Specific approaches are used by athletes to enhance muscle recovery and minimize the risk of overtraining (20) or injury (4). Whole-body vibration (WBV) is an emerging strategy used by athletes and active individuals to potentially accelerate muscle recovery, attenuate muscle soreness, and pain (23,32) and improve peripheral circulation (13,33), tissue oxygenation (22), arterial stiffness, or aortic hemodynamics (17).
Whole-body vibration elicits involuntary muscle stretch reflex contractions leading to increased motor unit recruitment and synchronization of synergist muscles (10). This increased muscle activity can lead to a greater leg blood flow (21,25) and metabolic waste disposal (16). These effects have been attributed to contraction-related vasodilatory factors, which, in turn, may increase the tissue oxygenation recovery rate (22) and attenuate the increase in leg arterial stiffness during metaboreflex activation after a bout of WBV (18).
Although previous studies have shown that acute vibration exposure can temporarily increase local blood flow in the muscle tissue (22,26), little is known on the effects of vibration as a recovery strategy after intense exercise. Lohman et al. (25) suggested that short-duration vibration without exercise can significantly increase skin blood flow for a minimum of 10 minutes after intervention. Zhang et al. (39) similarly found increased leg blood flow during and 2 minutes after vibration applied to the foot. Recently, a continuous 10-minute session of WBV without exercise on the legs decreases leg arterial stiffness and wave reflection during 15–30 minutes after vibration (38), suggesting local vasodilation. A possible factor involved in local arterial vasodilation induced by WBV without exercise may be increased nitric oxide production (27).
Another question that arises is whether WBV can maintain or even increase performance in subsequent efforts. Recent studies have found no benefit of exercise with WBV (12 Hz) on exercise performance recovery after high-intensity interval training (16) or isometric handgrip (19). We demonstrated that low-frequency vibration recovery (20 Hz–4 mm) applied during 15 minutes after short and intense exercise was not effective on blood lactate removal and on time to exhaustion (TTE) during cycling (11). Because increased leg artery blood flow and reduced leg arterial stiffness have been induced with vibration frequencies between 25 and 30 Hz (25,38), recovery from exercise may be enhanced at a vibration frequency higher than 20 Hz.
Elite competitive cycling is characterized by demanding training, which often occurs multiple times a day or on consecutive days. Because the riders are usually required to complete various stages on the same day, a faster recovery will enable the athlete to train more or to avoid overtraining (28). Fast recovery may also be important in track cycling where athletes are involved in a training program or who have a competition schedule that requires 1 or more high-intensity exercise sessions, games, or both in a 24-hour period. It has been shown that active recovery between bouts of intense exercise reduces muscle metabolites and consequently enhances anaerobic performance (1). It is possible to hypothesize that passive WBV after intense exercise may improve local blood flow and thereby subsequent maximal performance compared with inactive recovery. Therefore, the aim of this study was to determine the effect of acute passive WBV on leg blood flow after intense exercise and determine whether or not the improved local blood flow would positively influence subsequent maximal exercise performance.
Experimental Approach to the Problem
To examine the primary hypotheses of this investigation, participants performed a maximal graded exercise test on a cycle ergometer followed by an active recovery period using WBV (25 Hz–4 mm) or by a passive recovery period (noWBV; 0 Hz–4 mm). The protocol consisted of six 1-minute sets separated by 1-minute interset rest periods in the seated position with heart rate (HR) and blood flow measurements recorded at 1-minute interset rest periods. Thereafter, participants performed a bicycle exercise test-to-exhaustion and TTE and total distance covered (TDC) were recorded. Our study design (a randomized, counterbalanced, crossover design) addressed the issue of whether passive WBV would increase the leg blood flow during early recovery from intense exercise and find out whether or not these effects can influence subsequent maximal exercise performance.
Twenty-three healthy male volunteers (mean ± SEM; age, 24 ± 1 years; height, 1.72 ± 0.01 m; body mass, 71.8 ± 2.2 kg; body mass index, 24.4 ± 0.7 kg·m−2) were recruited for this study. All participants were recreationally active but have had no regular participation in structured conventional exercise programs or WBV during the last 12 months. Exclusion criteria were prescribed medications or supplements known to affect cardiovascular parameters and contraindications for WBV including osteoarticular conditions. All testing and training procedures were fully explained and written informed consent was obtained from all participants. The Local Research Ethical Committee approved the experimental protocol and the procedures involved.
A randomized, counterbalanced, crossover design was used to eliminate any order effects. Participants completed all trials in a quiet temperature-controlled room (23° C) at the same time of the day (±1 hour) to eliminate any influence of circadian variation. Participants rested at least 8 hours and were asked to abstain from caffeine, alcohol, and exercise for 24 hours and to fast for at least 4 hours before visiting the laboratory. Water intake was ad libitum. All participants visited the laboratory on 3 occasions separated by at least 48 hours: (a) a familiarization with laboratory equipment and maximal oxygen uptake power output (pV[Combining Dot Above]O2max) determination; an exercise test-to-exhaustion followed by either a recovery period (b) with WBV or (c) without WBV. After the recovery period, participants performed an exercise test-to-exhaustion on a cycle ergometer at pV[Combining Dot Above]O2max.
Maximal Graded Exercise Test
When participants arrived to the laboratory, they were asked to rest seated at least 20 minutes for the baseline measurements. Thereafter, all participants performed a brief warm-up (5 minutes, 50 W, 60 rpm) on a braked cycle ergometer (Kettler Axiom P2; GmbH & Co. KG, Ense-Parsit, Germany) before the incremental exercise test conducted to volitional exhaustion to determine pV[Combining Dot Above]O2max. Gas analyses were performed using a breath-by-breath gas analyzer (VO2000 Portable Metabolic System; MedGraphics, St. Paul, MN, USA). V[Combining Dot Above]O2max corresponded to the highest V[Combining Dot Above]O2max attained in 2 successive 15-second periods for the maximal graded test. The cycling exercise began at a workload of 50 W with a pedaling rate of 60 rpm, followed by increments of 25 W every minute until exhaustion. Maximal effort was confirmed by attainment of at least 3 criteria: (a) a respiratory exchange ratio >1.2; (b) HR >90% of age-predicted maximum; (c) a plateau in V[Combining Dot Above]O2max defined as no change (<150 ml·min−1) in V[Combining Dot Above]O2max from the previous stage; or (d) rating of perceived exertion >17 on the Borg's 6–20 scale (6).
Whole-Body Vibration and Recovery Period
After the incremental exercise test, all participants were seated with their feet on a synchronous vibratory platform (pro5 AIRdaptive; Power Plate, Badhoevedorp, The Netherlands) keeping their thighs abducted and the knees and hips flexed at 90° for either one of the following 12-minute recovery periods: (a) no vibration (noWBV, 0 Hz, 0 mm) and (b) passive vibration (WBV, 25 Hz, 4 mm) according to the randomly assigned order in the 2 testing days. The WBV protocol consisted of six 1-minute sets of squat separated by 1-minute interset rest periods in the seated position (Figure 1). The intensity of vibration (≈3.6 g) was chosen based on previous data showing that vibration frequency at ∼25 Hz can increase skin (22) and muscle blood flow in the exposed legs (25). This recovery period was applied because a complete muscle recovery can be achieved between 10 and 15 minutes after a fatigue test (24). Heart rate and blood flow measurements were recorded at 1-minute interset rest periods.
Blood Flow Measures
Alterations in blood flow of the popliteal artery were assessed with a Doppler ultrasound machine (Handydop Pro; Kranzbühler, Germany). Measurements of blood flow were recorded on the dominant leg in the sitting position. All blood flow outcomes were measured at the midpoint of the popliteal fossa at the medial condyle level and insonated at a 60° angle to standardize measurements. Recordings were made at rest and during minutes 2, 4, 6, 8, 10, and 12 of the recovery period in both conditions. In the WBV trial, these time points correspond to the six 1-minute interset rest periods. Blood flow outcomes measured were systolic peak frequency (FS), diastolic peak frequency (FD), HR, pulsatility index (PI), which is the result of (FS-FD)/time averaged mean, and resistance index (RI) which is the result of (FS-FD)/FS. The reproducibility of blood flow velocities and PI in the popliteal artery has been previously reported (14). The intraclass correlation coefficients (ICCs) for these outcomes recorded, which estimate the average correlation between all possible pairs within the subject taken by the same observer, varied from 0.765 (FD), 0.803 (FS), 0.814 (PI) to 0.848 (RI).
After the 12-minute recovery period, participants performed an exercise test-to-exhaustion on the cycle ergometer at pV[Combining Dot Above]O2max maintaining a pedaling frequency of 60 rpm. During the test, participants were instructed to exercise for as long as possible until they were unable to maintain the frequency of pedaling for more than 15 seconds. The TTE and TDC, defined as the time (s) and distance (km) at which the participant was unable to maintain the pedaling rate, were recorded to the nearest second with a chronometer and were rounded to the nearest meter, respectively. This type of fatigue test has been widely used to investigate the effectiveness of both warm-up and recovery strategies on exercise performance (29,37) and has been previously validated for measuring TTE and TDC (11).
All analyses were calculated using the SPSS software (version 16; SPSS, Inc., Chicago, IL, USA), with data examined for outliers and distribution. Normality of the data was confirmed via the Kolmogorov–Smirnov statistic with a Lilliefors significance correction. All variables were analyzed using 2-way repeated-measures (2 trials × 7 time points) analysis of variance to compare differences between the successive recovery minutes determined separately for each exercise. Post hoc test was used to examine the differences within and between trials when significant time and interactions were observed. Furthermore, comparison within the groups on TTE and TDC was performed using paired t-test. Statistical significance was accepted at p ≤ 0.05, and all data are presented as means ± SD.
Participant characteristics are presented in Table 1. Twenty-three adults were recruited and completed all procedures. Body mass index ranged from 20.14 to 27.46 kg·m−2. Maximal oxygen uptake power output reached a mean of 307.61 ± 39.5 W.
Table 2 shows the effect of WBV and noWBV recovery strategies on the blood flow indices. Heart rate was significantly higher than baseline in both groups (p < 0.001) throughout the recovery. During recovery, a similar trend was observed in both FS and FD dynamics in both WBV and noWBV strategies. The PI was decreased (p < 0.01) from baseline during the first recovery period in both trials. Significant between-group differences were observed after bouts 4 (p ≤ 0.05) with participants receiving vibration reaching lower scores than the noWBV. No significant differences in RI in both conditions in all the recovery time points were found.
The TTE and TDC after the exercise test-to-exhaustion on a cycle ergometer at pV[Combining Dot Above]O2max are reported in Figures 2A,B, respectively. Significant differences were found between the groups in both outcomes. Participants in WBV were pedaling more time (158.90 ± 57.17 seconds) compared with noWBV participants (138.21 ± 59.47 seconds). On the other hand, participants in the WBV group (1.64 ± 0.64 km) covered a 12% greater (p ≤ 0.05) distance than noWBV (1.45 ± 0.60 km).
To our knowledge, this study is the first to investigate the effects of vibration on the popliteal artery blood flow during the recovery phase after acute exhaustive exercise and on the exercise performance after a subsequent effort. The main findings of this study are that 12-minute lower vibration frequency stimuli (25 Hz–4 mm) were sufficient to increase muscle blood flow and distance and TTE during cycling at pV[Combining Dot Above]O2max.
Our study confirms the findings of previous work that have demonstrated an increase in leg blood flow after maximal cycling exercise (9). Reduced PI after acute exercise has been suggested as a marker of vasodilation induced by changes in peripheral vascular resistance (14,30). The decrease in PI after a bout of exercise has been attributed to increased production of vasodilatory metabolites from the exercised muscles (30). We found a significant reduction from baseline in PI after postbout 1 in both trials suggesting that 1 minute of WBV immediately after maximal leg exercise has no additional effect on the reduction in local vasodilation. Interestingly, although the reduction in PI was transient and did not persist after the second minute of recovery in both trials, further decreases in PI occurred after postbouts 4 and 5 of WBV. One possible explanation to these results can be the attributed upregulation in the nitric oxide dilator system and improved endothelial function reported after repeated bouts of WBV (15,27). These acute increments in local nitric oxide production after WBV may be responsible for the increased vasodilation in the inactive vibrated limb (27). The increase in leg blood flow after passive vibration on the feet could be because of production of vasodilatory metabolites secondary to reflexive muscle contraction (21,34).
Although WBV was suggested to increase muscle blood flow and blood velocity and enhance recovery (5,35), only a few studies involving healthy young adults have investigated the effect of vibration on leg blood flow. Kerschan-Schindl et al. (22) found that vibrations delivered with similar characteristics to the ones employed in this study (26 Hz–3 mm) increased leg blood flow after 9 minutes of vibration applied to the feet. Maloney-Hinds et al. (27) also found that 10 minutes of continuous vibration increased arm skin blood flow after 5-minute after vibration. Again in a group of young healthy adults, Lythgo et al. (26) assessed leg blood flow velocity after a set of 14 random vibration exercise bouts (between 5–30 Hz and 2–5 mm) on a rotary plate and concluded that low frequencies between 5 and 20 Hz produce greater increases in leg blood flow than isometric squatting without vibration. These results contrast in part with the ones showed in our study and 1 possible explanation may be the frequency stimuli selected as it was reported that frequencies between 20 Hz and 36 Hz might have not been sufficient to increase muscle blood flow or muscle temperature required for enhancing recovery (12).
Another explanation may be that none of the previous studies used WBV after intense exercise. The local vasodilation produced by exercise on the cycle ergometer was so intense that the effect of vibration could be minimized. However, our results showed that the vibration was able to maintain vasodilation after postbouts 4 and 5 while the leg arteries not exposed to vibration had recovered completely.
Another possible reason for these inconsistencies can be attributed to the type of vibration selected for this study (synchronous vibration) instead of the side-alternating way employed in the aforementioned studies as the transmission vibration to various body segments in sitting posture may be different and may selectively influence the electromyographic (EMG) activity of the affected muscles. Side-alternating vibration is capable of producing the highest EMG activities and showed moderate vibrations in the X-direction, whereas synchronous vibration induced very stable vibration patterns (completely moved in vertical direction) and it was recently reported that the transmission of vertical accelerations at 25 Hz was largest in the ankle (2.3 times) using side-alternating vibration (31). These authors concluded that the differences in mechanical behavior induced variations in transmissibility of vertical vibrations to the lower body, which may also be responsible for the differences in blood flow found in these studies. In addition, regional differences in muscle fatigue have also been reported when comparing both types of vibration (36), so it is possible that muscle fatigue may have influenced outcomes in this study but this is purely speculative and further research would be necessary to test this hypothesis.
Another innovation of this study is the analysis of an exercise test-to-exhaustion on the cycle ergometer at pV[Combining Dot Above]O2max after the recovery period to ascertain whether the vibration has a favorable effect on subsequent exercise performance. Significant differences were found in both TTE and TDC with WBV and participants exercised more time (13%) and covered a 12% more distance (188 m) compared with the noWBV trial. Despite that WBV was reported to act as a mechanical massage for athletes (12), little is known about it effects on successive efforts. It seems that if the intensity of WBV is high enough to stimulate the muscles, it might have the potential to improve the physiological recovery from strenuous exercise and authors such as Bakhtiary et al. (3) reported that administration of local vibration at 50 Hz to the lower limbs significantly increased the recovery of isometric force production. However, low-vibration frequencies have had no benefits on running performance recovery following a high-intensity interval training (16). Bullock et al. (8) also suggested that a frequency ≤30 Hz is too small of a stimulus to produce meaningful benefits in elite athletes, possibly because of advanced training status and neuromuscular adaptations. In addition, we previously demonstrated that low-frequency vibration applied during 15 minutes after short and intense exercise does not seem to be effective to remove blood lactate and to increase TTE during cycling at pV[Combining Dot Above]O2max (11), which contrast with the results reported in this study. It is reasonable to think that higher vibration frequencies (36 Hz) may have a greater effect on performance by facilitating the return of oxygen consumption to resting level after maximal exercise (12) or by increasing muscle preactivation (greater number of motor units and muscle fibers recruited), which may lead to a reduced myofibrillar stress during repeated muscle contractions and therefore to an accelerated recovery (7).
In conclusion, WBV decreased PI in the popliteal artery after maximal cycling exercise suggesting that a decrease in leg arterial resistance may be the mechanism underlying the increase in blood flow. Our results demonstrate that acute WBV effectively increased performance in a later exercise test-to-exhaustion. This preliminary information provides new strategies to improve recovery after intense exercise via increased peripheral vasodilation in the vibrated limb.
Previous studies investigated the acute effects of vibration on leg blood flow (21,25) and found that WBV can lead to vascular effects and therefore be considered as a potential intervention to accelerate muscle recovery during intense exercise training (2,12). This study showed that using six 1-minute sets of WBV (25 Hz–4 mm) immediately after maximal cycling exercise decreased PI in the popliteal artery. This is a first attempt to better understand how new recovery strategies may influence the performance of a subsequent exercise session and from a practical point of view, our findings suggest that WBV have potential benefits to enhance muscle recovery by improving leg artery vasodilation. Coaches and athletes may consider WBV as a possible recovery technique before training and high-intensity cycling competitions because this approach is correlated with greater local blood flow and better subsequent exercise performance. Further studies are needed to clarify whether different vibration protocols (frequency, duration, and exposed area) can obtain better results.
The authors declare that they have no conflicts of interest. This research was not supported by any funding source and the results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association. This work was supported by University of Seville.
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