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Muscle stiffness and injury effects of whole body vibration
Physical Therapy in Sport 5 (2004) 68–74 www.elsevier.com/locate/yptsp
Original research
Muscle stiffness and injury effects of whole body vibration John B. Cronina,*, Melody Olivera, Peter J. McNairb a Sport Performance Research Centre, Auckland University of Technology, Private Bag 92006, Auckland 1020, New Zealand Neuromuscular Research Unit, School of Physiotherapy, Auckland University of Technology, Private Bag 92006, Auckland 1020, New Zealand
b
Received 7 April 2003; revised 7 December 2003; accepted 11 January 2004
Abstract Objective. The interest in vibration as a method of enhancing muscular performance is receiving considerable attention from sport scientists, physiotherapists, coaches and athletes. Increases in strength, power and jump height have been reported after a single session of whole body vibration. In some cases, this immediate change has been attributed to changes in the stiffness of the musculotendinous complex without directly measuring the stiffness properties of muscle. The aim of this study was to determine whether whole body vibration had an effect on the stiffness of the triceps surae muscle group. Design. The stiffness of the right and left leg of 11 relatively untrained subjects was measured prior to a warm-up, post-warm-up and postvibration using a damped oscillation technique. After warm-up, one leg was vibrated (frequency 26 Hz, amplitude 6 mm) whilst the other leg acted as a control. The subjects were exposed to the vibration five times for a duration of 60 s with a 60 s rest between each repetition. Results. No significant differences (P . 0:05) in stiffness were found between the baseline measures and the post-warm-up and postvibration measures of stiffness. Following vibration a number of subjects experienced pain to the lower leg muscles and a loss of function. In some subjects, the pain was experienced at the jaw and the neck. These symptoms required treatment in six subjects but the pain had resolved within 7 – 10 days in all subjects. Conclusions. It appears that vibratory stimulation loading parameters used to enhance performance do not significantly alter muscle stiffness in untrained individuals. Considering the injury potential associated with whole body vibration further research into safe doseresponse relationships is recommended. q 2004 Elsevier Ltd. All rights reserved. Keywords: Performance; Gravitational forces; Active component; Passive component
1. Introduction Vibration has been widely used as a therapeutic tool for rehabilitation, pain management and physiotherapy. Early research reported the prescription of vibration for the treatment of ‘acute neuralgic pain’ (Granville, 1881). Physiotherapists have used vibration for many years both in the treatment of pain and in the acute relief of spastic/ rigid muscle (Bishop, 1974). Vibration at a frequency of 27 Hz has also been shown to improve lower limb neuromuscular function as demonstrated by the improved co-ordination and balance of 35 elderly subjects performing a standardised chair-raising test (Runge et al., 2000). With regards to the work environment, research has predominantly focused on the harmful and/or performance * Corresponding author. Fax: þ 61-9-917-9960. E-mail address: [email protected] (J.B. Cronin). 1466-853X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ptsp.2004.01.004
diminishing effects of vibration. Studies have shown that low frequency (2 –12 Hz) chronic vibration, such as that produced by hand held machines and transport devices might be detrimental to health (Mester, Spitzenfeil, Schwarzer, & Seifriz, 1999). Research of this kind brings attention to the potential dangers associated with vibration, i.e. translation of vibration to the head (Paddan & Griffin, 1998), lumbar spine injury (Griffin, 1998), low back pain and vibration induced muscular fatigue (Pope, Wilder, & Magnusson, 1998). A relatively new and exciting area in sport science is the investigation of the effects of vibration on muscular function and functional performance. There is a growing body of evidence both anecdotal and scientific, which suggests that vibration can be used as a performance-enhancing tool. Studies have examined the kinematic and kinetic adaptations following a single vibration session (Bosco, Cardinale, & Tsarpela, 1999a; Bosco et al., 1999b, 2000;
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Clarkson, Zigon, & Kamen, 1980; Liebermann & Issurin, 1997; Rittweger et al., 2000; Warman, Humphries, & Purton, 2002) and after multiple vibration sessions (Bosco et al., 1998; Mester et al., 1999). In most cases vibration has been shown to positively influence maximal strength and force (Bosco et al., 1999b; Liebermann & Issurin, 1997; Warman et al., 2002), power (Bosco et al., 1999b), total work done (Bosco et al., 2000) and jump height (Bosco et al., 1998, 2000). Most of this research has used vibratory stimulation characterised by frequencies of 26 –44 Hz with amplitudes ranging from 1 to 10 mm and accelerations of 3 –17g. Mechanisms by which vibration affects functional performance are unclear, and to date, no succinct explanation has been defined. Candidate mechanisms for improved performance post-vibration have been attributed to both central and peripheral mechanisms. Vibration applied at different frequencies has been shown to influence the supplementary motor area, the caudal cingulate motor area and area 4a of the brain (Cardinale & Bosco, 2003), which is thought to increase excitatory outflow from these structures. In terms of peripheral responses, vibration is thought to initiate the tonic vibration reflex (TVR) (Bishop, 1974; Bosco et al., 1999a), cause vasodilation, which increases blood flow (Bosco et al., 1999a; Kerschan-Schindl et al., 2001) and increase intra-muscular temperature (Bosco et al., 1999a; Kerschan-Schindl et al., 2001). It has been suggested that each of these mechanisms affects the contractile and/or viscoelastic properties of muscle (Ettema & Huijing, 1994; Panjabi & White, 2001; Shellock & Prentice, 1985; Tancred & Tancred, 1995). Much of the research using vibratory stimulation has attributed both short-term and long-term changes in functional performance to changes in muscle stiffness (Bosco et al., 1999a,b, 2000; Cardinale & Bosco, 2003; Mester et al., 1999). Muscle stiffness can be defined as the change in force a muscle has to an increase in length (Wilson, Wood, & Elliott, 1991). This can be assessed in either an active or passive state, as stiffness is found within the contractile (cross-bridges) and passive viscoelastic elements of a muscle (Wilson et al., 1991; Wilson, Elliott, & Wood, 1992; Wilson, Murphy, & Pryor, 1994). Realising this, it is very likely vibratory stimulation has the potential to affect these structures as described previously. For example, vibration causes small length changes in muscle fibres, and its spindles activate neural pathways via Ia afferent fibres causing agonist muscle contraction, and decreasing action potential activity to the antagonist. The net result would be increased stiffness due to greater crossbridges interaction (Bosco et al., 1999a). If vibration results in vasodilation and increased blood flow, this also is likely to increase stiffness by increasing blood volume within the working muscle. However, if increased muscle temperature also occurs, it is quite likely that the viscoelastic and hence damping properties of muscle are altered resulting in a decrease in stiffness. As stated previously much of
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the research using vibration has attributed both short-term and long-term changes in performance to changes in muscle stiffness. However, this assumption has been made without muscle stiffness being directly measured (Cardinale & Bosco, 2003; Mester et al., 1999). Whether stiffness is affected by vibration is, as yet, unclear. Therefore, the aim of this study was to investigate whether triceps surae stiffness alters in response to whole body vibration. It was hypothesised that vibration would not significantly alter stiffness due to the opposing effects of the TVR as against vasodilation, and the intra-muscular temperature changes.
2. Method 2.1. Subjects A total of seven males and four females volunteered to participate in the research project. The subjects’ mean (SD) age, mass, and height were 24.6 (3.6) years, 69.27 (12.96) kg and 176 (9.9) cm, respectively. Training status of the subjects varied from club-level athletes to subjects with little or no training experience. Subjects were excluded from vibratory stimulation if any of the following conditions were reported: pregnancy, acute thromboses, acute inflammations, malignomae, implants, fresh fractures, acute tendinopathies, urolithiasis, and cholelithiasis (as per the manufacturers instructions). The Ethics Committee of the Auckland University of Technology approved all of the procedures undertaken, and all subjects read a participant information sheet and signed an informed consent form prior to their participation in the research. 2.2. Equipment 2.2.1. Force platform A force platform (AMTI Force Plate and Amplifier; Advanced Technology Inc., Washington: USA) was used to measure muscle stiffness. The force plate was mounted according to the manufacturer’s specifications and was calibrated before testing. The force signal was linear (, 0.5%) over a range of force 0 –10 kN, with an accuracy of ^ 1%. 2.2.2. Whole body vibration device Subjects were exposed to vertical sinusoidal whole body vibration using the Galileoe 2000 (Novotec, Pforzheim, Germany). The device is a mechanical teeterboard, that is the teetering surface rocks up and down about a sagittal shaft (Fig. 1). The Galileoe 2000 has a vibration range of 0 –30 Hz and an amplitude of 1 –6 mm. 2.3. Stiffness testing protocol The technique used to measure muscle stiffness of the plantar flexor muscles was similar to that of McNair
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2.4. Intervention
Fig. 1. Whole body vibration device (Galileoe 2000).
and Stanley (1996). Familiarisation with the testing procedures occurred prior to data collection. Subjects were instructed to sit on a chair with their greater trochanter in line with a marker 10 cm from the edge of the chair. One foot rested on the side of the force platform, while the other was placed on a block on the force platform. The heads of the first and fifth metatarsals of the foot were level with the block edge and the subject was asked to hold a 908 angle at the ankle. Chair height was set so that when the leg being measured was resting, the angle at the knee was 908. Each subject was instructed to sit upright with arms crossed and hands resting on their shoulders (Fig. 2). Segment mass of the lower leg was measured three times on the force platform, averaged and used for stiffness calculations. Stiffness for both legs was measured by lightly perturbating—approximately 100 N (Wilson et al., 1991, 1992, 1994) the knee six times with a 5-s rest between perturbations.
Prior to vibration a 3-min warm-up was performed on a Powerjog GX100 treadmill (Powerjog, Birmingham, UK) at a speed which required the subject to jog just above walking pace. One leg was randomly assigned to the vibration treatment, comprising vertical sinusoidal whole body vibration. Subjects stood flat-footed on the Galileoe 2000 supported by their treatment leg which was slightly flexed, and stabilised themselves by holding onto the handlebars with both hands. Subjects were instructed not to put weight through their hands, but to use the supports for balance only. The subjects were then vibrated according to the protocol described by Bosco et al. (1999b, 2000), which included vibratory stimulation at a frequency of 26 Hz, a displacement of 6 mm and an acceleration of 15g. The duration of stimulation was 60 s ( £ 5), with 60 s rest between treatments. The other leg hung freely and acted as a control. The subjects of this study, however, performed less repetitions compared to Bosco’s study (10 £ 60 s). Stiffness measurements using the protocol outlined above were calculated prior to the warm-up, after the warm-up and after the vibratory stimulation. 2.5. Data analysis The data were sampled at 1000 Hz by a computer (MacIntosh G4 Computer, Cupertino, CA, USA) based data acquisition and analysis program (Superscope Version 3, GW Instruments, Boston, USA). The peak-to-peak force change divided by the peak-to-peak position change was measured for every clear oscillation as a result of the perturbation (Fig. 3). Stiffness was calculated for each of these oscillations. The formulae for calculating stiffness from the damped oscillation were k ¼ 4pi2 mf 2 þ c2 =4m
Fig. 2. Perturbation of the knee to measure triceps surae muscle stiffness.
Fig. 3. Example of a damped oscillation. The peak-to-peak area analysed is indicated by the arrows.
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Table 1 Means (^SD) and coefficients of variation for pre-warm-up, post-warm-up, and post-vibration stiffness Control limb
Treatment limb
Stiffness (N m)
Mean
SD
CV
Mean
SD
CV
Pre-warm-up Post-warm-up Post-vibration
5380.273 5506.545 5148.424
956.923 1202.491 1042.099
4.1 3.5 4.8
5923.061 5715.939 6162.273
1430.069 1055.890 1679.822
3.2 4.0 2.9
where k is the stiffness in Nm21; m is the mass, f is the damped frequency of oscillation, and c is the coefficient of damping (McNair, Wood, & Marshall, 1992). 2.6. Statistical analysis Mean values and standard deviations of the highest three results were calculated for the stiffness measures of each subject. The reliability of these measures were represented as a coefficient of variation (CV) using CV ¼ SD/Mean £ 100. Coefficients of variation were calculated for each individual and then the average CV for the sample was calculated. The percent change between baseline and warm-up, and baseline and post-vibration measures were calculated. These change scores were then analysed using paired t-tests to identify if the mean percentage change between treatment and control limbs was significantly different. The chances that the true value of the effect statistic (percent change) was clinically positive, trivial or harmful were calculated for each variable (Hopkins, 2002) by assuming the smallest practically important change in muscle stiffness was 5%. Five percent was chosen as the between trial variation of the stiffness measure were found to range between 2.9 and 4.8% (Table 1).
Fig. 4. The mean of individual percent change scores and absolute differences in stiffness post-warm-up and post-vibration. Absolute differences were calculated as the absolute sum of the percentage stiffness changes for the treatment and control legs.
percent change (8.1%) between pre- and post-vibration for the treatment and control limbs was not significantly different (P ¼ 0:118). Although not statistically significant, vibration did have an observable effect on the muscle stiffness of the subjects (Fig. 4). The chance that the true effect of vibration was positive/trivial/negative for the average athlete is 74/25/1%. That is, if the smallest practically important increase in muscle stiffness is 5%, the chance that training with vibration increases stiffness is 74% and the chance it decreases stiffness is 1%. Some subjects during the course of the vibratory stimulation described an itching sensation in the lower limb, but no subjects withdrew from the intervention. A number of the subjects, however, experienced pain following the prescribed vibration treatment in the jaw, neck and lower extremity, particularly tibialis posterior.
4. Discussion 4.1. Whole body vibration and stiffness
3. Results The means, standard deviations and coefficients of variation for pre-warm-up, post-warm-up and post-vibration stiffness are presented in Table 1. Standard deviations provided relate to the group results, whilst the coefficient variables provided are associated with the means of each individual. Coefficients of variation ranged from 2.9 to 4.8% for the three testing occasions for each leg, indicating that there was little variation in the stiffness measures between trials. The percentage change between stiffness measures (Fig. 4) was calculated for the control and treatment legs and thereafter paired t-tests were used to identify if the mean percentage change between limbs was significantly different. The mean percent change (4.6%) between pre- and post-warm-up for the treatment and control limbs was not significantly different (P ¼ 0:402). Similarly the mean
The decrease in stiffness observed (Fig. 4) after the warm-up was to be expected and has been reported previously (McNair & Stanley, 1996). As mentioned earlier, increased intra-muscular temperature is thought to improve performance by enhancing neural efficiency, reducing viscous resistance and increasing muscle elasticity (Ettema & Huijing, 1994; Shellock & Prentice, 1985; Tancred & Tancred, 1995). Increased temperature will also affect the damping properties, influencing the resistance of the muscle to motion and hence decrease stiffness. It has been proposed that as a result of vibration muscle stiffness is altered, which may explain the improved functional performance reported in the research cited previously. Bosco et al. (1998) has discussed the role of vibration in improving performance via stiffness regulation, and in later studies (Bosco et al., 1999a, 2000) noted increased neuromuscular efficiency post-vibration. This may involve reduced EMG activity for a given force output
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due to increased motor unit synchronisation and improved synergist co-contraction and antagonist inhibition, which in turn would influence muscle stiffness (Bosco et al., 2000). Mester et al. (1999) stated that muscle tension and stiffness increased in response to vibration. However, none of these studies directly measured muscle stiffness. It can be observed from the results of this study that no significant (P ¼ 0:118) change between pre- and post-vibration stiffness for the treatment and control limbs was found. These results suggest that the muscle stiffness of relatively untrained subjects was not affected by the vibratory stimulation protocol used in this study. Although not statistically significant, vibration did have an observable effect on the muscle stiffness of the subjects (Fig. 4). If this study was replicated with a larger sample, the chance that the true effect, that is the change in stiffness due to the vibration treatment, was positive/trivial/negative for the average athlete was 74/25/1%. That is, if the smallest practically important increase in muscle stiffness is 5%, the chance that training with vibration increases stiffness is 74% and the chance it decreases stiffness is 1%. Furthermore, although no significant differences were detected within one session, this does not exclude the possibility that further treatments may have induced significant changes in the stiffness properties of the muscle. Research is needed that maps changes in stiffness after multiple sessions of vibratory stimulation. Further research is warranted investigating if the differences in stiffness result in significant changes to functional performance. The absence of any significant change in stiffness may be explained by a number of factors. First, the subjects of this study performed only half the repetitions prescribed by Bosco et al. (1999b, 2000). As a result the vibration stimulus may have been of inadequate duration for adaptations to occur. Second, vibration may have induced an increase in muscle temperature, which may have countered any shortterm neurogenic adaptation. It may be that vibration also elicits a certain level of pre-synaptic inhibition, which reduces the further recruitment of motoneurons. De Ruiter, van der Linden, van der Zijden, and Hollander (2003) proposed such an explanation, subsequent to finding no significant changes in performance, after replicating the protocol of Bosco used in this research. Furthermore de Ruiter and colleagues questioned whether whole body vibration would lead to neural adaptations that could enhance performance, after finding that there was no increase in voluntary muscle activation post-vibration. 4.2. Whole body vibration and injury Gravitational forces (g) of 1– 1.2g (1 –2 Hz) have been reported during walking and 3 –5g (2 – 3 Hz) whilst running (Hamill & Knutzen, 1995). Injury potential associated with walking is minimal, however, the increased intensity associated with running has commonly been linked to stress fractures in the tibia and fibula, and ITB friction syndrome
(Norris, 1993). Higher g forces (11g) have been associated with 60 cm drop jumps (Arampatzis, Schade, Walsh, & Bru¨ggemann, 2001). However, training using such high intensity plyometrics usually necessitates the athlete to be well trained (Chu, 1998) and requires substantial rest between jumps, which results in frequencies of less than 1 Hz. In comparison to these activities, the gravitational forces associated with vibration on the Galileoe 2000 has been reported to be as high as 17g at frequencies of 26– 30 Hz (Bosco et al., 1999b, 2000; Rittweger et al., 2000). These forces far exceed the gravitational forces associated with walking, running and jumping, and no doubt will cause discomfort or injury if an individual is unaccustomed to such forces. A major finding of the study was the injury potential associated with whole body vibration. Whilst work science research has reported the detrimental effects of prolonged exposure to vibration, the detrimental effects of short-term exposure to vibration as used in sport science research has not been documented. Although the intervention used in this study was less to that used in previous research (Bosco et al., 1999b, 2000), a number of the subjects experienced pain following the prescribed vibration treatment in the jaw, neck, and lower extremity, particularly tibialis posterior. Six subjects required physiotherapy treatment, all subjects reporting, however, that their pain had subsided within 7– 10 days. It is proposed that the frequency and high forces associated with the vibration treatment provided a training stimulus that was too intense for the relatively untrained subjects of this research. It also should be noted that research in this area in terms of controlled training studies is in its infancy. The appropriate dose-response relationships and principles for training such as progression, frequency and recovery are yet to be determined. For example, Bosco et al. (2000) stated that 10 min of whole body vibration at 17g was equivalent to 200 drop jumps from a height of 100 cm (5g), performed twice a week for a period of five months. Given these statistics one should err on the side of caution when prescribing vibration as a training stimulus. At the very least, subjects need to undergo a pre-screening that not only includes a review of injury status but also each subject’s training history, prior to vibratory stimulation. With this in mind one should remain cognizant of the limitations of whole body vibration as a training stimulus. Although Issurin, Liebermann and Tenenbaum (1994) recommended more research into the possible adverse effects of vibratory stimulation, no other research mentions the injury potential associated with whole body vibration, nor have subject discomfort or injuries been reported. The majority of sport science vibration research to date has involved subjects who are involved in athletic activities, competing in sports at least at club or university/college level. The training status of the subjects participating in this study varied, and included individuals who did not participate in physical activity of any kind. Although no pre-existing injuries or conditions were
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reported, this lack of physical conditioning prior to vibration may have predisposed these subjects to injury. Also the subjects of this study stood flat-footed on the Galileoe 2000. It may be that a plantar flexed position may attenuate force transmission to better effect and decrease injury potential. Given that the muscles are active in this position, however, one could also speculate that the transmission of forces may be greater. The literature is unclear on this point and further investigation into the responses of the musculoskeletal system to vibratory stimulation using different postures is warranted.
5. Conclusion Although no significant differences were found between the mean percent change between pre- and post-vibration stiffness for the treatment and control limbs, vibration did have an observable effect on the muscle stiffness of the subjects. Future research needs to compare the stiffness changes between subjects of different training status so as a better appreciation of the effects of whole body vibration can be established. This will necessitate a better understanding of whether adaptation to vibration is mediated by central or peripheral factors or a combination of both. It is also quite probable that the neuromuscular system of the trained athlete will respond differently to vibratory stimulation as compared to the untrained athlete. Furthermore the relationship between stiffness and performance, and what percentage change in stiffness results in a significant change to performance needs to be identified. A major finding of the study was the injury potential associated with whole body vibration using the Galileoe 2000. A number of the subjects experienced pain during and/or following the prescribed vibration treatment in the jaw, neck, and lower extremity, particularly tibialis posterior. Forces experienced on the Galileoe 2000 are far greater than those produced during traditional training methods, due to the high gravitational forces and frequencies associated with vibration (up to 17g at 26 – 30 Hz). It is proposed that the high forces associated with vibratory stimulation provide a training stimulus that can be detrimental to a subject’s well being. It is recommended that caution be exercised when using whole body vibration and that the intensity and duration of the vibratory stimulation be applied progressively. It is suggested that more research be conducted on the effects of whole body vibration across divergent samples (e.g. training status, age, gender) and a dose-response model formulated. This in the first instance will necessitate a better understanding of how the different foot positions (flat-footed vs plantar flexed), amplitudes (foot position on teetering vibration platform) and frequencies affect the g forces associated with the use of the Galileoe 2000. Thereafter identifying how these forces are damped at different locations of the body would provide useful insights into the loading parameters for this doseresponse relationship.
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Original research
Muscle stiffness and injury effects of whole body vibration John B. Cronina,*, Melody Olivera, Peter J. McNairb a Sport Performance Research Centre, Auckland University of Technology, Private Bag 92006, Auckland 1020, New Zealand Neuromuscular Research Unit, School of Physiotherapy, Auckland University of Technology, Private Bag 92006, Auckland 1020, New Zealand
b
Received 7 April 2003; revised 7 December 2003; accepted 11 January 2004
Abstract Objective. The interest in vibration as a method of enhancing muscular performance is receiving considerable attention from sport scientists, physiotherapists, coaches and athletes. Increases in strength, power and jump height have been reported after a single session of whole body vibration. In some cases, this immediate change has been attributed to changes in the stiffness of the musculotendinous complex without directly measuring the stiffness properties of muscle. The aim of this study was to determine whether whole body vibration had an effect on the stiffness of the triceps surae muscle group. Design. The stiffness of the right and left leg of 11 relatively untrained subjects was measured prior to a warm-up, post-warm-up and postvibration using a damped oscillation technique. After warm-up, one leg was vibrated (frequency 26 Hz, amplitude 6 mm) whilst the other leg acted as a control. The subjects were exposed to the vibration five times for a duration of 60 s with a 60 s rest between each repetition. Results. No significant differences (P . 0:05) in stiffness were found between the baseline measures and the post-warm-up and postvibration measures of stiffness. Following vibration a number of subjects experienced pain to the lower leg muscles and a loss of function. In some subjects, the pain was experienced at the jaw and the neck. These symptoms required treatment in six subjects but the pain had resolved within 7 – 10 days in all subjects. Conclusions. It appears that vibratory stimulation loading parameters used to enhance performance do not significantly alter muscle stiffness in untrained individuals. Considering the injury potential associated with whole body vibration further research into safe doseresponse relationships is recommended. q 2004 Elsevier Ltd. All rights reserved. Keywords: Performance; Gravitational forces; Active component; Passive component
1. Introduction Vibration has been widely used as a therapeutic tool for rehabilitation, pain management and physiotherapy. Early research reported the prescription of vibration for the treatment of ‘acute neuralgic pain’ (Granville, 1881). Physiotherapists have used vibration for many years both in the treatment of pain and in the acute relief of spastic/ rigid muscle (Bishop, 1974). Vibration at a frequency of 27 Hz has also been shown to improve lower limb neuromuscular function as demonstrated by the improved co-ordination and balance of 35 elderly subjects performing a standardised chair-raising test (Runge et al., 2000). With regards to the work environment, research has predominantly focused on the harmful and/or performance * Corresponding author. Fax: þ 61-9-917-9960. E-mail address: [email protected] (J.B. Cronin). 1466-853X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ptsp.2004.01.004
diminishing effects of vibration. Studies have shown that low frequency (2 –12 Hz) chronic vibration, such as that produced by hand held machines and transport devices might be detrimental to health (Mester, Spitzenfeil, Schwarzer, & Seifriz, 1999). Research of this kind brings attention to the potential dangers associated with vibration, i.e. translation of vibration to the head (Paddan & Griffin, 1998), lumbar spine injury (Griffin, 1998), low back pain and vibration induced muscular fatigue (Pope, Wilder, & Magnusson, 1998). A relatively new and exciting area in sport science is the investigation of the effects of vibration on muscular function and functional performance. There is a growing body of evidence both anecdotal and scientific, which suggests that vibration can be used as a performance-enhancing tool. Studies have examined the kinematic and kinetic adaptations following a single vibration session (Bosco, Cardinale, & Tsarpela, 1999a; Bosco et al., 1999b, 2000;
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Clarkson, Zigon, & Kamen, 1980; Liebermann & Issurin, 1997; Rittweger et al., 2000; Warman, Humphries, & Purton, 2002) and after multiple vibration sessions (Bosco et al., 1998; Mester et al., 1999). In most cases vibration has been shown to positively influence maximal strength and force (Bosco et al., 1999b; Liebermann & Issurin, 1997; Warman et al., 2002), power (Bosco et al., 1999b), total work done (Bosco et al., 2000) and jump height (Bosco et al., 1998, 2000). Most of this research has used vibratory stimulation characterised by frequencies of 26 –44 Hz with amplitudes ranging from 1 to 10 mm and accelerations of 3 –17g. Mechanisms by which vibration affects functional performance are unclear, and to date, no succinct explanation has been defined. Candidate mechanisms for improved performance post-vibration have been attributed to both central and peripheral mechanisms. Vibration applied at different frequencies has been shown to influence the supplementary motor area, the caudal cingulate motor area and area 4a of the brain (Cardinale & Bosco, 2003), which is thought to increase excitatory outflow from these structures. In terms of peripheral responses, vibration is thought to initiate the tonic vibration reflex (TVR) (Bishop, 1974; Bosco et al., 1999a), cause vasodilation, which increases blood flow (Bosco et al., 1999a; Kerschan-Schindl et al., 2001) and increase intra-muscular temperature (Bosco et al., 1999a; Kerschan-Schindl et al., 2001). It has been suggested that each of these mechanisms affects the contractile and/or viscoelastic properties of muscle (Ettema & Huijing, 1994; Panjabi & White, 2001; Shellock & Prentice, 1985; Tancred & Tancred, 1995). Much of the research using vibratory stimulation has attributed both short-term and long-term changes in functional performance to changes in muscle stiffness (Bosco et al., 1999a,b, 2000; Cardinale & Bosco, 2003; Mester et al., 1999). Muscle stiffness can be defined as the change in force a muscle has to an increase in length (Wilson, Wood, & Elliott, 1991). This can be assessed in either an active or passive state, as stiffness is found within the contractile (cross-bridges) and passive viscoelastic elements of a muscle (Wilson et al., 1991; Wilson, Elliott, & Wood, 1992; Wilson, Murphy, & Pryor, 1994). Realising this, it is very likely vibratory stimulation has the potential to affect these structures as described previously. For example, vibration causes small length changes in muscle fibres, and its spindles activate neural pathways via Ia afferent fibres causing agonist muscle contraction, and decreasing action potential activity to the antagonist. The net result would be increased stiffness due to greater crossbridges interaction (Bosco et al., 1999a). If vibration results in vasodilation and increased blood flow, this also is likely to increase stiffness by increasing blood volume within the working muscle. However, if increased muscle temperature also occurs, it is quite likely that the viscoelastic and hence damping properties of muscle are altered resulting in a decrease in stiffness. As stated previously much of
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the research using vibration has attributed both short-term and long-term changes in performance to changes in muscle stiffness. However, this assumption has been made without muscle stiffness being directly measured (Cardinale & Bosco, 2003; Mester et al., 1999). Whether stiffness is affected by vibration is, as yet, unclear. Therefore, the aim of this study was to investigate whether triceps surae stiffness alters in response to whole body vibration. It was hypothesised that vibration would not significantly alter stiffness due to the opposing effects of the TVR as against vasodilation, and the intra-muscular temperature changes.
2. Method 2.1. Subjects A total of seven males and four females volunteered to participate in the research project. The subjects’ mean (SD) age, mass, and height were 24.6 (3.6) years, 69.27 (12.96) kg and 176 (9.9) cm, respectively. Training status of the subjects varied from club-level athletes to subjects with little or no training experience. Subjects were excluded from vibratory stimulation if any of the following conditions were reported: pregnancy, acute thromboses, acute inflammations, malignomae, implants, fresh fractures, acute tendinopathies, urolithiasis, and cholelithiasis (as per the manufacturers instructions). The Ethics Committee of the Auckland University of Technology approved all of the procedures undertaken, and all subjects read a participant information sheet and signed an informed consent form prior to their participation in the research. 2.2. Equipment 2.2.1. Force platform A force platform (AMTI Force Plate and Amplifier; Advanced Technology Inc., Washington: USA) was used to measure muscle stiffness. The force plate was mounted according to the manufacturer’s specifications and was calibrated before testing. The force signal was linear (, 0.5%) over a range of force 0 –10 kN, with an accuracy of ^ 1%. 2.2.2. Whole body vibration device Subjects were exposed to vertical sinusoidal whole body vibration using the Galileoe 2000 (Novotec, Pforzheim, Germany). The device is a mechanical teeterboard, that is the teetering surface rocks up and down about a sagittal shaft (Fig. 1). The Galileoe 2000 has a vibration range of 0 –30 Hz and an amplitude of 1 –6 mm. 2.3. Stiffness testing protocol The technique used to measure muscle stiffness of the plantar flexor muscles was similar to that of McNair
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2.4. Intervention
Fig. 1. Whole body vibration device (Galileoe 2000).
and Stanley (1996). Familiarisation with the testing procedures occurred prior to data collection. Subjects were instructed to sit on a chair with their greater trochanter in line with a marker 10 cm from the edge of the chair. One foot rested on the side of the force platform, while the other was placed on a block on the force platform. The heads of the first and fifth metatarsals of the foot were level with the block edge and the subject was asked to hold a 908 angle at the ankle. Chair height was set so that when the leg being measured was resting, the angle at the knee was 908. Each subject was instructed to sit upright with arms crossed and hands resting on their shoulders (Fig. 2). Segment mass of the lower leg was measured three times on the force platform, averaged and used for stiffness calculations. Stiffness for both legs was measured by lightly perturbating—approximately 100 N (Wilson et al., 1991, 1992, 1994) the knee six times with a 5-s rest between perturbations.
Prior to vibration a 3-min warm-up was performed on a Powerjog GX100 treadmill (Powerjog, Birmingham, UK) at a speed which required the subject to jog just above walking pace. One leg was randomly assigned to the vibration treatment, comprising vertical sinusoidal whole body vibration. Subjects stood flat-footed on the Galileoe 2000 supported by their treatment leg which was slightly flexed, and stabilised themselves by holding onto the handlebars with both hands. Subjects were instructed not to put weight through their hands, but to use the supports for balance only. The subjects were then vibrated according to the protocol described by Bosco et al. (1999b, 2000), which included vibratory stimulation at a frequency of 26 Hz, a displacement of 6 mm and an acceleration of 15g. The duration of stimulation was 60 s ( £ 5), with 60 s rest between treatments. The other leg hung freely and acted as a control. The subjects of this study, however, performed less repetitions compared to Bosco’s study (10 £ 60 s). Stiffness measurements using the protocol outlined above were calculated prior to the warm-up, after the warm-up and after the vibratory stimulation. 2.5. Data analysis The data were sampled at 1000 Hz by a computer (MacIntosh G4 Computer, Cupertino, CA, USA) based data acquisition and analysis program (Superscope Version 3, GW Instruments, Boston, USA). The peak-to-peak force change divided by the peak-to-peak position change was measured for every clear oscillation as a result of the perturbation (Fig. 3). Stiffness was calculated for each of these oscillations. The formulae for calculating stiffness from the damped oscillation were k ¼ 4pi2 mf 2 þ c2 =4m
Fig. 2. Perturbation of the knee to measure triceps surae muscle stiffness.
Fig. 3. Example of a damped oscillation. The peak-to-peak area analysed is indicated by the arrows.
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Table 1 Means (^SD) and coefficients of variation for pre-warm-up, post-warm-up, and post-vibration stiffness Control limb
Treatment limb
Stiffness (N m)
Mean
SD
CV
Mean
SD
CV
Pre-warm-up Post-warm-up Post-vibration
5380.273 5506.545 5148.424
956.923 1202.491 1042.099
4.1 3.5 4.8
5923.061 5715.939 6162.273
1430.069 1055.890 1679.822
3.2 4.0 2.9
where k is the stiffness in Nm21; m is the mass, f is the damped frequency of oscillation, and c is the coefficient of damping (McNair, Wood, & Marshall, 1992). 2.6. Statistical analysis Mean values and standard deviations of the highest three results were calculated for the stiffness measures of each subject. The reliability of these measures were represented as a coefficient of variation (CV) using CV ¼ SD/Mean £ 100. Coefficients of variation were calculated for each individual and then the average CV for the sample was calculated. The percent change between baseline and warm-up, and baseline and post-vibration measures were calculated. These change scores were then analysed using paired t-tests to identify if the mean percentage change between treatment and control limbs was significantly different. The chances that the true value of the effect statistic (percent change) was clinically positive, trivial or harmful were calculated for each variable (Hopkins, 2002) by assuming the smallest practically important change in muscle stiffness was 5%. Five percent was chosen as the between trial variation of the stiffness measure were found to range between 2.9 and 4.8% (Table 1).
Fig. 4. The mean of individual percent change scores and absolute differences in stiffness post-warm-up and post-vibration. Absolute differences were calculated as the absolute sum of the percentage stiffness changes for the treatment and control legs.
percent change (8.1%) between pre- and post-vibration for the treatment and control limbs was not significantly different (P ¼ 0:118). Although not statistically significant, vibration did have an observable effect on the muscle stiffness of the subjects (Fig. 4). The chance that the true effect of vibration was positive/trivial/negative for the average athlete is 74/25/1%. That is, if the smallest practically important increase in muscle stiffness is 5%, the chance that training with vibration increases stiffness is 74% and the chance it decreases stiffness is 1%. Some subjects during the course of the vibratory stimulation described an itching sensation in the lower limb, but no subjects withdrew from the intervention. A number of the subjects, however, experienced pain following the prescribed vibration treatment in the jaw, neck and lower extremity, particularly tibialis posterior.
4. Discussion 4.1. Whole body vibration and stiffness
3. Results The means, standard deviations and coefficients of variation for pre-warm-up, post-warm-up and post-vibration stiffness are presented in Table 1. Standard deviations provided relate to the group results, whilst the coefficient variables provided are associated with the means of each individual. Coefficients of variation ranged from 2.9 to 4.8% for the three testing occasions for each leg, indicating that there was little variation in the stiffness measures between trials. The percentage change between stiffness measures (Fig. 4) was calculated for the control and treatment legs and thereafter paired t-tests were used to identify if the mean percentage change between limbs was significantly different. The mean percent change (4.6%) between pre- and post-warm-up for the treatment and control limbs was not significantly different (P ¼ 0:402). Similarly the mean
The decrease in stiffness observed (Fig. 4) after the warm-up was to be expected and has been reported previously (McNair & Stanley, 1996). As mentioned earlier, increased intra-muscular temperature is thought to improve performance by enhancing neural efficiency, reducing viscous resistance and increasing muscle elasticity (Ettema & Huijing, 1994; Shellock & Prentice, 1985; Tancred & Tancred, 1995). Increased temperature will also affect the damping properties, influencing the resistance of the muscle to motion and hence decrease stiffness. It has been proposed that as a result of vibration muscle stiffness is altered, which may explain the improved functional performance reported in the research cited previously. Bosco et al. (1998) has discussed the role of vibration in improving performance via stiffness regulation, and in later studies (Bosco et al., 1999a, 2000) noted increased neuromuscular efficiency post-vibration. This may involve reduced EMG activity for a given force output
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due to increased motor unit synchronisation and improved synergist co-contraction and antagonist inhibition, which in turn would influence muscle stiffness (Bosco et al., 2000). Mester et al. (1999) stated that muscle tension and stiffness increased in response to vibration. However, none of these studies directly measured muscle stiffness. It can be observed from the results of this study that no significant (P ¼ 0:118) change between pre- and post-vibration stiffness for the treatment and control limbs was found. These results suggest that the muscle stiffness of relatively untrained subjects was not affected by the vibratory stimulation protocol used in this study. Although not statistically significant, vibration did have an observable effect on the muscle stiffness of the subjects (Fig. 4). If this study was replicated with a larger sample, the chance that the true effect, that is the change in stiffness due to the vibration treatment, was positive/trivial/negative for the average athlete was 74/25/1%. That is, if the smallest practically important increase in muscle stiffness is 5%, the chance that training with vibration increases stiffness is 74% and the chance it decreases stiffness is 1%. Furthermore, although no significant differences were detected within one session, this does not exclude the possibility that further treatments may have induced significant changes in the stiffness properties of the muscle. Research is needed that maps changes in stiffness after multiple sessions of vibratory stimulation. Further research is warranted investigating if the differences in stiffness result in significant changes to functional performance. The absence of any significant change in stiffness may be explained by a number of factors. First, the subjects of this study performed only half the repetitions prescribed by Bosco et al. (1999b, 2000). As a result the vibration stimulus may have been of inadequate duration for adaptations to occur. Second, vibration may have induced an increase in muscle temperature, which may have countered any shortterm neurogenic adaptation. It may be that vibration also elicits a certain level of pre-synaptic inhibition, which reduces the further recruitment of motoneurons. De Ruiter, van der Linden, van der Zijden, and Hollander (2003) proposed such an explanation, subsequent to finding no significant changes in performance, after replicating the protocol of Bosco used in this research. Furthermore de Ruiter and colleagues questioned whether whole body vibration would lead to neural adaptations that could enhance performance, after finding that there was no increase in voluntary muscle activation post-vibration. 4.2. Whole body vibration and injury Gravitational forces (g) of 1– 1.2g (1 –2 Hz) have been reported during walking and 3 –5g (2 – 3 Hz) whilst running (Hamill & Knutzen, 1995). Injury potential associated with walking is minimal, however, the increased intensity associated with running has commonly been linked to stress fractures in the tibia and fibula, and ITB friction syndrome
(Norris, 1993). Higher g forces (11g) have been associated with 60 cm drop jumps (Arampatzis, Schade, Walsh, & Bru¨ggemann, 2001). However, training using such high intensity plyometrics usually necessitates the athlete to be well trained (Chu, 1998) and requires substantial rest between jumps, which results in frequencies of less than 1 Hz. In comparison to these activities, the gravitational forces associated with vibration on the Galileoe 2000 has been reported to be as high as 17g at frequencies of 26– 30 Hz (Bosco et al., 1999b, 2000; Rittweger et al., 2000). These forces far exceed the gravitational forces associated with walking, running and jumping, and no doubt will cause discomfort or injury if an individual is unaccustomed to such forces. A major finding of the study was the injury potential associated with whole body vibration. Whilst work science research has reported the detrimental effects of prolonged exposure to vibration, the detrimental effects of short-term exposure to vibration as used in sport science research has not been documented. Although the intervention used in this study was less to that used in previous research (Bosco et al., 1999b, 2000), a number of the subjects experienced pain following the prescribed vibration treatment in the jaw, neck, and lower extremity, particularly tibialis posterior. Six subjects required physiotherapy treatment, all subjects reporting, however, that their pain had subsided within 7– 10 days. It is proposed that the frequency and high forces associated with the vibration treatment provided a training stimulus that was too intense for the relatively untrained subjects of this research. It also should be noted that research in this area in terms of controlled training studies is in its infancy. The appropriate dose-response relationships and principles for training such as progression, frequency and recovery are yet to be determined. For example, Bosco et al. (2000) stated that 10 min of whole body vibration at 17g was equivalent to 200 drop jumps from a height of 100 cm (5g), performed twice a week for a period of five months. Given these statistics one should err on the side of caution when prescribing vibration as a training stimulus. At the very least, subjects need to undergo a pre-screening that not only includes a review of injury status but also each subject’s training history, prior to vibratory stimulation. With this in mind one should remain cognizant of the limitations of whole body vibration as a training stimulus. Although Issurin, Liebermann and Tenenbaum (1994) recommended more research into the possible adverse effects of vibratory stimulation, no other research mentions the injury potential associated with whole body vibration, nor have subject discomfort or injuries been reported. The majority of sport science vibration research to date has involved subjects who are involved in athletic activities, competing in sports at least at club or university/college level. The training status of the subjects participating in this study varied, and included individuals who did not participate in physical activity of any kind. Although no pre-existing injuries or conditions were
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reported, this lack of physical conditioning prior to vibration may have predisposed these subjects to injury. Also the subjects of this study stood flat-footed on the Galileoe 2000. It may be that a plantar flexed position may attenuate force transmission to better effect and decrease injury potential. Given that the muscles are active in this position, however, one could also speculate that the transmission of forces may be greater. The literature is unclear on this point and further investigation into the responses of the musculoskeletal system to vibratory stimulation using different postures is warranted.
5. Conclusion Although no significant differences were found between the mean percent change between pre- and post-vibration stiffness for the treatment and control limbs, vibration did have an observable effect on the muscle stiffness of the subjects. Future research needs to compare the stiffness changes between subjects of different training status so as a better appreciation of the effects of whole body vibration can be established. This will necessitate a better understanding of whether adaptation to vibration is mediated by central or peripheral factors or a combination of both. It is also quite probable that the neuromuscular system of the trained athlete will respond differently to vibratory stimulation as compared to the untrained athlete. Furthermore the relationship between stiffness and performance, and what percentage change in stiffness results in a significant change to performance needs to be identified. A major finding of the study was the injury potential associated with whole body vibration using the Galileoe 2000. A number of the subjects experienced pain during and/or following the prescribed vibration treatment in the jaw, neck, and lower extremity, particularly tibialis posterior. Forces experienced on the Galileoe 2000 are far greater than those produced during traditional training methods, due to the high gravitational forces and frequencies associated with vibration (up to 17g at 26 – 30 Hz). It is proposed that the high forces associated with vibratory stimulation provide a training stimulus that can be detrimental to a subject’s well being. It is recommended that caution be exercised when using whole body vibration and that the intensity and duration of the vibratory stimulation be applied progressively. It is suggested that more research be conducted on the effects of whole body vibration across divergent samples (e.g. training status, age, gender) and a dose-response model formulated. This in the first instance will necessitate a better understanding of how the different foot positions (flat-footed vs plantar flexed), amplitudes (foot position on teetering vibration platform) and frequencies affect the g forces associated with the use of the Galileoe 2000. Thereafter identifying how these forces are damped at different locations of the body would provide useful insights into the loading parameters for this doseresponse relationship.
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