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Improvement in Running Economy After 8 Weeks of Whole-Body Vibration Training

Cheng, Ching-Feng1; Cheng, Kuo-Hui2; Lee, Yu-Ming2; Huang, Hsin-Wei2; Kuo, Yu-Hsuan2; Lee, Heng-Ju2

The Journal of Strength & Conditioning Research: December 2012 - Volume 26 - Issue 12 - p 3349–3357
doi: 10.1519/JSC.0b013e31824e0eb1
Original Research

Cheng, C-F, Cheng, K-H, Lee, Y-M, Huang, H-W, Kuo, Y-H, and Lee, H-J. Improvement in running economy after 8 weeks of whole-body vibration training. J Strength Cond Res 26(12): 3349–3357, 2012—The purpose of this study was to investigate the effects of 8-week whole-body vibration (WBV) training on running economy (RE) and power performance. Twenty-four male collegiate athletes were recruited and randomly assigned to experimental (WBV) and placebo (PL) groups. The WBV subjects performed semisquat vibration training (30 Hz, ±1−2 mm, 3 times per week), whereas PL subjects performed identical training without vibration. The isometric maximum voluntary contraction tests were used to evaluate maximal isometric force (F max) and rate of force development (RFD) of lower extremities, before and after the intervention, and RE was measured on a level treadmill at 3 velocities (2.68, 3.13, and 3.58 m·s−1). The F max of the lower leg (plantar flexion, from 80.8 ± 24.5 to 99.0 ± 33.9 N·m, p < 0.05, η2 = 0.567; dorsiflexion, from 38.1 ± 6.5 to 43.0 ± 7.7 N·m, p < 0.05), and the RFD of 0–200 milliseconds during plantar flexion (from 186.0 ± 69.2 to 264.6 ± 87.2 N·m·s−1, p < 0.05, η2 = 0.184) were significantly increased in the WBV group after training. The averaged RE values for the 3 running velocities were significantly improved after WBV training (pretraining vs. posttraining, 4.31 ± 0.33 vs. 4.65 ± 0.34 m·ml−1·kg−1, p = 0.001, η2 = 0.654); however, no significant differences were found in the PL group (pretraining vs. posttraining, 4.18 ± 0.26 vs. 4.26 ± 0.44 m·ml−1·kg−1, p = 0.476). The WBV training significantly improved RE at selected speeds (∼5.0−8.5%, p < 0.05). These results indicated that short-term WBV training could be an effective stimulus to enhance RE and lower extremity power performance in competitive athletes.

1Department of Athletic Performance, National Taiwan Normal University, Taipei, Taiwan

2Department of Physical Education, National Taiwan Normal University, Taipei, Taiwan

Address correspondence to Heng-Ju Lee, hjlee@ntnu.edu.tw.

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Introduction

Running economy (RE) is typically defined as the energy demand for a given velocity of submaximal running and is determined by measuring the steady-state oxygen uptake (V[Combining Dot Above]O2max) and respiratory exchange ratio (RER) (17,31). The relationship between RE and distance running performance is documented in previous studies (26,31,33). The RE may be a valuable predictor of endurance performance in endurance athletes and is likely to be influenced by a number of factors, such as the training (i.e., endurance and resistance training), environment (altitude and heat), aerobic and anaerobic capacities, and neuromuscular adaptation (26,31). Although the RE is a critical physiological factor of running performance, there were surprisingly few studies of strategies that might improve the economy of running (17,31). However, Paavolainen et al. (26) found that concurrent explosive-strength training improved the 5-km running performance of highly trained endurance athletes because of the improved neuromuscular characteristics that were transferred into improved muscle power and RE. Several studies have also shown that RE in runners could be improved by the short-term plyometric training program (32–34).

Plyometric training enhances the ability of muscle to generate power by exaggerating the stretch-shorten cycle (SSC) and the storage and release of elastic energy (20,34). Komi (20) suggested that stretch reflexes play an important role in an SSC and contribute to force production during the touch down phase in activities such as running and hopping. Paavolainen et al. (26) indicated that the improvement of RE after explosive-strength training might be because of the adaptation of the neuromuscular system.

In the last decade, whole-body vibration (WBV) has been increasingly examined for improving strength and explosive performance in athletes after a regimen of acute stimulation (4,5,8,10,28) or long-term training (2,9,11,12,15,23). Mechanical vibration, applied directly to the muscle or tendon, can elicit a reflex muscle contraction called “tonic vibration reflex” (18). The vibration-induced tonic vibration reflex involves activation of muscle spindle, mediation of the neural signals by Ia afferents, and activation of α-motor neurons and leads to an enhancement of the stretch-reflex loop (6,7,29,30). Therefore, WBV may also have the potential to enhance RE by a similar underlying mechanism of plyometrics, that is, the activations of muscle spindle. In addition, WBV may be a less time-consuming training tool with lower impacts of tissue damage and perceived exertion in comparison to plyometrics (4,6). However, there is currently no evidence as to whether or not such WBV training could be successfully applied to improve the RE in well-trained athletes.

Despite the increasing scientific interest, the results of WBV training on the neuromuscular performance in athletes widely vary between studies (6,7,21,29,35). The discrepancies in study design and WBV protocols across the literature contribute to these inconsistent results (6,7,21,29,35). The power production of muscle can be defined as the rate of the rise in contractile force at the onset of contraction, that is, the rate of force development (RFD) employed during the early period of muscle force generation (1). The contractile RFD is calculated as the slope of the joint-moment-time curve (Δmoment/Δtime) and is an important parameter in relation to instantaneous maximal power production, which is required during explosive activities such as hopping and jumping (13). Wilcock et al. (35) suggested that studies needed robust methodological structures by using the experimental, placebo, and control groups to clarify the exact effects of the short-term WBV training in athletes. Therefore, we hypothesized that the strength and RFD could be enhanced after short-term WBV training, and RE could also be improved by the chronic stimulation of WBV on the neuromuscular system, compared with an identical exercise program performed in the absence of vibration. The purpose of this study is to determine the effects of WBV training on the economy of running. The maximal isometric force (F max) and RFD during isometric maximum voluntary contraction (MVC) were also investigated, before and after WBV training.

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Methods

Experimental Approach to the Problem

Previous studies (2,15) have found that 8 weeks of WBV training program could effectively improve the muscular strength and power performance in trained subjects. Wilcock et al. (35) suggested that inclusion of placebo and control groups is considered of critical relevance when investigating the effects of WBV training program in athletes. This study thus used a randomized experimental design, with measures conducted before and after 8 weeks of a training intervention period on a group of competitive athletes (24 men, 18–23 years old). All the subjects were randomly assigned to 2 groups: a WBV group that underwent 8 weeks of WBV training with a static semisquat position on the vertical vibration device (AV-001A; Body Green Technology Co. Ltd., Taiwan; approximate cost = 8,900 US dollars) and a placebo (PL) group that underwent 8 weeks of isometric semisquat training without any vibration stimulations. Each subject completed a series of tests at 3 different times: test session 1, at beginning of the study; test session 2 (48 hours after session 1), immediately before the start of the 8-week intervention period; and test session 3, within 3 days after the 8-week intervention period. The following variables were measured in each subject in every test session: isometric MVC test and running economy test. The procedures used to measure these dependent variables are described below. The MVC test was chosen as the dependent variable in this study because this test is able to examine if the addition of WBV to isometric semisquat training results in a larger neuromuscular performance, compared with an identical training program without vibration. The independent variables were the time at which the measurement was taken (i.e., before and after the 8-week period), and the group of subjects (i.e., WBV and PL groups).

To avoid the impact of fatigue from 1 test to the next, half the subjects were asked to perform the MVC test first and to execute the RE test 24 hours after the MVC test in each test session. In test session 1, all the subjects were familiarized with the MVC and RE test protocols, and all of the measured data in this test session were unused in the subsequent data analysis. The subjects performed a brief warm-up (5 minutes at 6.4 km·h−1) on a level treadmill (Mercury 5.0, h/p/cosmos, Germany) before each test.

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Subjects

Twenty-four male Division I collegiate athletes volunteered to participate in this study and were randomly assigned to experimental (WBV, n = 12) and placebo (PL, n = 12; age 20.0 ± 1.7 years; height 177.7 ± 6.6 cm; weight 68.5 ± 7.5 kg) groups. The average daily and weekly training time for all the subjects was approximate to 2–3 and 8–15 hours, respectively, in the 3 months preceding the beginning of the experiments. The subjects had trained and competed regularly in their own sporting events (volleyball, tennis, taekwondo, and track and field) for at least 5 years. For each sports activity, half of the subjects were randomized to the WBV group and half to the PL group, so that the number of subjects would be equal in both groups. One experimental subject was selected by the national team of Asian Game and, therefore, discontinued his participation. The final sample consisted of 11 subjects in the WBV group (age 20.3 ± 1.5 years; height 177.7 ± 8.9 cm; weight 75.8 ± 12.4 kg). No significant difference in age, height, or body weight was observed between groups either at pretraining or at posttraining. The study was conducted in the off-season period (summer vacation) of the annual training plan of the athletes. All the subjects continued their regular training (∼2 hours per session; 5–8 sessions per week) without any organized power training programs, such as plyometrics and resistance training, during the experimental period. A requirement of this study was that subject did not previously undertake a structured WBV training program. The subjects refrained from drinking alcohol or caffeine-containing beverages for 24 hours before testing, and fasted for at least 4 hours before visiting the laboratory to reduce any interference from food on the experiment. Each subject completed all the trials at the same time period of testing days to eliminate any influence of circadian variation. The Institutional Review Board of National Chiayi University reviewed and approved the protocol used in this study to protect the human rights of subjects.

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Procedures

Training Protocol

The participants were subjected to a regimen of intervention training program, with or without vibration stimulation, 3 times a week for 8 weeks. The subjects who completed the study attended all 24 scheduled training sessions (100%). Because there were still no specific WBV training programs available for athletes, the training variables (amplitude and frequency) chosen for this study were based on previous studies (2,9,11,35). Each training session comprised 10 sets of vibration or nonvibration exposures with 60-second rest intervals (Table 1). In the WBV group, the training intensity, excluding the vibration frequency that was suggested by Wilcock et al. (35) for athletes, was progressively increased over the 8-week period by increasing the duration (from 30 seconds to 1 minute) and amplitude (from ±1 to ±2 mm) of the vibration according to the overload principle. The training duration and exercise position in the PL group was similar to the WBV group. However, the vibration device was not switched on. All the subjects wore nonslippery socks during the training session and maintained an upright position with their knees flexed at 120° (full knee extension = 180°) and their feet in full contact with the vibration platform. The knee angle during vibration or nonvibration stimulation was measured by a manual goniometer (Zimmer, Warsaw, Indiana, USA). The subjects completed a personal exercise diary during all training sessions, under the strict supervision of a conditioning trainer.

Table 1

Table 1

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Isometric Maximum Voluntary Contraction Test

All the subjects performed the MVC test using an isokinetic dynamometer (Biodex system 4, Biodex Medical Systems, Shirley, New York, NY, USA) after a 5-minute warm-up. The MVC test, modified from a previous study (1), was used to assess the F max and RFD for the knee extensors, knee flexors, ankle plantar flexors, and dorsiflexors. The MVC tests for the knee extensors and flexors were executed in a seated position at a knee joint angle of 120° (full knee extension = 180°). The rotational axis of the dynamometer was visually aligned to the lateral femoral epicondyle, and the lower leg was attached to the dynamometer lever arm above the medial malleolus, with no fixation of the ankle joint. The maximal isometric strength of the ankle plantar flexors and dorsiflexors were measured in a seated position, with the hip and knee joint angles maintained at 75° (neutral hip position = 180°) and 150°, respectively, and in a neutral ankle position with the shank paralleled the ground. The foot to be tested was secured into the plantar flexion/dorsiflexion boot. Testing angles of knee and ankle joints chosen in this study were mimicked to the angle during vibration or nonvibration intervention. The positioning of the seat, backrest, dynamometer head, and lever arm length for each subject was similar in pretraining and posttraining.

All the measurements were performed on the right leg. The subjects were instructed to contract as fast and forcefully as possible for a period of 5 seconds, and a resting period of 60 seconds was given between repetitions to prevent fatigue. Each subject performed 3 repetitions of maximal isometric contraction for each muscle group with positive verbal encouragement. The subjects were not informed of the instantaneous dynamometer force on the computer screen, or the elapsed time. Trials with an initial countermovement, defined as a visible drop in the force signal, were always disqualified, and a new trial was performed. The force data were sampled at 1,000 Hz and recorded on a computer. The F max was calculated as the peak of the force curve. The contractile RFD was determined from the trial with maximal isometric force. The contractile RFD was derived as the average slope of the moment-time curve (Δmoment/Δtime) over time intervals of 0–100, 0–200, and 0–300 milliseconds relative to the onset of muscle contraction. The onset of muscle contraction was defined as the time point at which the moment curve exceeded baseline moment by >5 N·m. The intraclass correlation coefficient (ICC) was moderate to good for the F max (ICC = 0.73–0.95, p < 0.05) and for the contractile RFD (ICC = 0.50–0.78, p < 0.05).

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Running Economy Test

The subjects were requested to run on a horizontal treadmill (grade = 0%) for evaluating the running economy after the brief warm-up. The test of running economy, based on a previous study (34), was measured in the order of the following 3 running velocities: 2.68, 3.13, and 3.58 m·s−1. The treadmill speed was calibrated and verified before each RE test. The subject ran for 6 minutes at each velocity and rested for 5 minutes between consecutive velocities. Pulmonary gas exchange and ventilation were measured breath by breath throughout the RE test, with the subjects wearing a face mask (Hans Rudolph, Kansas City, MO, USA) through an online gas analysis system (Cortex Metamax 3B; Cortex Biophysik, Leipzig, Germany). The system was calibrated before every test in accordance with the manufacturer's guidelines, against known concentrations of cylinder gases (15% oxygen, 5% carbon dioxide) and a 3-L calibration syringe (Hans Rudolph, Kansas City, MO, USA). Heart rates were monitored using a wireless chest strap telemetry system (Polar S810i; Polar Electro Inc., Oy, Kempele, Finland) and continuously measured through a link to the Cortex gas analysis system during the RE test. The 6–20 Borg rating of perceived exertion (RPE) scores were also measured at rest, and immediately after each running velocity.

The per-minute oxygen uptake and RER were calculated during each of the last 3 minutes of each velocity. The average of the 2 values that least differed from each other was used as the economy measure. Running economy in this study was expressed as both a gross distance unit cost and a gross caloric unit cost. The gross distance unit cost was presented as meters run per milliliter of oxygen uptake per kilogram of body weight (meters per milliliter per kilogram). The gross caloric unit cost was calculated as caloric unit cost (kilocalories per kilogram per kilometer) = V[Combining Dot Above]O2 × caloric equivalent × running velocity−1 × body weight−1 × K (16), where V[Combining Dot Above]O2 is measured in liters per minute, caloric equivalent is in kilocalories per liter, running velocity is in meters per minute, body weight is in kilograms, and K is 1,000 m·km−1. The caloric equivalent of V[Combining Dot Above]O2 was determined from the following formula: H (kcal·L−1 O2) = (15.480 + 5.55 × RER)/4.186 (14). The average running economy over all 3 running velocities was also calculated for each subject. The ICC was moderate for the V[Combining Dot Above]O2 (ICC = 0.66–0.78, p < 0.05), RER (ICC = 0.67–0.75, p < 0.05), distance unit cost (ICC = 0.67–0.73, p < 0.05), and for the caloric unit cost (ICC = 0.71–0.83, p < 0.05) at selected running speeds.

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Statistical Analyses

All data are expressed as mean ± SD. The independent variable was the type of exercise training: isometric training with WBV for experimental group and isometric training only for placebo group. The null hypothesis was that the WBV training would have no effect on the dependent variables: F max, RFD, and running economy. Possible effects of the independent variable on the dependent variables were statistically evaluated by using a 2 × 2 (between-within) analysis of variance (time [pretraining and posttraining] × group [WBV and PL]) with Bonferroni-adjusted t-tests. To minimize the violation of the assumption of homogeneity of variance, the Greenhouse–Geisser correction was used when sphericity was violated. If the significant differences were found in data at pretraining between groups, 1-way analysis of covariance (ANCOVA) was used, using the respective pretraining measures as the covariate. The ICC was used to assess the test reproducibility of all measurements. According to Portney and Watkins (27), the magnitude of difference between changes (pretraining and posttraining) or groups in key variables were expressed as an eta square (η2) using the following criteria: small η2 = 0.01, medium η2 = 0.06, large η2 = 0.14. The SPSS software package was used for statistical analysis (SPSS for Windows 17.0, SPSS, Inc., Chicago IL, USA). The statistical significance was denoted by a p value ≤0.05.

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Results

Maximal Isometric Force

Significant increases in F max of plantar flexors (p < 0.05, η2 = 0.567) and dorsiflexors (p < 0.05, η2 = 0.415) were observed in the WBV group after training (Table 2). The F max of plantar flexors in the PL group was also significantly improved (p < 0.05, η2 = 0.649) after training. However, there were no significant changes in F max of knee extensors and knee flexors in the both groups after intervention. No significant differences were observed on the F max between groups for knee extensors, knee flexors, ankle plantar flexors, and dorsiflexors during pretraining.

Table 2

Table 2

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Rate of Force Development

The RFD of knee extension in each time interval in both groups did not significantly change after intervention (Figure 1). Figure 1 shows that after 8 weeks of WBV training, the RFDs of 0–200 milliseconds (p < 0.05, η2 = 0.184) and 0–300 milliseconds (p < 0.05, η2 = 0.188) during knee extension were significantly higher than those in the PL group. No significant differences were observed on the RFD of knee extension in each time interval, except 0–100 milliseconds, between groups before training. The RFD of 0–100 milliseconds during knee extension in the WBV group at pretraining was significantly higher than that in the PL group; ANCOVA was used and revealed that there was no significant difference after training between the groups (F = 0.602, p = 0.447).

Figure 1

Figure 1

Figure 2 illustrates that, except 0–100 milliseconds, the RFDs of 0–200 milliseconds (p < 0.05, η2 = 0.463), and 0–300 milliseconds (p < 0.05, η2 = 0.610) during plantar flexion in the WBV group were significantly increased after training. The RFD of 0–300 milliseconds during plantar flexion in the PL group was also significantly increased after training (p < 0.05, η2 = 0.649). However, there were no significant differences on the RFD in each time interval between groups at either pretraining or posttraining. No significant changes were observed on the RFD of 0–100 milliseconds during plantar flexion between pretraining and posttraining in both groups.

Figure 2

Figure 2

No significant differences were observed on the RFD in each time interval between pretraining and posttraining and between groups during knee flexion in WBV (0–100 milliseconds, pretraining vs. posttraining, 406.8 ± 164.1 vs. 451.0 ± 189.5 N·m·s−1; 0–200 milliseconds, pretraining vs. posttraining, 354.3 ± 125.4 vs. 373.8 ± 193.3 N·m·s−1; 0–300 milliseconds, pretraining vs. posttraining, 279.5 ± 92.2 vs. 299.8 ± 126.4 N·m·s−1) and PL (0–100 milliseconds, pretraining vs. posttraining, 349.5 ± 150.0 vs. 378.5 ± 178.8 N·m·s−1; 0–200 milliseconds, pretraining vs. posttraining, 307.8 ± 98.7 vs. 326.1 ± 122.6 N·m·s−1; 0–300 milliseconds, pretraining vs. posttraining, 248.9 ± 65.8 vs. 256.0 ± 78.4 N·m·s−1) groups. No significant differences were observed on the RFD in each time interval between pretraining and posttraining and between groups during dorsi flexion in WBV (0–100 milliseconds, pretraining vs. posttraining, 125.4 ± 52.5 vs. 149.8 ± 79.2 N·m·s−1; 0-200 milliseconds, pretraining vs. posttraining, 110.9 ± 39.9 vs. 129.3 ± 40.2 N·m·s−1; 0–300 milliseconds, pretraining vs. posttraining, 89.7 ± 28.0 vs. 103.0 ± 26.5 N·m·s−1) and PL (0–100 milliseconds, pretraining vs. posttraining, 178.3 ± 48.0 vs. 178.8 ± 41.8 N·m·s−1; 0–200 milliseconds, pretraining vs. posttraining, 136.5 ± 30.0 vs. 143.9 ± 24.2 N·m·s−1; 0–300 milliseconds, pretraining vs. posttraining, 101.7 ± 19.9 vs. 108.7 ± 17.4 N·m·s−1) groups. However, the RFD of 0–300 milliseconds for dorsi flexion was slightly increased (p = 0.053) after WBV training.

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Running Economy

The V[Combining Dot Above]O2 at each running velocity was significantly decreased after WBV training; however, no significant changes were found in PL group after intervention (Table 3). The V[Combining Dot Above]O2 at 2.68 m·s−1 in WBV group was significantly lower than that in PL group (p < 0.05, η2 = 0.221). Respiratory exchange ratio over all 3 running velocities was significantly increased after WBV training (pretraining vs. posttraining, 0.91 ± 0.06 vs. 0.97 ± 0.06, p < 0.05, η2 = 0.734); however, changes in PL group were nonsignificant (pretraining vs. posttraining, 0.95 ± 0.06 vs. 0.95 ± 0.03, p > 0.05, η2 = 0.026) (Table 3).

Table 3

Table 3

The caloric unit cost at 2.68 m·s−1 (p < 0.05, η2 = 0.410), 3.13 m·s−1 (p < 0.05, η2 = 0.583), and 3.58 m·s−1 (p < 0.05, η2 = 0.446) were significantly improved after WBV training; however, no significant changes were observed in PL group (Table 3). Furthermore, the caloric unit cost at 2.68 m·s−1 (p < 0.05, η2 = 0.191) and 3.13 m·s−1 (p < 0.05, η2 = 0.204) at posttraining in WBV group was significantly lower than that in the PL group.

Figure 3 demonstrates that the distance unit cost at average velocity (p < 0.05, η2 = 0.654), 2.68 m·s−1 (p < 0.05, η2 = 0.560), 3.13 m·s−1 (p < 0.05, η2 = 0.644), and 3.58 m·s−1 (p < 0.05, η2 = 0.593) were significantly improved after 8 weeks of WBV training. Furthermore, there were no significant differences on the distance unit cost at each running velocity between pretraining and posttraining in the PL group. The distance unit cost at 2.68 m·s−1 at posttraining in WBV group was significantly higher than that in the PL group (p < 0.05, η2 = 0.198). The distance unit cost at 3.58 m·s−1 at posttraining in the WBV group was slightly higher than that in the PL group (p = 0.061), although there was no statistical significance. The RPE across the 3 running speeds was unaltered with training at approximately 11–12. The WBV and PL subjects did not demonstrate different in V[Combining Dot Above]O2max, RER, caloric unit cost, and distance unit cost at each running velocity before the training period.

Figure 3

Figure 3

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Discussion

The major finding of this study is that the 8-week WBV training program improved the economy of running (∼5.0–6.2% in caloric unit cost and ∼7.2−8.5% in distance unit cost) at selected speeds in male collegiate athletes. The maximal isometric force of the lower leg and the RFD of knee extensors and plantar flexors were also increased by the short-term WBV training program. It is assumed that this is the first study to demonstrate that a regimen of WBV training improves running economy.

The results of this study, which are consistent to previous reports (12,13,23), showed that there were no significant improvements on the F max of the upper leg after WBV training. However, the isometric muscular strength of plantar flexion and dorsiflexion were significantly increased to approximately 22 and 14%, respectively. De Ruiter et al. (13) found that 2 weeks of WBV training (30 Hz, ± 4 mm) with the semisquat position could not improve the maximal isometric knee extensor force production in healthy untrained students. Delecluse et al. (12) reported that the WBV training protocol of 5 weeks (35–40 Hz, ±0.85–1.25 mm) had no significant effects to improve the isometric and isokinetic knee-extensor and knee-flexor strength in sprint-trained athletes. Mahieu et al. (23) showed that 6 weeks of WBV training (24–28 Hz, ±1–2 mm) for competitive skiers resulted in a significantly greater gain in plantar flexor isokinetic strength at low speed, compared with equivalent resistance training. However, the isokinetic strength of upper legs after WBV training was not significantly different from those after the equivalent resistance training. Luo et al. (21) suggested that the amplitude and frequency of WBV may be attenuated during its transmission through soft tissues. That is, the muscle groups close to the vibration source may be sufficiently activated by the stimulation of vibration, when comparing with the muscle far away from vibration source. Therefore, for the athletes, this study provided partial evidence that the 8 weeks of WBV training (30 Hz, ±1–2 mm) may be more effective in improving the isometric muscle strength of the lower leg than that of the upper leg.

A previous study (25) showed that the RFD correlated with the countermovement jump in athletes (r = 0.83, p < 0.05) and in untrained men (r = 0.79, p < 0.05). De Ruiter et al. (13) found that 2 weeks of WBV training could not improve the RFD of the knee extensor in untrained students. Our results indicated that the RFD of the plantar flexor was significantly increased, and the RFD of the knee extensor was slightly improved in male collegiate athletes after 8 weeks of WBV training. Colson et al. (9) suggested that higher vibration loading (additional loads, different frequency or peak-to-peak displacement, and WBV protocol) or a longer WBV training program than the 4-week WBV training program (40 Hz, ±2 mm) should be proposed to athletes to attain significant improvement in explosive physical performance, such as countermovement jump and 10-m sprint performance. Annino et al. (2) showed that the 8 weeks of WBV training, which has a higher amplitude (30 Hz, ±5 mm), could effectively enhance the countermovement jump in elite ballerinas. Fagnani et al. (15) also found that the 8 weeks of WBV training (35 Hz, ±4 mm) could improve the isokinetic knee extension strength and countermovement jump in female athletes. As such, it is assumed that the 8 weeks may be the minimal WBV training period and that the higher vibration stimulus is required for the improvement of explosive performance in athletes. However, because of the special consideration of the daily vibration exposures in the International Organization for Standardization standards (19), the higher vibration stimulation probably incurred a health and safety risk. Further studies for clarification of the effects of higher intensity WBV, which was performed under the safe vibration exposures of the ISO 2631-1 standards, for improving the explosive performance in athletes are recommended.

The principal finding of this study is that the RE, included the caloric and distance unit cost, at 3 different running velocities was improved after WBV training in comparison to the isometric semisquat training. Running economy is influenced not only by aerobic and anaerobic power but also by the neuromuscular capacity (26,31). Previous studies (26,32–34) also found that the explosive-strength and plyometric training program could effectively improve the RE in runners because of the adaptation of the neuromuscular system (i.e., the stretch reflex loop). The stimulation of the stretch reflex loop may be the primary reason for improving explosive performance after the WBV training (6,30). Muscle hypertrophy may also contribute to the improvement in explosive performance. Although previous studies (22,24) found that 10 and 13 weeks of WBV training could induce muscle hypertrophy in the thigh of older women and in the abdomen of untrained adults, respectively, Artero et al. (3) indicated that the muscle mass in young adults was not significantly increased by the 8 weeks of additional WBV training. Our results showed that there were no significant differences in the body weight of young athletes between before (75.8 ± 12.4 kg) and after (76.8 ± 12.3 kg) WBV training. The results of our study also showed that the RFD of the knee extensor and plantar flexor was increased by the WBV training, and the F max of the lower leg was also improved after vibration training. Aagaard et al. (1) reported that the increases of contractile RFD after heavy-resistance strength training could be explained by the enhanced neural drive. Although no electromyographic measurements in the muscles were conducted in this study, the improvement of RE after WBV training might be because of the adaptation of the neuromuscular system. Nevertheless, the changes in RFD and F max between before and after WBV training were not significantly correlated with the changes in distance (r = 0.01–0.45, p > 0.05) or caloric unit cost (r = 0.03–0.55, p > 0.05) in this study. Further studies are required to examine the possible neuromuscular mechanism induced by the WBV training to the improvements of the RE.

A limitation of this study is that the subjects were not highly trained distance runners. Although the subjects had running experience and training in their routine training program, the enhanced extent of RE found in this study may be attenuated in highly trained runners. Our training regimen could be completed in 14–19 minutes and could easily be included in warm-up or cool-down activities associated with the training sessions. Running economy is vital to running performance (17). The improved RE might also be valuable in improving the quality and quantity of training sessions, such as the long slow distance training. Therefore, the short-term WBV training program is recommended for competitive athletes to improve explosive performance and economy of running. Further research is required to establish whether such a regimen of WBV training could be successfully applied to highly trained runners.

In conclusion, this study demonstrated that 8 weeks of WBV training led to improvements in F max of the lower leg, the contractile RFD of knee extensors and plantar flexors, and RE in male collegiate athletes. The mechanism underlying this change might be the stimulation of muscle spindle; however, further research focusing on the relationships between the adaptation of neuromuscular system and improvement of RE should be conducted. This study provides support for including the least time consuming and low impacts training tool in the training program of competitive athletes to improve the RE and neuromuscular characteristics. Further studies are also required to confirm the effects of WBV on RE in highly trained runners and to determine whether the higher intensity of WBV has a further impact on RE and explosive performance.

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Practical Applications

Given the importance of RE to running performance (17), there are still few documented training methods that might improve the economy of running. Whole-body vibration training has been used as an exercise modality to enhance explosive performance for untrained and trained subjects. In this study, a short-term WBV training program not only significantly increased the muscular strength and power, but it also effectively improved the economy of running in collegiate athletes. Therefore, a training protocol included 30-Hz vibration frequency, and ±1–2 mm vibration amplitude could be used for inducing neuromuscular adaptation. Whole-body vibration stimuli could be provided in the form of 10 sets of 30–60 seconds. Recovery time between sets was suggested to be not <60 seconds. Finally, it can be suggested to coaches and players that an 8-week WBV training program can be easily added to warm-up activities during the off-season period without interfering with regular training. The overall effects of WBV could enhance muscular strength and power of lower extremities and improve the running economy by inducing the neuromuscular adaptation.

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Acknowledgments

This study was supported by grants from National Taiwan Normal University, Taiwan. The authors would like to thank the subjects who gave their time and effort to undertake the study. The authors would also like to thank Polypact International Co., Ltd., who sponsored the Cortex metabolic analysis system. Our gratitude goes to the Academic Paper Editing Clinic, National Taiwan Normal University. The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.

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Keywords:

muscular strength; muscular power; oxygen consumption

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