Whole-body vibration (WBV) training can be effective for cultivating substantial muscle strength (13) and power (6,22) among the general population. It has also been proven effective for enhancing the neuromuscular performance of athletes (1) and improving lower-limb muscle strength and power (4,8,11). However, WBV training does not significantly improve speed performance (5,18,23). The reason for this limited effect may be because athletes already possess superior neuromuscular function; therefore, the intensity of standard WBV training is relatively insufficient for athletes because it provides little stimulation to their neuromuscular system and consequently limits training effectiveness (19). Therefore, enhancing the benefits of WBV training for athletes is crucial, particularly for sprinters whose physical fitness primarily comprises power and speed. Sprint performance significantly correlated with the high-velocity contraction force of their lower limbs (15). Therefore, a sprinter must produce exceptional high-velocity contraction force to generate the substantial ground reaction force required to rapidly move the body and thereby complete the running distance in the shortest amount of time. Modifying the format of WBV training, which significantly enhances functions such as muscle strength, power, and neuromuscular function, to simultaneously improve muscle strength and speed may significantly contribute to enhancing sprint performance.
The complex training models most frequently used in WBV training involves performing static or dynamic squats with or without a load on a vibrating platform (3–6,8,10,11,19,20). Previous studies have indicated that a complex training model combining WBV with additional load may increase the vibration stimulus applied to the target muscles of the athlete, thereby achieving the desired neuromuscular adaptation. The complex training model for trained athletes or people can potentially improve strength-related performance; however, previous studies have reported that the strength enhancement provided by this combined training did not greatly exceed that of conventional resistance training in isolation (18,21). The inferred reason was that the extra load was equivalent to only 30% of 1 repetition maximum (RM) (18) or 30–36% of the body weight of the participants (21). For trained athletes or people who already possess superior neuromuscular function, the extra load might be insufficient.
To date, depending on various conditions, the effectiveness of protocols involving WBV training is an equivocal but important topic. Hazell, Kenno, and Jakobi (9) mentioned that during dynamic squatting, using a light external load (30% body weight) with or without WBV training had the same effect on increasing muscle activity; however, Marin, Bunker, Rhea, and Ayllon (12) discovered that while performing static half-squatting without shoes, using unload training combined with WBV induced greater muscle activation than that induced without performing WBV. Chen, Chang, and Shiang (2) investigated the difference in muscle activation when the isometric contraction strength was at a load of 25 and 75% maximum voluntary contraction (MVC) and when it was stimulated by an equal amount of vibration. They discovered that during the vibration stimulation process, the integrated electromyogram value induced by a 75% MVC load was significantly higher than that induced by a 25% MVC load. Therefore, increased neuromuscular adaptation may be induced during isometric contraction using a 75% MVC load combined with vibration stimulus.
Moreover, short-term WBV training (3–5 weeks) has been commonly used by trained athletes or people (5,14,22) to achieve enhanced performance levels. A related study demonstrated that performing WBV training combined with no load before resistance training did not enhance strength and speed performance for trained sprinters (5) because of the insufficient training intensity. Using an additional load of 75% MVC in WBV training as a complex training method for increasing intensity may improve both strength and speed in a short training period for trained sprinters during preseason. Therefore, the purpose of this study was to investigate whether a 4-week WBV training program including an additional load of 75% MVC considerably enhanced the strength and speed of trained athletes compared with the results of performing isolated WBV training or loaded training (LT) alone. The study hypothesis was that WBV training combined with an additional load of 75% MVC may benefit elite sprinters who already possess excellent strength and speed.
Experimental Approach to the Problem
A pretest-posttest equivalent-group design was used to investigate whether WBV combined with extra-load training can enhance the strength and speed of trained athletes compared with the results of using either isolated WBV training or LT alone for 4 weeks. All of the participants, trained elite sprinters, were recruited from the same track and field team and randomly assigned to the vibration combined with extra-load training group, the isolated vibration training group, and the LT group. The vibration frequency was 30 Hz, and the amplitude was 4 mm. The training protocol for both groups involved performing five 30-second bursts of WBV 3 days a week for 4 weeks. The independent variables were loaded vibration (LV) training, unloaded vibration (ULV) training, and LT, and the dependent variables were isometric strength, isokinetic concentric strength, isokinetic eccentric strength, and sprint speed.
Twenty-one elite male track and field athletes (age range: 19-23 years), who had not sustained lower-limb neuromuscular injury in the previous 6 months, volunteered to participate in this study. All of the participants were sprinters who had participated in the final round of a 100- or 200-m sprint event in a national competition within the past 3 years. They were trained athletes and routinely engaged in 3-hour track and field training sessions 4 times per week. All of the participants were recruited from the same track and field team; thus, all of them used an identical training program and applied the same workflow. To develop explosive muscle strength for improving speed, this experiment was conducted during the preseason. In addition, all the participants continued their routine sprint training and resistance training without squat-related training.
The participants were randomly assigned to the LV group (n = 7), ULV group (n = 7), and LT group (n = 7). The physical characteristics of the participants in each group are shown in Table 1. The experimental procedures used in this study were approved by the Institutional Review Board of the Taipei Physical Education College. All of the participants were informed of the experimental risks and signed an informed consent form before participating in this study.
The 3 groups performed the 4-week training program in a weight training room. As shown in Figure 1, both the LV and ULV groups engaged in WBV training that required the athletes to maintain a static squat position without wearing any shoes and bend their knees at an angle of 120°, which was standardized using a goniometer, while standing on a synchronous vibration platform (custom designed by Magtonic, Corp., Tainan City, Taiwan), either under a load of 75% MVC for the LV group or under no load for the ULV group. This study adopted a 30-Hz frequency with 4-mm amplitude as the vibration protocol when the participants maintained a static half-squatting position (12). The range of the peak acceleration was between 10.68 and 10.90 g. To ensure that the 30-Hz vibration frequency and the 4-mm amplitude established using the training formula were uniform for the participants in both the groups and that neither the body weight of the participant nor the additional load influenced the vibration intensity, metal plates weighing 300 kg were placed atop the vibration platform, and a 25 g triaxial accelerometer (CXL25GP3; Crossbow Technology, Inc., San Jose, CA, USA) was attached to the center of the platform during vibration, as shown in Figure 1. The vibration frequency, peak-to-peak amplitude, and peak acceleration in the vertical direction were obtained from the z-axis of the triaxial accelerometer. While the participants with the greatest total weight (body weight + load) were performing the movement, the triaxial accelerometer at the center of the vibration platform measured the vibration parameters, which remained at 30 Hz and 4 mm. Thus, the 2 groups experienced identical vibration intensity. After 4 weeks of training, all of the participants had completed the training sessions.
The training protocol for both groups involved performing five 30-second bursts of WBV with a 3-minute rest interval in between, 3 days a week for 4 weeks. The training protocol is shown in Table 2. The weight of the extra load for the LV group participants was determined by measuring the right knee extensor MVC strength during monoarticular exercise using a Biodex isokinetic dynamometer (Biodex System 3 Pro, Shirley, NY, USA), while the participants' knees were bent at a 60° angle, and by calculating and doubling the 75% MVC values of the right knee extensor to estimate the additional load for the bilateral in both the LV and LT groups. During the training session, metal plates were installed on the horizontal bar of the Smith machine to achieve 75% MVC load. When the participants stood on the vibrating platform with the horizontal bar on their shoulders, they were instructed to maintain the following position specifications when receiving the vibration stimulus: a knee angle of approximately 120°; feet positioned shoulder width apart, with the tip of each foot positioned directly below the barbells; and a weight distribution focused on the forefoot.
The pre- and posttests were performed 2 days before and after training, respectively. All of the participants performed standardized warm-up activities (jogging and stretching) before the measurements. The isometric strength, isokinetic strength, and sprint speed were tested by performing 3 trials using the same procedure before and after the training period. To avoid muscle fatigue, all of the participants were allowed a 3-minute rest between trials and a 5-minute rest between tests. The order of the testing procedures was 30-m sprints, isometric contractions, concentric contractions, and eccentric contractions, and they received verbal encouragement during each test. These assessments were conducted as follows: (a) isometric strength was assessed using a Biodex isometric dynamometer to measure the largest torque (in newton meter) resulting from a 3-second maximal isometric contraction at a knee angle of 60°, divided by body weight as normalization (in newton meter/body weight), (b) isokinetic strength was assessed using a Biodex isokinetic instrument to measure the largest torque (in newton meter) resulting from the concentric and eccentric knee extensor contractions at an angular velocity of 300°·s−1, divided by body weight as normalization (in newton meter/body weight), and (c) sprint speed was assessed by asking the participants to perform a starting action without using starting blocks and then begin sprinting immediately, and an infrared interval timer was placed at the starting and finish lines for a 30-m distance to measure the sprint time.
The baseline between-group differences were determined using an independent t test. The test-retest reliability of all the measurements was assessed using the intraclass correlation coefficient (ICC). A 2-way mixed analysis of variance (time × group) was designed and used to assess the differences among the groups. When the group × time interaction produced significant F values, a simple main effect test was performed for post hoc analysis. Statistical significance was set as p ≤ 0.05. The partial eta squared (η2) and observed power (OP) were also calculated to complete the analysis. All the statistics were analyzed using SPSS 17 software for Windows (SPSS, Inc., Chicago, IL, USA).
No significant differences among the groups were detected for the dependent variables at baseline. Excluding isokinetic eccentric strength, all the measurements indicated good reliability; the ICCs for the dependent variables were 0.661 (isomeric strength), 0.785 (isokinetic concentric strength), 0.388 (isokinetic eccentric strength), and 0.880 (sprint speed). Significant time × group interactions were observed for isometric strength (p = 0.024; η2 = 0.338; OP = 0.704), isokinetic concentric (p = 0.006; η2 = 0.434; OP = 0.873) and eccentric strength (p = 0.001; η2 = 0.550; OP = 0.977), and speed (p = 0.000; η2 = 0.607; OP = 0.994). Concerning the post hoc analysis of the results, the LV group demonstrated significant improvements in all the dependent variables after training (p ≤ 0.05), whereas the ULV group exhibited a significant reduction in sprint speed, and the LS group demonstrated no significant improvements in all of the dependent variables. Furthermore, the LV group exhibited significantly superior isokinetic eccentric strength compared with the ULV and LT groups (p ≤ 0.05) and exhibited significantly superior sprint speed compared with the ULV group (p ≤ 0.05) (Figures 2–5).
The primary result of this study was that 4 weeks of LV training significantly enhanced isometric strength, isokinetic concentric strength, isokinetic eccentric strength, and sprint speed compared with ULV training or LT training. This result supports the hypothesis that combining WBV with 75% MVC weight training increases the intensity of muscular stimulation and enhances the muscle strength and speed of elite track and field athletes within a short period. Furthermore, this combined training model is more effective than isolated WBV training or LT alone in enhancing the high-velocity eccentric strength of the quadriceps and sprint speed, which are key factors that influence sprint performance.
This study demonstrated that LV training significantly enhanced isokinetic concentric strength and isokinetic eccentric strength. Additionally, isokinetic eccentric strength after LV training was superior to that after ULV or LT training. Although this study involved static squatting combined with WBV training, the results of this study are similar to those of previous studies on training untrained or trained individual using dynamic squatting combined with WBV training. Osawa and Oguma (17) discovered that after 13 weeks of WBV training involving additional loads weighing 10 and 15% of the participants' body mass, untrained participants' maximal isometric strength, concentric strength, and countermovement jump height were significantly enhanced. However, previous studies involving athlete participants have determined that a complex training model combining WBV and resistance training does not produce superior effects compared with those produced by resistance training in isolation (18,21), which may have been caused by insufficient loads. Similarly, Rønnestad (22) observed that 5 weeks of WBV combined with squat training with a high intensity of 6–10RM notably enhanced the 1RM squat and countermovement jump height of recreationally resistance-trained male participants; however, a trend toward greater enhancement was observed in the group that performed WBV combined with resistance training compared with that in the group that performed resistance training in isolation.
This study adopted a high intensity of 75% MVC (approximately 10RM) as the additional load weight, which was established according to the participants' individual muscle strength rather than body weight. In addition, a 75% MVC can enhance the neuromuscular response during WBV (2). Thus, a complex training model that combines WBV with a 75% MVC load can effectively enhance isokinetic eccentric strength and sprint speed. Isokinetic strength was assessed at an angular velocity of 300°·s−1, thereby corresponding to high-velocity contraction strength intensity, which is an essential element for sprinters who are required to move their body rapidly. Achieving a fast sprint speed depends on propulsive ground reaction force, which is produced by knee extensor contractions accompanied by high-velocity eccentric contractions (16). Previous studies have reported a significant correlation between eccentric knee extensor strength and speed (7). Based on these findings, the complex training model adopted in this study that combines WBV and a 75% MVC load significantly enhanced key performance factors in achieving excellent sprint performance, such as high-velocity eccentric strength and speed.
The WBV without additional load training implemented in this study did not improve sprint speed. Furthermore, sprint performance significantly deteriorated, which caused neuromuscular adaptation after 4 weeks of training. Delecluse, Roelants, Diels, Koninckx, and Verschueren (5) provided sprinters with WBV training without additional load for 5 weeks using a vibration frequency ranging between 35 and 40 Hz and an amplitude ranging between 1.7 and 2.5 mm. They determined that the knee extensor and flexor strengths, maximal knee extension velocity, jump performance, force-time characteristic of the start action, and sprint running velocity demonstrated during a 30-m sprint exhibited no significant differences, suggesting that WBV training without additional load is as effective as traditional sprint training. Comparatively, Owen (18) reported that WBV, at 30 Hz and 6.0–12.5 mm, combined with 30% of 1RM squat exercise did not improve the 5-, 10-, and 20-m sprint times of elite rugby players. Although exceptions to the mentioned results exist (4), WBV training without additional load improved sprint speeds when provided to sprinters with a certain muscle strength capacity (19). Paradisis and Zacharogiannis (19) trained previously active athletes for 6 weeks using WBV at a frequency of 30 Hz and an amplitude of 2.5 mm, and observed that the 10 and 60-m sprint speeds were significantly increased. This result implied that the effectiveness of WBV training without additional load or with small loads is limited and insufficient for enhancing the speed performance of athletes. Although the vibration protocol adopted in this study (e.g., frequency, amplitude, stimulation time, training amount, and exercise movement) did not differ drastically from that of other studies, the WBV without additional load training implemented in this study significantly decreased the speed of athletes during the 4-week training period.
Performing a 30-m sprint requires the exertion of muscle power using the lower extremities. Theoretically, power = force × velocity, and sprint velocity = stride frequency × stride length. Therefore, the speed required for sprinting 30 m is determined by the integrated performance of power, stride frequency, and stride length. Regarding the improvement of muscle strength, WBV training combined with an additional load of 75% MVC was determined to effectively enhance knee extensor strength during high-velocity eccentric contraction, thereby increasing the 30-m sprint speed. The stride frequency is negatively correlated with the stride length exerted during a sprint event (20). Although stride frequency and stride length were not analyzed in this study, the results obtained by Paradisis and Zacharogiannis (19) suggested that after WBV training, stride length increases by approximately 5.6%, which is approximately −3.9% greater than the stride frequency decrease. Based on this finding, WBV training can increase stride length and thereby increase sprint speed. Thus, the sprint speed significantly improved after 4 weeks of WBV training with additional load because the isokinetic strength and stride length increased.
In conclusion, a 4-week training program involving WBV combined with 75% MVC loads can effectively enhance the knee extensor isometric strength, isokinetic concentric strength, isokinetic eccentric strength, and speed of elite track and field athletes. This combined training method had a significant positive effect on lower-limb maximal muscle strength, high-velocity contraction strength, and sprint speed during the 4 weeks of training compared with that of isolated WBV training or LT alone. However, to reduce confounding factors by restricting all the sprinters to the same training conditions, this study was limited by the use of a small sample size. Furthermore, the load of squat training may have been underestimated by applying the MVC test to the unilateral leg rather than to the bilateral leg.
At the end of the preseason, effectively developing the strength and speed performance of sprinters who have undergone high-intensity training is challenging. Previous studies have determined that WBV training can effectively enhance the lower-limb muscle strength and power of athletes; however, this type of training does not significantly improve speed because the intensity of isolated WBV is inadequate for athletes who already possess superior neuromuscular function. Consequently, this study verified that applying a complex training model, combining WBV training and additional load, can significantly enhance lower-limb high-velocity contraction strength and sprint speed after 4 weeks of training for sprinters who must perform rapid lower-limb movements. A crucial element of this complex training model is that the additional load applied should be 75% of the MVC for each athlete. Therefore, WBV training combined with an additional load of 75% MVC can increase training intensity, which can shorten the training period to 4 weeks for elite athletes.
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