Whole-body vibration (WBV) has been promoted as an alternative exercise intervention able to affect neuromuscular performance (4,6) in young and old individuals. This new neuromuscular training method consists of squatting on specially designed plates producing sinusoidal oscillations of different frequencies and amplitudes (6). It has been suggested that the sinusoidal vibration generated by the plate oscillations elicits reflex muscle activity in the lower limbs (5,9,26), mainly via monosynaptic pathways.
Muscle activation during squat exercise on vibrating surfaces is still a controversial topic. In fact, although recent work from De Ruiter et al. (8) indicates that vibration elicits only low levels of muscle activation in leg extensor muscles, Rittweger et al. (25,24) have shown that both frequency and amplitude of vibration increase specific oxygen uptake and thus muscle activity. Very recently, Yamada et al. (34) have reported that squatting exercise with low-frequency, small-amplitude vibration performed on a tilting vibrating plate reduced muscle-oxygenation levels of vastus lateralis (VL) more than squatting without vibration as measured by near-infrared spectroscopy (NIRS). An increase in oxygen use was reported as the most likely cause.
Vibration exercise intensity can be determined by manipulating two parameters: amplitude and frequency (4,6). In most of the vibrating plates currently available on the market, vibration frequency is the only parameter that can be changed, and notwithstanding manufacturers' instructions, there is no real evidence to suggest the optimal training frequency to be adopted. Therefore, the determination of the effects of different WBV frequencies on muscle oxidative metabolism represents an important aspect to be analyzed to provide guidelines for WBV training programs.
The possibility of noninvasively studying local muscle oxidative metabolism during exercise has been enhanced in the last few years, thanks to the use of NIRS (2,10,12,15,21,31). For the quantification of muscle O2 saturation, different NIRS methods are available; so far, one of the most largely used in muscle studies is represented by near-infrared, spatially resolved spectroscopy (NIRSRS). NIRSRS provides an average (from small vessels, such as the capillary, arteriolar, and venular bed) tissue O2 saturation and concentration changes in oxyhemoglobin (O2Hb), deoxyhemoglobin (HHb), and total hemoglobin (tHb = O2Hb + HHb). Tissue O2 saturation represents a dynamic balance between O2 supply and consumption in the investigated tissue volume.
During exercise, both blood flow and oxidative metabolism in skeletal muscle respond to meet increased oxygen demand. For this reason, NIRS measurements can provide an indication of localized muscle activities (21). Few studies have been conducted using NIRS during WBV exposure. In particular, recent work from Maikala et al. (16) shows a decrease in lumbar erector spinae oxygenation and blood volume in healthy males during seated WBV, independently of vibration dose. The authors show that prolonged exposure to such a form of WBV could lead to fatigue in erector spinae, and they suggest this as a possible underlying factor for low-back pain. Considering the lack of information regarding muscle oxygenation during WBV exercise, we aimed to investigate the effects of different WBV frequencies on oxygenation of VL and gastrocnemius medialis (GM) muscles during static squatting in sedentary and physically active healthy males. We hypothesized that vibration would determine a decrease in muscle oxygenation in GM and VL greater than in the control condition, because of an increase in muscle activation. Furthermore, we hypothesized that the oxygenation of GM muscle, because of its proximity to the vibration source, would be more affected than the oxygenation of VL during WBV.
Twenty volunteers (age: 24.6 ± 2.9 yr; body mass: 80.6 ± 11.8 kg; height: 178.1 ± 7.6 cm) participated in this study. Ten subjects were sedentary individuals (body mass: 78.6 ± 7.1 kg; height: 177.1 ± 6.9 cm), and 10 were athletes (body mass: 83.6 ± 16.7 kg; height: 179.4 ± 9.0 cm) practicing different sports: road cycling (1), weight lifting (2), mountain biking (2), rugby (1), volleyball (2), full contact (1), and triathlon (1). All subjects gave their written informed consent before participation. The study was approved by the ethics committee of the University of L'Aquila.
Muscle oxygenation was assessed by NIRSRS using a two-channel tissue oximeter (NIRO-300, Hamamatsu Photonics, Japan). The design and the features of this device have been previously described (28). The theory behind the NIRSRS approach and the reliability to measure tissue O2 saturation has been reported (28). The NIRO-300 provides tissue O2-saturation data as tissue-oxygenation index (TOI, expressed in percentage), which has been tested both in vitro and in vivo (3,28). The TOI value reflects predominantly the mean of arteriolar, capillary, and venular O2 saturations, with a minor (less than 20%) contribution from myoglobin (23). In addition, the NIRO-300 provides, independently of TOI, changes in O2Hb and HHb (expressed in ΔμM·cm) and the derived changes in total hemoglobin volume (ΔtHb = Δ[O2Hb] + Δ[HHb]). The tHb changes, being strictly related to blood volume changes, can be considered an indirect measure of local blood flow changes. After the probe test on a phantom to analyze the total probe sensitivity and the sensitivity difference between the three sensors of the detection probe, the two optical probes (each consisting of an emitter and a detector kept at a constant geometry and distance of 4.5 cm by a rigid rubber probe holder) were firmly attached to the skin overlying the belly of the GM and VL muscle groups of the right leg. In the case of hairy skin, the skin was carefully shaven before the experimentation. The optical probes were further secured by an elastic band. No downward sliding of the optical probes was observed at the end of the measurements in any subject. After fixing the probe holders on the subjects, an initialization procedure was carried out. The latter sets each laser power, automatically establishing the optimum measurement condition. The zero-set procedure (carried out just before the beginning of each baseline condition) was adopted to return the O2Hb, HHb, and tHb parameters to the zero value. This procedure does not affect the TOI value, because TOI is measured as absolute value instead of a change with respect to the arbitrary initial zero value. The NIRSRS data were recorded with a sampling rate of 6 Hz.
Adipose tissue thickness underlying the monitored VL and GM area was measured with a Harpenden skinfold caliper. Adipose tissue thickness of sedentary subjects was 4.6 ± 1.2 and 3.4 ± 0.7 mm for VL and GM, respectively. Adipose tissue thickness of athletes was 3.6 ± 0.9 and 2.9 ± 0.9 mm for VL and GM, respectively.
For each subject, TOI and tHb data are reported as mean values for the last 5 s (30 data points) in each considered condition (vibration and control) and time (baseline and at 30, 60, 90, and 110 s).
Subjects were asked to attend the experimental session after 24 h of complete rest from any physical activity. The subjects were familiarized with the protocol 1 d before the study. After the skinfold measurement, the two optical probes were positioned. Before each treatment, the subjects performed a 10-min warm-up, cycling at 50 W on an electronically braked cycle ergometer (QTE Biomedica Ergocard II, Esaote Biomedica, Italy).
The subjects (wearing their socks) were then asked to stand in half-squat position (knee angle 110°) on a vibration platform (Fitwave, Medisport, Italy), with their arms on the front handlebar, their feet parallel, and their foot stance similar to the distance between shoulders. A custom-made goniometer, fixed by velcro straps to the contralateral leg, was used to maintain the joint angle. The subjects were asked to maintain the squat position for 110 s (the maximum duration allowed by the vibrating platform) on the balls of their feet in the following randomized conditions: no vibrations, and 30-, 40-, and 50-Hz WBV. Each condition was preceded by a 60-s baseline in half-squat position. All subjects were able to hold the required position for the duration of the condition throughout the protocol. The vibration amplitude in all conditions was ± 4 mm (peak-to-peak displacement of the platform). Twenty minutes of rest were observed between conditions to allow for full recovery and to avoid the occurrence of fatigue. The NIRO-300 recordings were interrupted 30 s after the end of the randomized condition. Then, the subjects were allowed to move freely and, eventually, to sit on a chair. During the interval between the different conditions (rest period), the optical probes were not removed. During the 20 min of rest, the cardiac frequency was monitored by a pulse oximeter (N-200; Nellcor, Puritan Bennet, St. Louis, MO), with the sensor attached to the forehead. The cardiac frequency was completely recovered to the baseline values at the end of the rest period (data not shown).
Data are reported as mean ± standard error of measurement of TOI and changes in tHb. A four-way analysis of variance (treatment (4) × time (5) × groups (2) × muscle (2)) was used to compare changes in TOI and in tHb. When a significant interaction was found, a repeated-measures ANOVA with Dunnet comparisons was used. Significance level was set at P < 0.05. The statistical analyses were performed with SPSS (Chicago, IL).
Some typical tracings of TOI and ΔtHb are shown in Figure 1. The statistical analysis revealed no differences between treatments in the baseline values of TOI, indicating that full recovery occurred between the treatments (Fig. 2). The statistical analysis revealed no significant differences in changes of TOI and tHb between groups (athletes and sedentary), suggesting that a short exposure to vibration in isometric squat position affects muscle oxygenation independently of the training level of the individual. Because no significant difference was identified between trained and sedentary individuals, all data were pooled and statistical analyses were performed on all 20 subjects participating in the study.
The data analysis revealed no significant treatment-by-time interactions in TOI or ΔtHb in VL and GM muscles. A significant main effect of time in TOI of VL as well as GM muscles was identified (P < 0.001; Fig. 2). The repeated-measures ANOVA with Dunnet comparisons revealed a significant decrease from baseline in TOI of VL in control conditions after 90 and 110 s (P < 0.05 and P < 0.05, respectively), a significant decrease from baseline in TOI in the 30-Hz condition after 110 s (P < 0.01), a significant increase in TOI after 30 s (P < 0.05) in the 40-Hz condition, and a significant increase in TOI after 30s in the 50-Hz condition.
GM TOI was found to be significantly lower than baseline at 60 s (P < 0.05), 90 s (P < 0.01), and 110 s (P < 0.01) in the control condition, and at 110 s (P < 0.05) in the 30-Hz condition. The results suggest, again, that vibration does not determine the same level of desaturation observed in the control condition. A significant main effect when comparing VL and GM TOI was identified (P < 0.001). The repeated measure revealed significant differences at all time points between GM and VL, suggesting different desaturation between plantar flexors and leg extensors during the task.
No significant difference was identified in ΔtHb over time in VL muscle. A significant increase for ΔtHb over time was identified in GM (P < 0.001; Fig. 3). The repeated-measures ANOVA with Dunnet comparisons revealed a significant increase in ΔtHb after 90 and 110 s only in the control conditions.
WBV training is becoming a popular form of exercise because it is relatively user friendly and does not require advanced instructions or complicated movements. However, few studies have been conducted on this novel exercise modality, and they all have shown contradicting results, particularly when acute responses were identified (7,8,29,30). Recent technological developments have made it possible to noninvasively study local muscle oxidative metabolism by means of NIRS (2,10,21). To the best of our knowledge, only one study thus far has been conducted using NIRS to investigate the oxygenation of lower-limb muscles during squatting exercise on a vibrating plate (34).
In the study by Yamada et al. (34), subjects performed 3 min of continuous dynamic squatting in the control and vibration conditions (15 Hz, ± 2.5-mm amplitude). The results of their study show that muscle-oxygenation levels of VL were significantly reduced after 90 s. Our results did not show any significant difference between the diverse vibration conditions and the control condition. Differences in exercise modality and vibration magnitude between the two studies could explain the disparity. In fact, Yamada et al. (34) exercised their subjects on a vibrating plate while performing dynamic squatting with a relatively large range of motion (from full extension to 60°), whereas our subjects performed a static squatting exercise. Different studies have shown that dynamic muscle contraction generated by electrical stimulation causes greater energy turnover and fatigue than static contractions performed at equal force levels (1,33). The same can hold true for WBV exercise. Furthermore, they used a similar amplitude (± 5 vs ± 4 mm in our study), a completely different frequency (15 vs 30 Hz in our study), and a different vibration device (alternating vibration side to side vs whole-plate oscillation). This suggests that WBV performed on a platform oscillating side to side can be more demanding for the lower limbs, because for brief periods during the oscillations, the body is almost entirely supported on one leg. It is also possible that the static position used in our study produced a very low level of muscle tension, and for this reason, significant differences probably would have been identified only after a much longer duration than 110 s. Furthermore, considering that Yamada et al. (34) used an incorrect statistical procedure (repeated Student's t-tests with no Bonferroni adjustment), it is possible that, even during dynamic squatting on a vibrating plate for short periods of time, there is no significant alteration of local oxidative metabolism, because of the low level of muscle stimulation generated by low-amplitude vibration.
As supportive evidence, recent work from Mileva et al. (20) has shown that even after four sets of eight repetitions with a load equal to 35 or 70% of the one-repetition maximum of leg extension on a vibrating leg-extension device (10-Hz vibration frequency; total duration approximately 80 s), no significant muscle-deoxygenation rate could be observed. Muscle-oxygenation level is affected by the balance between oxygen use and oxygen supply in human skeletal muscle. The lack of difference in muscle-oxygenation response between WBV exposure for 110 s and the control condition observed in our study seems to suggest that when low-amplitude vibration is applied for a short duration, it is unlikely to alter local muscle metabolism significantly.
However, it is interesting to remark on the different effects of vibration on GM and VL over time. The results of our study show a significant desaturation in both GM and VL after 60 and 90 s of static exercise in the control condition. WBV exposure determined a significant desaturation only after 110 s in VL and GM. Considering also the significant increase in VL TOI% after 30 s at 40 and 50 Hz, and the lack of significant changes over time in GM, it seems that when vibration is superimposed to mild levels of muscle activation, some alterations in muscle metabolism and blood flow occur, delaying the hypothesized desaturation.
Previous studies have reported that muscular blood circulation in the calf and thigh was significantly increased after one bout of a 9-min WBV exercise on a tilting plate (26 Hz, 3-mm amplitude; (14)). The authors attributed the peripheral changes they observed to an exercise-induced widening of small vessels. Because TOI patterns reflect the balance between oxygen supply and demand in the target muscles, it seems feasible to suggest that the increased blood flow caused by vibration could provide an adequate oxygen supply, delaying the desaturation observed when no vibration is applied. Furthermore, considering that plantar vibration has been shown to increase supine blood flow in the calf measured by strain-gauge plethysmography by 20% (27), and that similar increases were observed by the same authors in peripheral lymphatic flow and venous drainage, it is possible that the relatively short duration of our vibration protocol might have actually delayed muscle desaturation through an improvement in muscle perfusion.
WBV exposure previously has been shown to determine higher levels of muscle activation by EMG in VL and GM muscles as compared with standing on the vibrating plate with no vibration delivered to the body (5,9). Many authors have suggested that this neuromuscular response is similar to the tonic vibration reflex (11) previously observed with direct applications of vibration to either muscles or tendons (5,6,9,13,26). The increased muscle activation observed in previous studies was hypothesized as able to produce a larger muscle desaturation. Furthermore, Rittweger et al. (25) have reported a higher oxygen uptake while performing dynamic squatting in vibration conditions as compared with nonvibration conditions, and a shorter time to exhaustion. Local muscle oxidative metabolism with vibration exercise may be sustained despite an increase in EMG activity that is possibly attributable to changes in blood flow and intramuscular pressure, and this may change with dynamic and static WBV exercise. In this light, a recent study from Vedsted et al. (32) shows no differences in the reduction of muscle-oxygenation tension between dynamic and static muscle contraction despite a marked difference in EMG and mechanomyography. The authors suggest that the lower intramuscular pressure measured during dynamic exercise was responsible for the discrepancy between muscle activation and oxygenation. No study has been conducted so far on intramuscular pressure during vibration exercise, but other mechanisms such as a reduction in blood viscosity (14), an increase in total peripheral resistance and opening of more capillaries (19), and an enhanced peripheral blood and lymphatic flow (27) could be responsible for maintaining oxygen supply despite an increase in motor unit recruitment.
The results of our study seem to suggest that short-duration WBV performed in static conditions with small amplitudes does not cause the same level of muscle desaturation that is observed from squatting without vibration, and could also delay the desaturation observed in similar conditions without vibration. Considering the lack of desaturation observed in our study and the possibility for vibration to increase blood flow in the exercising muscles, we suggest that WBV exercises with a total duration shorter than 2 min could be used as a warm-up procedure in athletes. Needless to say, more studies are needed to elucidate the vascular mechanisms and to evaluate what happens when long-duration vibration exercise is performed, and also whether there is any value to using vibration to increase muscle perfusion.
At the moment, it is not clear whether WBV represents a training stimulus strong enough to influence force-generating capacity and/or muscle metabolism (6). However, because many manufacturers are advertising the use of vibration as an exercise tool capable of positively influencing the overall fitness of an individual, it is clear that more well-controlled studies are needed to provide safe and effective guidelines for its use. Considering that our study and few other previous studies have shown that there are no potential limitations of using NIRS to analyze muscle and cerebral oxygenation in WBV research (16-18), and considering the successful application of NIRS to sports science research (21,22), future studies might be carried out employing this technique to improve the understanding of muscle oxidative metabolism in response to different vibration protocols.
In conclusion, this study has shown that WBV exercise with frequencies of 30, 40, and 50 Hz and small amplitudes does not affect muscle oxygenation of VL and GM muscles to a higher degree than a nonvibration condition. More studies are needed to elucidate the physiological responses to WBV exercise, with particular reference to the interactions between neuromuscular and metabolic demands of this novel and promising exercise intervention.
This research was supported in part by Hamamatsu Photonics K.K, Japan. The authors wish to thank Medisport s.r.l. (Borgo Podgora, Latina, Italy) for providing the vibration platform used in the study (Fitwave); Dr. S. Crisostomi, Dr. A. Franco, and Dr. L. Fallavollita, who assisted with NIRS measurement setup; and Ms. J. Erskine for carefully reviewing the manuscript. The results of the present study do not constitute endorsement of the product by the authors or ACSM.
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Keywords:©2007The American College of Sports Medicine
VIBRATION EXERCISE; MUSCLE OXYGENATION; NEAR-INFRARED SPECTROSCOPY; OXIDATIVE METABOLISM