Methodological quality is considered a very significant criterion for
the inclusion or exclusion of studies in systematic reviews, so as to
provide reliable outcomes. The use of this criterion provides strength
in the present review. The quality evaluation was performed by the use
of a standardised model, the PEDro scale, proposed as a rapid and accurate
tool for clinicians (Maher et al., 2003).
Most of the reviewed studies were of high quality (18 RCTs and 4 pseudo-RCTs).
The mean PEDro score was above average for either neurological or musculoskeletal
conditions but not for metabolic conditions, providing a total score of
5.14 ± 1.46. Nevertheless, only a few studies presented a sound research
design, that is, a score > 7, and methodological rigour, that
is, fulfilling at least one of the blinding factors, based on PEDro scale
(Arias et al., 2009; Gusi et al., 2010;
Van Nes et al., 2004). Comparisons were, therefore, difficult to make, because
of the different conditions that have been assessed.
Methodological limitations included insufficient randomisation (Arias
et al., 2009;
Haas et al., 2006b; Turbanski et al., 2005;
Van Nes et al., 2004),
lack of sample homogeneity (size, age variability) and poor blinding (Ahlborg
et al., 2006;
Alentorn-Geli et al., 2009;
Baum et al., 2007;
Broekmans et al., 2010; Ebersbach et al., 2008; Haas et al., 2006b;
Johnson et al., 2010;
Rittweger et al., 2002;
Trans et al., 2009;
Turbanski et al., 2005; Van Nes et al., 2004). Indeed, there were studies where the sample size was
small (Ahlborg et al., 2006; Johnson et al., 2010; Schuhfried et al., 2005; Tihanyi et al., 2007) or no control group was used (Brogardh et al., 2010; Jackson et al., 2008; Ness and Field-Fote, 2009; Rietschel et al., 2008; Roth et al., 2008; Schyns et al., 2009), the sample's age varied (Haas et al., 2006b;
Jackson et al., 2008;
Ness and Field-Fote, 2009)
and the severity of the condition differed (Jackson et al., 2008).
Furthermore, most researchers faced practical complications with regards
to blinding factors, that is, a difficulty in determining similar intervention
parameters (frequency, duration) either among groups or assessors. This
is a limitation which could negatively impact the PEDro scale scores.
In order to address "blinding" sufficiently, placebo interventions
were used in two studies, with participants either standing on the platform
without vibration (Arias et al., 2009) or using TENS to simulate the vibration (Schuhfried et
affecting WBV outcomes
There is a large number of factors identified which could produce a WBV
effect. Those are grouped into the vibration characteristics (vibration
type, vibration frequency and amplitude, eVDV) and the subjects' characteristics.
WBV characteristics: The information regarding dose-response of WBV
parameters (frequency, amplitude, duration, eVDV) in pathological populations
is rather limited. Indeed, there is only one study which explored the
acute effects of WBV using either 2Hz or 26Hz and did not find any significant
differences in strength of MS patients between the two frequencies (Jackson
et al., 2008). Furthermore, there is no research to compare different
amplitudes or durations in pathological populations. Moreover, the eVDV
index has not been examined in any of the reviewed studies, although its
importance is highly stressed in ISO guidelines (Merriman and Jackson,
2009). It is, therefore, not known whether WBV training is
harmful for the pathological populations that have been examined. It is
important to note, however, that the focus of ISO health guidelines lies
on chronic exposures of healthy adults to daily vibration and may, consequently,
have limited value when assessing risk associated with exposures in clinical
populations (Merriman and Jackson, 2009). For example, there is evidence to support that WBV,
exceeding the safety values mentioned earlier, can relieve pain in LBP
rather than cause it. It is, therefore, necessary to modify the ISO standards
and include information on vibration within the rehabilitation field (Rittweger,
2010). Despite the fact that no serious side effects have been
reported in any of the reviewed studies, it is essential for researchers
in the future to calculate eVDV so as to provide clinicians with information
on the appropriateness of WBV on pathological populations.
The type of vibration used while training may also be considered a possible
factor influencing WBV effects. In the reviewed studies, three different
vibration types have been used: the AV (n = 10), the SV (n = 11) and the
MV (n = 4). In the literature, comparisons only between AV and SV platforms
have been identified in healthy populations. The results show that SV
proves to be more effective than AV, regarding the enhancement of muscular
performance, probably due to the greater frequencies that are used in
these platforms (Marin and Rhea, 2010). Similar conclusions in patient populations are not easy
to make. Indeed, the results of the current review are inconsistent, irrespectively
of the vibration type. Specifically, either no significant effects (4SV
vs. 4AV), positive effects (3SV vs. 2AV) or positive effects with no between-group
differences (4SV vs. 4A) were identified regarding the under study variables.
This disparity in the results could be attributed to the different pathologies
that have been examined, the different variables that have been measured
and the different assessment tools that have been used to evaluate the
same variables (e.g. balance). It is possible that different pathologies
respond differently to WBV, therefore, future research using consistency
in the assessment tools might provide comparable results.
Subjects' characteristics: The use of WBV for therapeutic purposes
has only recently been examined, with neurological conditions being the
main focus in a population consisting of young adults (20-34 years), young
middle-aged (35-44 years), middle-aged (45-64 years) and/or older subjects
(> 64 years) according to Shephard's age classification (Shephard,
1998). It is difficult to reach conclusions regarding the effects
of WBV in relation to the sample's age. In the current review, only a
few studies explored the WBV effects on defined age groups. Indeed, in
most studies selected age groups included a combination of middle-aged
and older subjects (n = 10) while only a few studies included solely adults
(n = 4) or middle-aged (n = 5) participants. No research studies investigated
older patients. The results from either population groups are controversial
with studies reporting no effect (n = 3), statistically significant effect
(n = 4) or significant effect with no between-group differences (n = 7).
It is, therefore, possible that the large variation observed in the samples'
age for the majority of the reviewed studies, explain the controversial
results. Since WBV exercise seems to be more beneficial in healthy aged
adults in relation to younger adults (Vipond et al., 2008), future research should focus on the exploration of WBV
in relation to age, in pathological populations.
In the current review, the acute as well as the long-term effects of WBV
on neurological conditions have been investigated. Furthermore, only studies
exploring the long-term effects of WBV on patients diagnosed with musculoskeletal
and metabolic conditions have been identified in the literature. Physical
capabilities such as strength, gait, balance and mobility have been examined.
Variables such as proprioception, spasticity, motor impairment, cardiovascular
performance and respiration have also been reported.
Strength and power: There is no consensus in the literature with
regards to the long-term effects of WBV on strength. All studies apart
from Trans et al.'s (2009) used SV platforms and reported no effects of WBV on patients'
strength (Broekmans et al., 2010; Brogardh et al., 2010; Schyns et al., 2009) or whenever there was an effect, it was not statistically
significant between groups (Ahlborg et al., 2006; Johnson et al., 2010). Only in one study, strength has been improved irrespectively
of the intervention used, that is, a resistance, flexibility, stretching
or WBV exercise (Baum et al., 2007). Furthermore, in the study of Trans et al., 2009, strength was improved significantly in the WBV and not
in the balance group, in relation to a control group, although information
on the comparison between interventions was not provided. Moreover, Roth
et al., 2008, in their study on CF patients, claimed to have assessed
power and lung function using two different intervention protocols (12
Hz and 26 Hz), both of which were applied in the same 6-month period.
Nevertheless, it seems impossible to distinguish the effectiveness of
each protocol on power and lung function due to methodological inadequacy.
A few studies revealed positive acute effects on strength either in high
(26 Hz) or low (2 Hz) frequency (Jackson et al., 2008) using AV or only in the vibration in relation to an exercise
group (Tihanyi et al., 2007). Therefore, the positive effects of either acute or chronic
exposure of WBV vs. other interventions on strength could not be established.
Future, more concise research, in terms of the methodology used, is necessary.
Moreover, further research on the effects of WBV on strength and power
of different pathological populations, using an AV platform is essential.
This stems from the fact that SV has been proved to have a higher eVDV
and a greater transmission of mechanical energy to the head, thus, having
greater health risks than AV (Abercromby et al., 2007).
Gait, balance and mobility: With regards to gait, during long-term
exposure, no effects or positive effects with no between-group differences
have been observed on MS, SCI and PD patients, independently of the platform
type and the protocol used. It should be noted, however, that the lack
of effects could be attributed to the program duration which was not over
eight weeks long. It can, therefore, be assumed, that gait cannot be affected
to a greater extent by WBV compared to other interventions, during chronic
exposure. Future research should explore the effects of WBV on gait following
exercise protocols of longer duration. Furthermore, since no study referring
to the acute effects of WBV on gait has been identified, research focusing
on the acute effects of WBV on gait of PD, MS, CP and SCI patients as
well as on patients diagnosed with musculoskeletal or metabolic conditions,
should also be conducted in order to identify possible differences in
relation to the program duration.
Research referring to the long-term effects of WBV on balance, mobility
and motor impairment of several clinical populations, revealed that, in
most studies, WBV had equal effects on the parameters that were examined
compared to alternative interventions (Ahlborg et al., 2006; Arias et al., 2009;
Ebersbach et al., 2008; Gusi et al., 2010; Johnson et al., 2010; Schuhfried et al., 2005; Schyns et al., 2009; Van Nes et al., 2006). This lack of differences was evident, irrespectively
of the vibration type and the program duration. Only three studies (two
referring to musculoskeletal and one in metabolic conditions) identified
statistically significant improvement in balance of patients with FM (Gusi
et al., 2010) and ACLS (Moezy et al., 2008) as well as mobility of patients with CF (Rietschel et
al., 2008). The number of studies conducted on several clinical
populations is rather limited; it is therefore, not clear whether WBV
is more effective than any other exercise intervention or a control condition.
It may be assumed, however, that WBV might produce greater positive effects
in musculoskeletal compared to neurological conditions, concerning balance.
Furthermore, the results with regards to the acute effects of WBV on balance
are not clear since there are occasions in which it improves in relation
to the control group, e.g. tandem standing (Turbanski et al., 2005)
or both groups improve equally (Schuhfried et al., 2005),
e.g. narrow standing (Turbanski et al., 2005). Further research on neurological as well as on musculoskeletal
and metabolic conditions would provide more objective conclusions.
Spasticity, pain and fatigue: WBV appears to have positive long-term
effects on decreasing spasticity (Ahlborg et al., 2006; Schyns et al., 2009). The limited data on this topic indicates that WBV may
be successfully used to reduce muscle spasms. Pain also seems to be reduced
in conditions such as MS, LBP, FM, TKA and osteoporosis (Alentorn-Geli
et al., 2008; Iwamoto et al., 2005; Johnson et al., 2010; Rittweger et al., 2002). The results are more evident in FM since the combination
of vibration and exercise appears to have greater effects than exercise
alone (Alentorn-Geli et al., 2008). The rather limited data on the effects of WBV on proprioception,
depression and fatigue levels of patients suffering from specific pathological
conditions as well as several methodological flaws do not allow one to
reach solid conclusions. Nevertheless, it can be assumed that WBV can
be used as a less fatiguing and less time-consuming training method since
it seems to reduce fatigue levels (Alentorn-Geli et al., 2008) and produce positive effects in less training time (30s-5min)
than conventional exercise. The above assumption is of great clinical
importance considering that in rehabilitation, therapists seek to improve
their patients' physical condition with minimum cost of energy.
Respiratory and cardiovascular capacity, hormonal secretions and bone
density: The long-term effects of WBV on the respiratory and cardiovascular
capacity have only been examined in metabolic conditions, where no effects
on WBV were identified with regards to respiration (Rietschel et al.,
2008; Roth et al., 2008) and cardiovascular parameters (Baum et al., 2007; Roth et al., 2008). Systolic blood pressure was found to be reduced irrespectively
of the intervention, with the vibration group being less improved than
the other two treatments (flexibility and strengthening training), (Baum
et al., 2007). Furthermore, assumptions regarding hormonal secretion
and bone density cannot be made. Although it seems that there are positive
effects of WBV on the above parameters, there is not much evidence to
support that vibration is better compared to drugs (Iwamoto et al., 2005) or other exercise interventions (Baum et al., 2007). Similar results derive from research in healthy populations
as well. Indeed, there is rather limited data regarding the long-term
effects of WBV on the hormonal system, presenting increase in parathormone
(Martin et al., 2009) or no changes in hormones such as testosterone and cortisol
(Kvorning et al., 2006). The only exception is growth hormone which seems to
show a greater response due to a combination of weight training and WBV
training protocol (Kvorning et al., 2006). Conflicting evidence also exist in relation to bone
density, which seems to improve in healthy elderly (Gusi et al., 2006;
Verschueren et al., 2004) while it remains unchanged in young adults (Torvinen
et al., 2003). Therefore, further research in relation to chronic exposure
of patients on WBV is needed, before reaching a conclusion.
Although the results regarding the additional effect that WBV has on other
interventions are quite clear, this is not the case in studies where no
intervention at all was used. Indeed, irrespectively of the program duration
and the vibration type used, the results of some studies showed no significant
effect (n = 3) or positive effects but no between-group differences (n
= 3) when WBV was compared to no intervention. Similar results were derived
from healthy populations' studies where the positive effects of WBV in
relation to no intervention at all could also not be established (Bautmans
et al., 2005; Cheung et al., 2007; Delecluse et al., 2005).