. 2000; Kvorning et al. 2006).
• Other studies contradict this notion and even report an increase in cortisol levels in response to whole body vibration (Erskine et al. 2007; Cardinale et al. 2008).

Blood lipids

• An increase in serum free fatty acids, but not in glycerol, has been reported 150 min after the vibration exercise (Goto and Takamatsu 2005).
• As proposed by the authors, mainly because of the considerable time lag, this is more likely to be related to the non-significant increase in GH observed in that study than to catecholamine action.

Blood glucose

• Evidence suggests that vibration exercise may lead to reduced blood glucose levels (Di Loreto et al. 2004).
• Given that insulin and glucagon levels were not affected in that study the most likely explanation would be an enhanced glucose uptake from the blood, probably into the musculature.

Adaptive and training effects

• The literature available on the effects of vibration training programs upon the musculature has been summarized in Table 1.
• The table discloses some common features.
• Secondly, training programs were mostly constructed with a few bouts lasting between 1 and 2 min.
• Moreover, most studies encouraged participants to change between different exercises and postures, e.g. between different types of squats and lunges.
• As a general rule, smaller vibration amplitudes have been applied with synchronous than with side-alternating vibration.

Effects by vibration training

• Looking at the outcome in Table 1 it seems that jump height is consistently enhanced by vibration training in elderly people (Bogaerts et al.
• In line with this view, all of the improvement was achieved after 12 weeks in the 24-week study by Roelants et al. (2004b), and no substantial change in muscle power was observed in Torvinen’s study between 4 months (Torvinen et al. 2002b) and 8 months (Torvinen et al. 2003).
• Two different philosophies have been employed to justify the application of vibration for enhancement of bone strength (Cardinale and Rittweger 2006).
• Nevertheless, these results are encouraging, even though the observed increase was moderate in absolute terms.
• As for the leg muscles, resistive vibration exercise had positive effects for the back extensor muscles, as their atrophy, as well as spinal lengthening and intervertebral disk swelling were all significantly mitigated, though not entirely prevented, in the training group (Belavy et al. 2008).

Clinical application

• It seems clear from the above that vibration induces specific responses in the body that could potentially be exploited for therapeutic purposes.
• Focussing on studies that have investigated vibration as an exercise modality, there seems to be an acute reduction in postural sway (Torvinen et al. 2002a), which, however, does not translate into any persistent changes after an 8- months training program in young women (Torvinen et al. 2003).
• The question arises in how far the response to vibration exercise will be affected by age.
• Potentially, vibration-induced pain relief may involve supra-spinal mechanisms (Gay et al. 2007), as passive vibration can improve the range of motion and reduce pain severity in complex regional pain syndrome type I, formerly known as reflex sympathetic dystrophy syndrome or Sudeck’s atrophy.
• It was found that patients improved in their functional abilities, which also led to greater independence in almost all cases (Semler et al. 2008).

Safety aspects

• ISO 2631-1 (1997) defines the limits of vibration that people can be expected to tolerate in industrial exposure (see Fig. 5).
• The important question arises in how far the ISO 2631-1 standard is applicable when vibration is applied as a means of exercise and rehabilitation.
• Vibration transmissibility to the head and trunk can be reduced by knee flexion (Abercromby et al. 2007b).
• 1994; Lundborg et al. 1990), which physiologically are involved in the axon reflex (Bruce 1913) and thus promote vasodilation through release of endothelin-1 and histamin (Dowd et al. 1998).
• It is unclear whether an equivalent to the vibration white finger disease can also occur in the lower extremity.

Conclusions

• Whilst vibration exposure has traditionally been regarded as perilous only, it is now seen as potentially beneficial in certain areas of sports, exercise, rehabilitation and preventive medicine.
• There is an emerging profile of application of vibration as an exercise modality.
• Acknowledgment I would wish to express my gratitude to Dag Linarsson for inviting me to write this article—complying with his request has been even more enjoyable (but also more exhaustive) than anticipated.
• Most importantly, thanks to the two Hans’ (Schiessl & Degens), who are entirely different characters, but equally good characters, and who hopefully will continue to en-Hans me.

Figures

Fig. 1 Rating of perceived exertion (RPE) on a scale of 20 (Borg and Borg 1976) during either simple squatting exercise (ordinate) or whole body vibration exercise (abscissa) on a Galileo Fitness platform (Novotec Medical, Pforzheim, Germany). Both conditions were set up so that the metabolic rates were matched (squatting: 11.4 ml O2 kg -1 min-1 (SD 0.7), vibration 10.7 ml O2 kg -1 min-1 (SD 1.0), p = 0.3). By contrast, RPE was significantly greater in vibration (mean 11.9, SD 2.4, dashed line) than in squatting (mean 9.3, SD 2.1, solid line) exercise, suggesting that at perceived exertion is at least partly dominated by factors unrelated to metabolic rate. Interestingly, there seems to be no relationship between RPE values for vibration and squatting conditions, indicating that different people perceive both exercise modalities as differently exerting. Data are from an unpublished experiment by Rittweger & Degens

Fig. 2 Illustration of the two principle modes of vibration transmission in whole body vibration exercise. In the synchronous mode, both legs extend and stretch at the same time, and a purely linear acceleration is directed to the trunk. In the side-alternating mode, conversely, the right and left leg operate anti-phase, which introduces a rotary component to the lumbar spine and therefore is expected to reduce vibration transmission to the trunk. Evidence suggests, that greater aPeak levels can be tolerated in the side-alternating as opposed to the synchronous vibration mode (Abercromby et al. 2007b)

Fig. 3 Demonstration of the behaviour of a rigid body that is attached to a vibration platform by gravity only. The upper diagram shows the displacements of the platform and the rigid body, respectively. In this example, where f = 15 Hz, A = 2 mm and aPeak = 1.81 g, the rigid body will be lifted off the platform just before the platform reaches its highest position, and the rigid body hits the platform just after it reaches its lowest position. This causes a distinct impact, as demonstrated by the acceleration depicted in the lower diagram. It is obvious, however, that the human body is not completely rigid, but rather has elastic and damping properties. Nevertheless, the feet can sometimes be lifted off the platform during whole body vibration exercise, which leads to a loss of contact and skidding

Fig. 5 Safety limits for vertical sinusoidal vibration exposure at 18 Hz and at 25 Hz, as defined by the ISO 2361-1 standard. The ISO 2361-1 safety-limit for 18 Hz has been compared to the exposure in a study by Rittweger et al. (2002b), in which whole body vibration (f = 18 Hz, A = 6 mm, side-alternating) has been practiced for 7 min per day by patients with chronic lower back pain. Patients reported significant pain alleviation as a result of the intervention, despite the fact that vibration exposure exceeded the ISO 2361-1 safety-limit by threefold. Moreover, ISO 2361-1 permits greater values for 25 Hz than for 18 Hz, although perceived exertion in whole body vibration exercise is greater for 25 Hz than for 18 Hz vibration. It is apparent, therefore, that ISO 2361-1 limits cannot be transferred to whole body vibration exercise in a straightforward way

Full text

INVITED REVIEW
Vibration as an exercise modality: how it may work,
and what its potential might be
Jo
¨
rn Rittweger
Accepted: 13 November 2009 / Published online: 12 December 2009
Ó Springer-Verlag 2009
Abstract Whilst exposure to vibration is traditionally
regarded as perilous, recent research has focussed on
potential benefits. Here, the physical principles of forced
oscillations are discussed in relation to vibration as an
exercise modality. Acute physiological responses to iso-
lated tendon and muscle vibration and to whole body
vibration exercise are reviewed, as well as the training
effects upon the musculature, bone mineral density and
posture. Possible applications in sports and medicine are
discussed. Evidence suggests that acute vibration exercise
seems to elicit a specific warm-up effect, and that vibration
training seems to improve muscle power, although the
potential benefits over traditional forms of resistive exer-
cise are still unclear. Vibration training also seems to
improve balance in sub-populations prone to fall, such as
frail elderly people. Moreover, literature suggests that
vibration is beneficial to reduce chronic lower back pain
and other types of pain. Other future indications are
perceivable.
Keywords Mechanical oscillation  Training 
Rehabilitation  Physical medicine  Safety  ISO standard
Nomenclature
a Acceleration, m s
-2
a
Peak
Peak acceleration, i.e. largest acceleration within
one vibration cycle. For sinusoidal oscillations
given by x
2
A (Eq. 1), m s
-2
a
RMS
Mean acceleration, i.e. average acceleration over an
entire vibration cycle. For sinusoidal oscillations
given by a
Peak
/H2 (Eq. 2), m s
-2
A Amplitude of the oscillation. In other words, the
displacement of the oscillating actuator is between
-A and A,m
BMD Bone mineral density
D Damping factor, given by the ratio of the
attenuation coefficient and x
0
EMG Electromyography
f Frequency of the oscillation, i.e. the number of
vibratory cycles per unit time. Therefore, 1/f gives
the duration of a single cycle, Hz, which is
equivalent to s
-1
g Gravitational acceleration on Earth, 9.81 m s
-2
k Stiffness, i.e. the resistance to deformation, N/m
m Mass that is inert to acceleration as well as being
accelerated by gravity, kg
p 3.1415
x Angular frequency. x is proportional to f, as it is
given by 2pf, Hz, which is equivalent to s
-1
x
0
Resonance frequency of the resonator, i.e.
frequency at which mechanical energy will
be accumulated, expressed as angular frequency,
Hz
x
A
Angular frequency of the actuator, Hz
Communicated by Susan Ward.
J. Rittweger (&)
Institute of Aerospace Medicine,
German Aerospace Center,
Linder Ho
¨
he 1, Ko
¨
ln 51147, Germany
e-mail: joern.rittweger@dlr.de
J. Rittweger
Institute for Biomedical Research into Human Movement
and Health (IRM), Manchester Metropolitan University,
Oxford Rd, Manchester M1 5GD, UK
123
Eur J Appl Physiol (2010) 108:877–904
DOI 10.1007/s00421-009-1303-3

Introduction
A visit at the local gym will demonstrate how popular
vibration exercise currently is, with numerous devices
available for exercise and physical therapy. However, the
notion that vibration can be beneficial is relatively new, as
it has traditionally been regarded as only detrimental. That
view originated from occupational exposure to vibration. In
the neuronal sciences, by contrast, vibration is a standard
tool of investigation (Hagbarth and Eklund 1969). It is
therefore fair to say that studies of vibration effects have a
long tradition.
Nevertheless, only few authors have postulated thera-
peutic effects by vibratory stimuli in the past. Among them,
the first were Sanders (1936) and Whedon et al. (1949),
who performed some studies on an oscillating bed, meant
to counteract cardiovascular and musculoskeletal de-con-
ditioning. Nazarov and Spivak (1985) were the first to
apply vibration as a training modality for athletes. With
some delay, this led to an emerging scientific interest in
vibration as an exercise modality (Bosco et al. 1998a, b,
1999a, b, 2000; Issurin and Tenenbaum 1999; Kerschan-
Schindl et al. 2001; Rittweger et al. 2000, 2002a), not least
driven by companies who started to market commercial
devices.
However, vibration exercise nowadays being broadly
available to exercisers and patients, it seems that this
exercise modality is still largely unknown to the scientific
community, and there is currently no comprehensive
review available on the topic to allow an in-depth under-
standing, which is the purpose of the current article. Two
fascinating fields, however, will go more or less unmen-
tioned, namely central nervous information processing of
vibration, as it is too complex to be covered here, and the
potential effects upon adipogenesis, for which the available
evidence is still preliminary.
When being vibrated on a platform, most people report
an unusual perception that is often compared to the ranging
and banging of the feet during downhill skiing. This sen-
sation is partly due to a movement illusion (see ‘‘Neuro-
physiological responses’’), and there is also a perception of
exertion, which is not explicable by metabolic rate (see
Fig. 1). Vibration applied to the tendons during isometric
contractions leads to an over-estimation of the force gen-
erated by 30% (Cafarelli and Kostka 1981), and, con-
versely, a 25% smaller than intended force is generated.
Quite confusingly, the discrepancy between intended and
actual force levels declines during prolonged contractions
(Cafarelli and Layton-Wood 1986). The important question
therefore is whether the current interest in vibration as an
exercise modality is only due to unduly perceived exertion,
or whether it can really constitute a physiological training
stimulus.
Physical principles
Vibration is a mechanical oscillation, i.e. a periodic alter-
ation of force, acceleration and displacement over time.
Vibration exercise, in a physical sense, is a forced oscilla-
tion, where energy is transferred from an actuator (i.e. the
vibration device) to a resonator (i.e. the human body, or
parts of it). In most vibration exercise devices, these
oscillations have sinusoidal shape, and they are therefore
described by amplitude A, frequency f, and phase angle u.
Here and in the following A denotes the mathematical
amplitude, i.e. half the peak-to-peak amplitude. The angular
frequency x is given as 2pf. During vibration exercise, the
human body is accelerated, which causes a reactive force by
and within the human body. Importantly, the peak accel-
eration (a
Peak
) in sinusoidal oscillation is given by
a
Peak
¼ x
2
A: ð1Þ
Peak forces are potentially harmful to the body. On the
other hand, forces also constitute a training stimulus.
Therefore, a
Peak
values are important to consider. Another
convention to quantify vibration exposure is by RMS
(meaning root-mean square), which for sinusoidal
oscillations relates to a
Peak
as
a
RMS
¼
a
Peak
ffiffiffi
2
p
: ð2Þ
Fig. 1 Rating of perceived exertion (RPE) on a scale of 20 (Borg and
Borg 1976) during either simple squatting exercise (ordinate) or whole
body vibration exercise (abscissa) on a Galileo Fitness platform
(Novotec Medical, Pforzheim, Germany). Both conditions were set up
so that the metabolic rates were matched (squatting: 11.4 ml
O
2
kg
-1
min
-1
(SD 0.7), vibration 10.7 ml O
2
kg
-1
min
-1
(SD 1.0),
p = 0.3). By contrast, RPE was significantly greater in vibration (mean
11.9, SD 2.4, dashed line) than in squatting (mean 9.3, SD 2.1, solid
line) exercise, suggesting that at perceived exertion is at least partly
dominated by factors unrelated to metabolic rate. Interestingly, there
seems to be no relationship between RPE values for vibration and
squatting conditions, indicating that different people perceive both
exercise modalities as differently exerting. Data are from an unpub-
lished experiment by Rittweger & Degens
878 Eur J Appl Physiol (2010) 108:877–904
123

The actuator: different device types
Vibration exercise is mostly practiced as whole body
vibration, i.e. while standing on oscillating platforms.
Among the different platforms, two different types of
energy transfer have to be discerned. One type transfers
vibration to both feet synchronously, whilst the second type
operates in a side-alternating way, so that the right foot is
lowest when the left foot is highest (and vice versa,
Fig. 2a). It has been argued that side-alternating vibration
would evoke rotational movements around the hip and
lumbo-sacral joints (Rittweger et al. 2001). This movement
introduces an additional degree of freedom (see next sec-
tion) and, accordingly, whole-body mechanical impedance
is smaller in side-alternating than in synchronous whole
body vibration (Abercromby et al. 2007b). It should be
mentioned here that attempts have been made to develop a
third device type with randomly varying movements in the
horizontal and vertical plane. However, there are only very
few studies on this random vibration system, and due to
technical difficulties only vibration frequencies well below
10 Hz have been tested so far.
Finally, vibrating dumbbells have been developed for
upper-body exercise, which can be suspended and com-
bined with weight stacks (Cardinale and Rittweger 2006).
Their mechanical action on the human body will depend on
the specific set-up. However, when controlling for the
effective stimulatory parameters (force, velocity, energy),
the physiological responses to these devices should
resemble the response to platform devices.
Devices also differ by way of energy generation. Whilst
some operate by direct mechanical transmission (e.g.
Galileo
Ò
), some rely upon electromagnetic transmission,
and most available machines are based on oscillating mass-
spring systems (e.g. PowerPlate
Ò
or FitVibe
Ò
). As one
would expect, not all systems perform equally well (see
http://www.massamagra.com/pedane-vibranti/
recensioni.htm), and scientists should convince themselves,
e.g. by using accelerometers, of an accurate setting.
The resonator: importance of stiffness, damping
and posture
What happens to the body when being vibrated? As a first
approximation, consider a rigid body with mass m. When
the attachment of such a rigid body to the vibration device
is firm, then it will simply follow the sinusoidal trajectory
imposed by the actuator. The force applied is then deter-
mined by the acceleration of m. On whole-body platforms,
however, there is no firm attachment and the only down-
ward force acting on the body is gravity (Yue and Mester
2002). As a consequence, the rigid body will lose contact
and become air-bound when the acceleration of the plat-
form is smaller than -1g (see Fig. 3). Practically, this is
the reason why people’s feet sometimes skid on the
vibration plate. Moreover, as can also be seen from Fig. 3,
a collision with the platform will occur towards the end of
the air borne phase. This leads to the generation of impact
forces. Spectral analysis will interpret such impacts as
frequencies greater than the vibration frequency, some of
which will impose as harmonics (i.e. spectral power at 2f,
3f,4f etc.). Conversely, being air-bound can also lead to
missing out one or several cycles of the vibration platform,
and thus generate sub-harmonic frequencies (i.e. f/2, f/3,
f/4) in the vibrated object. Hence, firm stance on the force
plate must be achieved in order to ascertain well-defined
vibration parameters.
Obviously, however, the human body is not a rigid body,
and muscles and tendons act as spring-like elements that
store and release mechanical energy. In such a spring-mass
system, compression occurs during the vibration upstroke,
and expansion during the down stroke. As a consequence,
the displacement is smaller at the body’s centre of mass
than at the platform level. Together, the stiffness and mass
determine the natural frequency (x
0
) of such a system.
More precisely, x
0
¼
ffiffiffiffiffiffiffiffiffi
k=m
p
; where k is stiffness and m
mass. Obviously, adjustment of x
0
must be through alter-
ation of k,asm cannot be changed.
It is important to understand that such a mass-spring
resonator can accumulate mechanical energy, namely when
Synchronous Side-Alternatin
g
Vibration Transmission
Fig. 2 Illustration of the two principle modes of vibration transmis-
sion in whole body vibration exercise. In the synchronous mode, both
legs extend and stretch at the same time, and a purely linear
acceleration is directed to the trunk. In the side-alternating mode,
conversely, the right and left leg operate anti-phase, which introduces
a rotary component to the lumbar spine and therefore is expected to
reduce vibration transmission to the trunk. Evidence suggests, that
greater a
Peak
levels can be tolerated in the side-alternating as opposed
to the synchronous vibration mode (Abercromby et al. 2007b)
Eur J Appl Physiol (2010) 108:877–904 879
123

the frequency of the actuator, x
A
, matches the frequency of
the resonator, x
0
. The accumulation of energy can lead to a
situation where the vibration amplitude is greater in the
resonator than in the actuator. This amplitude amplification
entails increased internal forces within the resonator, which
can lead to its destruction—the so-called resonance catas-
trophe. It is crucial to bear in mind that amplitude ampli-
fication can only occur if there is little damping (see
Fig. 4a). Moreover, not all amplitude amplification causes
resonance catastrophe, as this will happen only if the
generated forces exceed the resonator’s structural strength.
Nevertheless, resonance should be prevented in vibration
exercise, e.g. by alteration of stiffness and thus x
0
,orby
introducing a damping element (friction). Evidence sug-
gests that muscles have such damping properties (Wake-
ling et al. 2002). However, any mechanical damping will
lead to the absorption of energy and thus generate heat.
Finally, considering to the human anatomy, we have to
consider the way in which vibrations are transmitted. This
will occur from one segment to the next, i.e. from the foot
to the calf, from the calf to the thigh etc. The amount of
vibration energy transmitted will depend on musculoskel-
etal stiffness and damping (see subsection Vibration
transmission at the end of this article for specific details).
Importantly, effective axial body stiffness increases with
straightened limbs (Greene and McMahon 1979; Lafortune
et al. 1996), which also leads to an increase in resonance
frequency. Assuming an appropriate posture can therefore
help to avoid resonance. Equally important, standing on the
forefoot will involve the ankle joint actuators (i.e. the calf
muscles) in the damping process, which is not the case
when stance is on the mid-foot (see Fig. 4b). Posing weight
on the forefoot, therefore, can help to avoid resonance, and
also to increase damping by the musculature and thus
reduce vibration transmission to the trunk. Hence, assum-
ing an appropriate posture is the prerequisite to avoid
unpleasant head and trunk vibration, and also to provide
firm stance on the vibrating platform.
Acute physiological effects
Muscle and tendon mechanics
What happens to the muscle–tendon complex during
vibration exercise? Given that a
Peak
levels at the head
are usually much smaller than at the vibrating platform
(Abercromby et al. 2007b), one has to assume that muscles
and tendons will elongate at one time (stretch phase), to be
followed by a period of shortening (shortening phase). In
other words, vibration exercise should be characterized by
cyclic transition between eccentric and concentric muscle
contractions. A recent study suggests that this is indeed the
case, and that the gastrocnemius muscle tendon complex is
Fig. 3 Demonstration of the
behaviour of a rigid body that is
attached to a vibration platform
by gravity only. The upper
diagram shows the
displacements of the platform
and the rigid body, respectively.
In this example, where
f = 15 Hz, A = 2 mm and
a
Peak
= 1.81 g, the rigid body
will be lifted off the platform
just before the platform reaches
its highest position, and the
rigid body hits the platform just
after it reaches its lowest
position. This causes a distinct
impact, as demonstrated by the
acceleration depicted in the
lower diagram. It is obvious,
however, that the human body is
not completely rigid, but rather
has elastic and damping
properties. Nevertheless, the
feet can sometimes be lifted off
the platform during whole body
vibration exercise, which leads
to a loss of contact and skidding
880 Eur J Appl Physiol (2010) 108:877–904
123

elongated by 1% of its total length during 6 Hz vibration
cycles with a
Peak
= 0.6g (Cochrane et al. 2009).
Neurophysiological responses
Although no study has yet assessed the motor unit dis-
charge patterns during vibration exercise, the neurophysi-
ological responses of the muscle to isolated vibrations
applied are very well studied. Applying vibration directly
to the muscle belly or tendon elicits a phase-oriented dis-
charge from primary (Brown et al. 1967; Burke et al. 1976;
Homma et al. 1971; McGrath and Matthews 1973; Roll and
Vedel 1982) and also secondary spindle endings (Brown
et al. 1967; Burke et al. 1976; McGrath and Matthews
1973). Between the two, primary endings are more
responsive than secondary endings (Bianconi and van der
1963; Brown et al. 1967). The spindle discharge depends
on the pre-stretch of the muscle and generally increases
with muscle length or stretch (Burke et al. 1976; Cordo
et al. 1993). It is also enhanced during voluntary isometric
contraction (Burke et al. 1976). In addition to the spindle
afferents, Ib-afferents from Golgi tendon organs are like-
wise responsive to muscle vibration (Burke et al. 1976;
Hayward et al. 1986). Like the spindle endings, afferents
from Golgi tendon organs become more responsive to
vibration when the muscle is contracting (Brown et al.
1967). The Golgi organ is believed to measure tendon
elongation, and thus a surrogate of force. It elicits an
inhibitory effect upon motor output via polysynaptic spinal
pathways, and its information converges with that from
cutaneous receptors, spindle afferents, joint receptors and
others (Lundberg et al. 1975).
Importantly, spindle discharge will elicit an excitatory
effect upon the a-motoneurone, either by monosynaptic
(primary or Ia afferents) or polysynaptic (secondary or
II-afferents) pathways and thus foster contractions of the
homonymous muscle (Granit et al. 1956). Passive muscle
vibration therefore causes a reflex contraction, also known
as the tonic vibration reflex (Hagbarth and Eklund 1966;
Matthews 1966). It is characterized by a gradual onset, and
it can be voluntarily suppressed (Hagbarth and Desmedt
1973; Lance et al. 1973). Alongside the contraction, people
experience an illusion of movement during the tonic
vibration reflex (Goodwin et al. 1972a, b). During the
reflex contraction, the discharge from both primary and
secondary spindle endings seem to decline, whereas the
discharge from Golgi tendon organs is increased (Burke
et al. 1976). Microneurographic evidence in humans
suggests that the reflex contraction crucially depends on
fusimotor efferents (Burke et al. 1976), suggesting that
supra-spinal control is involved in it. In relation to this, it
has been pointed out that the reflex contraction bears
similarities with Kohnstamm’s phenomenon (Bergenheim
et al. 2000; Gilhodes et al. 1992
).
m
body
thigh
calf
foot
F
F’
(a) (b)
Fig. 4 Model of the human body as a resonator, composed of a single
(a) or multiple (b) segments with spring-like and damping behaviour.
a A mass is linked to a spring with stiffness k and a dashpot with
friction b (see inset). This system can act as a resonator with
resonance frequency -
0
. When driven by an actuator with angular
frequency -
A
, transmissibility is determined by -
A
/-
0
(=normalized
frequency) and the grade of damping D. Amplitude amplification is
defined by transmissibility [1, i.e. when the amplitude of the
resonator is larger than the amplitude of the actuator. This occurs
when D is below a critical value and when -
0
& -
A
. Adapted from
Hering et al. (2004). b The calf and thigh muscles are idealized as
spring-dashpot systems, which can store and absorb energy. Force F
applies at the foot and thus loads the calf muscles and potentially also
the thigh muscles. By contrast, force F’ applies at the rotational centre
of the ankle and will therefore load the thigh muscles, but not the calf
muscles. Posing weight on the forefoot or on the mid-foot can thus
alter the transmission of vibration and thus relative loading of calf and
thigh muscles in whole body vibration exercise. Conversely, locking
of the knees will reduce energy absorption in the thigh muscles and
lead to greater vibration transmission to the trunk. It should be
understood that this model is an over-simplification. However, it
constitutes a basis for understanding the physics of whole body
vibration. Several mechanical models have been developed that
provide more detail than the simple sketch in this figure (Cole 1978;
Ghista 1982; Yue and Mester 2002). They indicate that amplitude
amplification may occur under certain circumstances, and it is
therefore important to assess vibration transmission by physical
measurements (see also ‘‘Vibration transmission’’ )
Eur J Appl Physiol (2010) 108:877–904 881
123

Frequently asked questions

Q1. What is the definition of vibration exercise?

In other words, vibration exercise should be characterized by cyclic transition between eccentric and concentric muscle contractions. 

Q2. What are the contributions in "Vibration as an exercise modality: how it may work, and what its potential might be" ?

Here, the physical principles of forced oscillations are discussed in relation to vibration as an exercise modality. Possible applications in sports and medicine are discussed. Evidence suggests that acute vibration exercise seems to elicit a specific warm-up effect, and that vibration training seems to improve muscle power, although the potential benefits over traditional forms of resistive exercise are still unclear. Moreover, literature suggests that vibration is beneficial to reduce chronic lower back pain and other types of pain. 

Q3. What have the authors stated for future works in "Vibration as an exercise modality: how it may work, and what its potential might be" ?

Future research should address whether the latter two effects can be strong enough to reduce the risk of falls in the elderly population. There is also substantial evidence to suggest that bones can be affected by vibration, although the exact mechanism is currently unclear. However, clearly more research is needed in order to better understand the specific therapeutic potential of vibration as an exercise model. Moreover, there seems to be a certain need for studies to assess any potential longterm risks. 

Q4. What are the accepted ground-based models to study the effects of vibration on Earth?

Hind limb suspension in rat, and bed rest in humans are broadly accepted ground-based models to study those deconditioning effects on Earth. 

Q5. What is the effect of locking the knees on the body?

locking of the knees will reduce energy absorption in the thigh muscles and lead to greater vibration transmission to the trunk. 

Q6. What is the effect of low-magnitude vibration on the musculature?

With regards to humans, low-magnitude vibration (f = 30 Hz, aPeak = 0.2g) seems to counteract bone losses from the spine and perhaps also from the hip in women after menopause (Rubin et al. 2004). 

Q7. What is the reason for the idea to increase flexibility by vibration exercise?

Provided that vibration exercise involves mechanical stretching (see ‘‘Muscle and tendon mechanics’’), there is good justification for the idea to increase flexibility by vibration exercise (Atha and Wheatley 1976). 

Q8. What is the way to improve joint stability?

One could speculate that the alteration of reflex levels by vibration, as discussed above, might help to improve joint stability. 

Q9. Why does the study suggest that vibration exercise may help improve knee stability?

This may be due to enhanced efficacy of the monosynaptic pathway, resulting in improved knee stability and reduced anterior tibial displacement upon shock provocation, and implying potential protection from anterior cruciate ligament injury. 

Q10. What is the effect of being air-bound on the vibration plate?

being air-bound can also lead to missing out one or several cycles of the vibration platform, and thus generate sub-harmonic frequencies (i.e. f/2, f/3, f/4) in the vibrated object. 

Q11. What is the effect of vibration training on postural sway?

postural sway during perturbed stance was reduced in response to vibration training in the latter study (Verschueren et al. 2004). 

Q12. What is the influence of vibration amplitude?

The influence of vibration amplitude, by contrast, seems to be essentially non-linear and more pronounced with increasing amplitude. 

Q13. What is the main argument that vibration exercise is a good way to increase muscle power?

If this view holds true, then vibration exercise could be a favourable means of warming-up in sports where muscle power is crucial, provided that any inhibitory effects upon muscle power by vibration exercise are smaller than the positive temperature effects.