In the last decade, whole-body vibration (WBV), that is, entire body exposure to mechanical vibration while exercising in static or dynamic conditions, has been used as an alternative method to enhance physical performance in healthy individuals. Improvements have been observed in strength, jump ability, and sprint running velocity (1,2,3,18,24). Several studies have indicated that this training modality develops isometric and dynamic strength, sprint performance, jump ability, and agility, albeit with a great diversity in the reported gains (for reviews, see [16,23,25]). The discrepancies are generally assumed to be because of the specific characteristics of the WBV sessions (frequency and peak-to-peak displacement of the vibration), training protocols (number and duration of the sessions), and testing procedures, and the pretraining status and interindividual differences (16,23,25). Although, it has recently been suggested that vibration training may serve as a tool to develop explosive ability in athletes (15), it remains unclear whether WBV could enhance physical performance in athletes.
Some of the studies have investigated the effects of short-term WBV training programs, that is, lasting 8 weeks or less, on the physical performance of athletes from various sports. For example, Mester et al. (22) and Mahieu et al. (19), respectively, demonstrated the positive impact of WBV training on the physical performance of a well-trained alpine skier and in a group of young skiers. More recently, Annino et al. (1) concluded that WBV training was effective in improving knee extensor muscle explosiveness in well-trained ballet dancers when added to the conventional training. In contrast, no benefits to sprint performance were reported when WBV training was added to the conventional training of sprint-trained athletes (11). Although WBV programs have been developed for explosive athletes, surprisingly, it appears that no study has yet examined the effects of WBV training on the physical performance of basketball players.
Analysis of a typical basketball match reveals the importance of aerobic endurance, anaerobic power, endurance, muscular strength, jump ability, sprint performance, and agility (9,21). Basketball is an intermittent sport that requires explosive actions including quick and repeated accelerations, jumps, and changes in movement direction (21). It was also recently pointed out that the new basketball rules have led to an increase in the number of explosive actions during matches (9). Further, the jump performance of basketball players has been shown to be significantly correlated with maximal isometric strength during leg extension (13,30), indicating that the production of maximal strength may be important for explosive actions. It has, indeed, been reported that isometric strength increases were positively correlated with squat jump (SJ) improvements after 4 weeks of electrostimulation training in basketball players (17). In addition, significant improvements in maximal isometric strength (18) and jump performance (18,20) have also been observed in volleyball players after only 4 weeks of training. In light of these findings, and because short-term WBV programs have been reported to be effective to increase the physical performance of athletes from various sports requiring explosive actions (1,2,18), one might question whether WBV would be an efficient complementary training method to enhance physical performance in explosive athletes like basketball players.
Therefore, the purpose of the present study was to determine the influence of a 4-week WBV training period in a selected population of competitive basketball players. It was hypothesized that a WBV training program added to conventional basketball training would result in improved isometric strength and physical performance in these basketball players. We particularly focused on the knee extensor muscle strength of the players, and their physical performance as assessed by vertical jumps and sprint performance.
Experimental Approach to the Problem
Short-term strength training protocols (4 weeks) have been used to improve maximal isometric strength and physical performance in explosive athletes (17-19). Because WBV has been included in strength training protocols, it has been reported to exert a positive effect on both maximal strength and explosive actions with a short time of exposure per session (2,6,12). Moreover, WBV can be performed easily with limited familiarization (12) and several advantages (6) over strength training protocols that depend on weight machines and free-weight exercises, generally used as complementary methods to improve physical performance.
This study thus used a randomized experimental design to examine the effects of a 4-week WBV training program on a group of competitive basketball players (13 males and 5 females, 18-24 years old). With the approval of their respective coaches, volunteers were selected from basketball teams competing at the regional level of the French Basketball Federation League. The experiment was conducted during the 4-week preseason preparation period (noncompetitive period). The subjects continued their conventional basketball training supervised by their respective coaches (3 times a week). All took part in typical basketball sessions that were divided into warm-up (jogging with and without ball handling exercises, accelerations, jumps, shots, and stretching), the main phase (fundamentals of basketball like individual and collective defense and attack, tactics and strategy, specific situations, etc.), and recovery (low-velocity jogging and stretching). For all subjects, the work/rest ratio was close to 1.
The basketball players' maximal voluntary bilateral isometric strength of the knee extensor muscles, SJs, countermovement jumps (CMJs), drop jumps (DJs), 30-second rebound jumps, and 10-m sprint running time were considered as dependent variables in this study. These variables were chosen because they are representative of the physical performance of basketball players (9,21). Dependent variables were measured in 2 groups of subjects, one of which underwent an additional 4-week WBV training program (whole-body vibration group [WBVG]); the other, the control group (CG), received no additional exercise training (CG). All subjects were tested at baseline and after the 4-week period. The tests after the 4-week period were performed 4 days after the last training session. The independent variables were the time at which the measurement was taken (i.e., baseline and after the 4-week period) and the group of subjects (i.e., WBVG and CG).
In WBV studies, it has recently been suggested that the CG should follow a training program identical to that of the WBV group, only without vibration (23). Because the purpose of our study was to assess the potential positive effects of a WBV training program offered as a complement to conventional basketball training in a group of subjects (WBVG), we used a “passive” CG, which only participated in the basketball training sessions.
Eighteen regional-level competitive basketball players agreed to participate in this study. They were randomly allocated to WBVG (n = 10; 7 ♂ and 3 ♀; age 20.4 ± 1.8 years, height 184.2 ± 6.4 cm, body mass 75.9 ± 8.3 kg, and means ± SD) or to CG (n = 8; 6 ♂ and 2 ♀; age 19.3 ± 1 years, height 177 ± 6.4 cm, body mass 68.2 ± 9.1 kg, and means ± SD). The respective average (mean ± SD) weekly training time was 5 hours 06 minutes ± 1 hour 03 minutes for WBVG and 4 hours 45 minutes ± 0 hour 42 minutes for CG. All players had trained and competed regularly in basketball for at least 5 years, but none of them had engaged in systematic strength training or WBV in the 3 months preceding the beginning of the experiments. All were informed of the procedures that would be performed and the experimental risks. They signed an informed consent form before enrollment in the investigation. The study was approved by our Institutional Human Ethics Committee in accordance with the Declaration of Helsinki.
WBV Training Program
The WBV training program consisted of 12 × 20-minute sessions of unloaded static exercises on a vibration platform oscillating vertically (100 × 65 cm, Silverplatine first generation, Silver® Développement, Valbonne, France). Because there is still no specific WBV training program available for athletes, the parameters (peak-to-peak displacement and frequency) chosen for this study were based on previous reviews of the literature (16,23,25). Training intensity was the same throughout the 4 weeks of the program (peak-to-peak displacement = 4 mm and frequency = 40 Hz). Subjects trained 3 times a week (Mondays, Wednesdays, and Fridays) for 20 minutes. Throughout each session, they were exposed to WBV for 30 seconds followed by a 30-second rest period (duty cycle: 50% and total duration of vibration exposure: 10 minutes). For the first 2 weeks, the program consisted of alternating between 2 static positions: first, a high squat position (knee angle 110°; where 180° corresponds to full extension of the knee) and, second, the same high squat position while standing on the toes (with the same knee angle; where the ankle angle is fixed at 90°) over the total time of vibration exposure. For the last 2 weeks, subjects followed the same program with the knee angle set at 90° (instead of 110°). Knee angle was constantly monitored with a manual goniometer during the training sessions. During vibration exposure, the subjects' hands were positioned on their waist, and the trunk was leaning slightly forward (thigh/trunk angle fixed at 130°). During the 30-second rest periods, subjects adopted a relaxed standing position. Subjects were not allowed to wear shoes, but to avoid bruising, they wore socks throughout the vibration sessions.
Before and after the 4-week period, testing sessions were organized to assess the physical performance of the 2 groups. Performance testing started after a standardized 15-minute warm-up that included submaximal intensity running, single and rebound jumping at a progressively increasing intensity, and several acceleration runs. Test order was randomized to avoid the impact of fatigue from one test to another. In all tests, verbal encouragement was given to motivate the subjects to perform each test maximally. The testing sessions comprised the following: maximal voluntary bilateral isometric strength assessment of the knee extensor muscles; jump performance (SJs, CMJs, DJs, and 30-second rebound jumps); and 10-m sprint running test.
The subjects were tested for maximal voluntary bilateral isometric strength of the knee extensor muscles at a knee angle of 70° (0° corresponding to the knee extension). They were placed in a seated position with a trunk-thigh angle of 120° on a specific strength training apparatus (i.e., leg extension machine). Trunk stabilization was assured by straps placed around the chest, and the subjects were required to keep their arms crossed on their chest. The maximal isometric voluntary contraction was recorded with a commercial strength dynamometer (TSD 121C, Biopac® System Inc., Holliston, MA, USA) composed of a load cell secured to the frame of the leg extension machine and connected to a microprocessor, so that the direct line of strength was measured. Two trials (5-second duration) separated by a 60-second rest period were completed, and the best value was used for subsequent analysis.
Vertical SJs, CMJs, and DJs were performed with an electronic timer connected to an optical acquisition system for measuring the flight time of the different jumps (Optojump® system, Microgate, Bolzano, Italy). Two bars compose the system (interbar distance ∼1.2 m): one containing the reception and control unit, the other embedding the transmission electronics. The time onset was triggered by the unloading of the subjects' feet from the ground and was stopped at the moment of touchdown. This method assumes that the position of the jumper is the same in take-off and landing. The jump height (h = elevation of the subject's center of gravity) was then calculated by using the flight time (tf) of the respective jumps according to the following ballistic formula proposed by Bosco et al. (4): h = tf2g8−1, where g is the gravity pool constant (9.81 m·s−2). The concentric phase of the SJ started from a static semisquatting position with the knee flexed at ∼90° for ∼1 second and without any preliminary movement. The CMJ was performed with a preparatory movement from the extended position down to a freely chosen flexed-knee position (∼100°), followed by a concentric action. The DJ started from a standing position on a 40-cm high platform, with dipping and then knee extension in one continuous movement. Whatever the testing modality, subjects were asked to jump as high as they could while keeping their hands positioned on the waist and without lifting their knees during the flight and landing phases. Two trials were completed for each jump condition, and 90 seconds of rest was allowed between each jump. The highest SJ, CMJ, and DJ values were used for subsequent analysis.
On the same system, 30-second vertical rebound jump tests were performed. The subjects were instructed to perform continuously as many maximal vertical jumps as they could, over a 30-second period with hands positioned at the waist, to assess lower limb explosive endurance capacity (4). From these tests, the average height (AH) and the average power (AP) of the jumps were calculated.
The sprint running tests were performed on an indoor track to avoid variable weather conditions. They consisted of 2 maximal 10-m sprints, with a 120-second period rest between the sprints. Running time was recorded by means of 2 pairs of photocell gates (Wireless Sprint System, Brower Timing Systems, Draper, UT, USA) located 1.2 m above the ground with an accuracy of 0.01 seconds. The subjects started the sprint when ready from a standing start position, 0.3 m behind the first photocell gate. Stance for the start was consistent for each subject. The timer was automatically activated and stopped as the subject passed the first gate at the 0-m mark and the second gate at the 10-m mark, respectively. The best performance was included in consequent analysis.
Standard statistical methods were employed for calculating means, SDs, and SEMs. Normality of the data was checked and subsequently confirmed using the Kolmogorov-Smirnov test. Independent Student's t-tests were used to compare the baseline characteristics of the groups. Paired-sample Student's t-tests were calculated for within-group comparisons. Between-group differences were analyzed by means of independent Student's t-tests on the change scores of both groups (posttraining value − pretraining value) with Bonferroni correction. To assess the meaningfulness of pretraining to posttraining changes, the effect size associated with the change for each variable in each group was calculated by the following formula: (posttraining mean - pretraining mean)/pooled standard deviation of pretraining and posttraining. The effect size of the difference in change scores between the groups was calculated by the following formula: (mean WBVG change score - mean CG change score)/pooled standard deviation of the WBVG and CG change scores. According to Rhea (26), a value of less than 0.25 represents a trivial effect size; 0.25-0.50, a small effect size; 0.50-1.00, a medium effect size; and more than 1.0, a large effect size. Associations between 2 variables were quantified using Pearson's product-moment correlation coefficient. The test reproducibility of all measurements was assessed using the coefficients of variation (i.e., CV = standard deviation/mean × 100) and intercorrelation coefficients for each subject. The statistical analyses were performed using Statistica® software for Microsoft Windows (StatSoft, version 5.0, Tulsa, OK, USA). Significance was set at p ≤ 0.05. Intercorrelation coefficients were calculated using a downloadable spreadsheet (14). Unless specified, all data are expressed as means ± SEs in the entire manuscript and all figures and tables.
The test/retest reproducibility and variability of all the dependent variables are presented in Table 1. Intercorrelation coefficient and CV ranged from 0.770 to 0.992 and from 1.34 to 4.60%, respectively.
No significant differences between the 2 groups were observed at baseline for any of the measured variables.
After 4 weeks of WBV training, the maximal isometric knee extensor strength increased significantly (mean of relative increase +4.98 ± 1.26%, p < 0.001; effect size was small = 0.36) in WBVG (Figure 1A). No significant changes in maximal isometric knee extensor strength occurred in CG (991.8 ± 59.6 and 987.5 ± 59.1 N, at baseline and 4 weeks, respectively; Figure 1B). After the 4-week period, the increase in maximal isometric knee extensor strength of the WBVG was significantly greater (p < 0.01; effect size was large = 1.24) than the change in the CG (data not illustrated).
Squat jump performance significantly increased in WBVG at the end of the training period compared with baseline values (mean of relative increase +6.66 ± 3.02%, p < 0.05; effect size was trivial = 0.23; Figure 2A). No significant change was observed in SJ performance in CG (33.59 ± 3.07 cm and 33.01 ± 2.28 at baseline and 4 weeks, respectively; Figure 2B). For the WBVG, a significant positive correlation fitted with a linear function was found between the maximal voluntary isometric knee extensor strength and SJ performance at baseline and after the 4-week training period (r = 0.880, p < 0.001 and r = 0.844, p < 0.01, respectively; Figure 3).
Countermovement jump and DJ performances were not modified in either WBVG or CG after the 4-week period (Table 2). For the 30-second rebound jumps, no significant changes were found after the 4-week period in either group for the AH or the AP of the jumps (Table 2). After the 4-week period, no significant difference was observed between WBVG and CG for jump performance. No significant change in sprint performance was observed after the 4-week period in WBVG or CG (Table 2), nor was there a difference between groups.
This is the first training study to investigate the effects of WBV on physical performance in competitive basketball players. The main findings of the study indicated that a 4-week WBV training program, incorporated into preseason preparation as a supplement to conventional basketball training, significantly increased the players' maximal isometric strength in the knee extensor muscles, whereas performance of CMJs, DJs, 30-second rebound jumps, and sprint running was not modified. The data also indicated that SJ performance was improved although the individual responses were variable and may be associated with individual differences other than gender or training status.
The increase in isometric knee extensor strength observed after a short-term WBV training period is in line with the results of previous studies of untrained subjects with longer training periods (12). In contrast, de Ruiter et al. (10) reported no effects of 11 weeks of WBV training on isometric knee extensor strength, and a more recent study found no effects after a 5-week WBV protocol added to the conventional training program of sprint-trained athletes (11). To our knowledge, this is the first study to report increases in the SJ performance of trained athletes after a short-term WBV training period. However, individual differences were observed in WBVG, and in CG. In WBVG, 6 players improved their SJ performance, whereas the 4 others showed a decrease, and this observation was not related to gender difference. Indeed, because male and female subjects were enrolled in the study, it might be pointed out that some of the adaptations observed in the present study could be partly attributable to gender differences, whatever the variable analyzed. Because Cochrane et al. (8) did not report SJ improvement after a short-term training period in noncompetitive team sport players, further research is warranted to more accurately determine the effects of WBV training programs on SJ performance. As previously observed (13,30), another interesting finding of the current study was the correlation between maximal voluntary isometric knee extensor strength and SJ performance both at baseline and at the end of the WBV training period. Thus, the increases in maximal voluntary isometric knee extensor strength and SJ performance can be attributed to the effect of combined WBV training and basketball practice, because no positive change in these values was observed in CG, who only participated in the conventional basketball training.
Although SJ performance was slightly improved, the CMJ, DJ, and 30-second rebound jumps of our basketball players were not affected by the vibration training protocol. This lack of improved CMJ performance matches with previous studies in physically active subjects and sprint-trained athletes who, respectively, followed 11 and 5 weeks of WBV training (10,11). In contrast, CMJ performance was increased in both well-trained ballerinas and resistance-trained men (1,28). Moreover, although CMJ and DJ were not directly assessed, other studies have reported positive effects of WBV training on explosive strength in skiers (19,22). Last, although it was recently reported that sprint running velocity was increased in healthy active subjects after 6 weeks of WBV (24), the lack of improvement in sprint performance reported in the present study agrees with previous results of short-term WBV training periods (8,11).
The discrepancies in the reported effects of WBV training on physical performance between the present study and the above-mentioned reports can be attributed to the great variability in WBV training programs (frequency and peak-to-peak displacement), session durations, the initial training status of subjects, and the actions performed on the platform. In some training protocols, subjects were asked to stand on the vibrating platform in one or more isometric positions (1,8,10,24), whereas others performed both dynamic and isometric exercises (11,19). It should also be noted that the total training duration of the sessions was increased throughout the training period in some studies (10,12,24). Therefore, all these differences make comparisons of the results between studies very difficult.
It is generally accepted that increases in strength after short-term training programs are mainly because of neural adaptations (29). Although there is still no consensus on the mechanisms by which a WBV training program improves physical performance, neural adaptation is often advanced (6,27). No electromyographic measurements were made in the present study, so the likelihood of a neural adaptation underlying the observed strength gains cannot be assessed. However, previous WBV training studies, although also lacking in direct measurements (1,2,24), have ascribed the enhanced physical performance after a short-term training period to an adaptation of the neuromuscular system. In our study, whatever the underlying adaptation might be, a 4-week WBV training period was an effective stimulus to improve maximal isometric knee extensor strength and partly SJ performance, although no modification was observed for CMJ, DJ, 30-second rebound jump, or sprint running performance.
Jump performance in basketball players has been shown to be significantly correlated with the maximal isometric strength during leg extension (13,30). One might expect that the improvement in maximal isometric knee extensor strength and SJ performance observed in the present study would be matched by improvement in explosive performance. However, CMJ, DJ, and 30-second rebound jump performances were not improved after the training period, nor was sprint running. During these explosive actions, the muscles are lengthened and potential energy is stored in the series elastic component and then released during subsequent shortening (5,7). The present study suggests that movements involving this stretch-shortening cycle (SSC) were not improved by WBV training. This could partly be explained by the fact that our WBV training protocol was performed in isometric conditions and, therefore, would not have stimulated the SSC. This result contrasts with the finding of Annino et al. (1) in female ballet dancers, but these authors used a longer training program (i.e., 8 weeks). It is quite likely that the lack of any improvement in explosive performance observed in our study was related to the short duration of WBV training or to the delayed appearance of adaptations to the training stimulus (i.e., increases in performance are further amplified after an interruption of the training period). For example, it was reported that after 4 weeks of electrostimulation training, the CMJ performance of basketball players was not immediately improved, but enhancement was observed 4 weeks after the end of the program (17). Similar observations have recently been reported for the SJ and CMJ performance of volleyball players (20). Besides, it should be noted that although Maffiuletti et al. (17) used electrostimulation, they also reported that their 4-week training program induced increased maximal isometric strength and SJ performance, as in the present study. Thus, based on these observations and on the results of Annino et al. (1), it can be suggested that improvements in explosive performance involving SSC actions might require a longer period before beneficial effects can be observed.
In conclusion, this study demonstrated that 4 weeks of WBV training added to the conventional training of basketball players during the preseason period improved maximal isometric knee extensor strength and slightly SJ performance but did not modify explosive performance (CMJ, DJ, and 30-second rebound jumps) or sprint running performance. The gains observed in maximal isometric knee extensor strength and jump performance without SSC were achieved in a relatively short period of vibration training added to the conventional training. It can be speculated that higher vibration loading (additional loads, different frequency or peak-to-peak displacement, and WBV protocol) or longer WBV programs should be proposed to players to attain significant improvement in explosive physical performance, especially for trained athletes in sports that require SSC actions.
A preseason short-term WBV training program, added to the conventional basketball training sessions, may be an effective stimulus to increase isometric knee extensor strength in basketball players. The effect of WBV on SJ performance should nevertheless be interpreted with the understanding that there may be great variability among individuals. In any case, this study failed to show improvement in lower limb dynamic explosive performance involving SSC actions (like the countermovement, drop and 30-second rebound jumps) or sprint running performance. This observation could be related to the specificity of the basketball game, which requires skill in jumping, sprint running, and rapid changes in movement direction (20). Moreover, the new basketball rules have led to an increased number of these repetitive explosive actions during matches (9). Last, it can be suggested to coaches and players that (a) a 4-week WBV training program can be added to conventional basketball training during the preseason period to enhance strength and to partially enhance SJ performance without interfering with basketball training and (b) because of the high number of repetitive SSC actions during conventional basketball training sessions, 4 weeks of WBV does not seem long enough to enhance dynamic explosive physical performance.
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