Whole-body vibration (WBV) training is a relatively new approach for enhancing muscle strength, power, and physical performance in people of any age or with various physical disorders.1–3 In particular, WBV training has great potential in geriatric health care as a safe, low-impact training method with a low starting threshold for those who are not attracted to or are unable to perform traditional exercise; this activity can be performed without a heavy load or dynamic joint movement.4 Whole-body vibration training produces both a long-term effect with regular WBV training over several weeks or months4–8 and an short-term effect after one or several WBV bouts that last for several minutes.9–17 Research on long-term WBV training has been shown to improve muscle strength, power, and physical performance in older adults;4–8 a systematic review has also been reported.1 On the contrary, almost all participants in previous research on the short-term effects of WBV training were trained or untrained, healthy, young adults9–16 rather than older adults.18
The typical short-term effects of WBV training in younger adults are improved muscle function9–13,15,16 and flexibility,13,14 both of which play an important role as a warm-up to more strenuous exercise. If short-term WBV training had similar effects on older adults, it might help them perform activities of daily living by improving their mobility or help them perform some exercises more smoothly and steadily, with a decreased risk of injury.
Whole-body training devices are divided into 2 types on the basis of their vibration modality: rotational vibration or vertical vibration (VV) devices.1 With a typical rotational vibration device (Galileo Platform; Novotec GmbH, Pforzheim, Germany), the platform rotates about an anterior-posterior axis; it is similar to standing near the middle of a teeter-totter. With a typical VV device (Power Plate; Power Plate International Ltd, London, United Kingdom), on the contrary, a monolithic platform oscillates in all 3 planes (mainly up and down, but also left to right and front to back), producing the same stimulus at any point on the platform. Whole-body training by using this triple-plane oscillating device is described as acceleration training (AT).19 Although there are not enough data to support the effectiveness of one type of vibration unit over another, the asynchronous nature of the rotational vibration may make it difficult for older adults to perform some exercises, while training with the uniform stimulus produced by the VV platform may be easier and safer.1
This study examined whether short-term WBV training by using the triple-plane VV device (ie, AT) improves functional mobility and flexibility in community-dwelling older adults.
A power analysis, using G*Power version 3.1.3 (Franz Faul, Universitat Kiel, Germany), revealed that a sample size of 15 participants was needed to detect significant time × intervention interactions (α = 0.05; 1 − β = 0.80; medium effect size20: f = 0.25). For the present study, we recruited 18 older adults (9 men and 9 women; age range, 65-73 years; mean age, 69.1 years; standard deviation [SD], 2.0 years), taking into account the inevitable dropout. The body weights of men ranged from 52 to 75 kg with a mean of 65.6 (7.5) kg and 47 to 65 kg with a mean of 54.7 (6.5) kg for women. Heights of men ranged from 158 to 172 cm with a mean of 163.7 (4.5) cm and 146 to 163 cm with a mean of 155.0 (4.8) cm for women. The participants were already involved in low-intensity aerobic activities at least 1 day per week. The exclusion criteria included any cardiovascular, respiratory, neurological, musculoskeletal, or other chronic diseases and any prostheses or medications that could affect the musculoskeletal system. This study was approved by the ethics committee of the University of Tsukuba, and all participants gave written informed consent.
Figure 1 shows the protocol of this study with a crossover design. Research was conducted on 3 different days (days 0, 1, and 2) in a training room at our university. On the first day (day 0), we instructed the participants on how to perform the physical function tests and the AT intervention. During this preliminary part of the experiment, we encouraged them to practice on the VV device so that they would be capable of performing the stretching positions and to diminish the learning curve in performing the tests during the real intervention experiments (days 1 and 2). Nine days after the preliminary part of the experiment, we conducted the first actual experiment (day 1). Participants performed all 4 physical function tests (test 1-1) to be sure they were completely familiar with the procedures. After resting 5 minutes in a chair, participants performed the same 4 tests again as a pretest (test 1-2). We randomly placed 9 participants (5 men and 4 women) into the AT intervention group; the other 9 participants were in the control (Con) group.
Previous research, even in the younger adults, has not led to a consensus on the optimal frequency and amplitude that elicit maximal increase in muscle function and flexibility. For instance, some studies recommended a relatively high frequency (40-50 Hz),16,21 while another study recommended a relatively low frequency (30 Hz).11 To gain the greatest effects possible in lower extremity function and flexibility while ensuring the safety of these older adults, we used a moderate (40 Hz) frequency and a low (2-4 mm) amplitude on the VV platform (Power Plate pro5; Power Plate International Ltd, London, United Kingdom). The AT intervention included 2 positions for the participants: (1) half-squatting stretch and (2) hamstrings stretch (Figure 2). For the half-squatting position, participants stood with their feet shoulder-width apart and both knees in approximately 120° (180° = full extension) isometric knee flexion. For the hamstrings-stretching position, participants stood with their feet shoulder-width apart, leaned forward while bending at the pelvis, and pushed their hips back until they felt the stretch in their hamstrings while keeping their back and legs straight. They held these positions for 30 seconds, with 3 sets per position alternating from one position to the other and resting on the chair for 30 seconds between sets. The 30-second duration can provide adequate safety for the older adults who engage in AT for the first time.4,5,7 Participants in the Con group performed the same stretches in the same manner as those in the AT intervention group but on the platform without oscillation. After the AT or Con interventions, participants repeated the 4 tests in a random manner immediately (test 1-3) and 30 minutes (test 1-4) later. Seven days after the first actual intervention experiment on day 1, we conducted the second actual intervention experiment (day 2) but switched participants between the AT and Con interventions.
Timed Up and Go Test
The Timed Up and Go (TUG) test was used to determine functional mobility, which is closely linked to lower extremity muscle function. We measured the time a participant took to rise from a chair (40-cm high), walk 3 m, turn around, and sit down again as fast as possible. The original method of measuring TUG test results incorporates a regular walking speed,22 which was not suitable for our research characteristics (ie, assessing short-term effects) because the results could be affected by psychological factors, such as motivation. Therefore, we adopted a modified method to evaluate maximum capacity. Previous research23–25 also evaluated physical function by using the same method as in our present study and found it had a good test-retest reliability and criterion-related validity. The participants performed the test 2 times, and the faster time was recorded.
Ground Reaction Force Parameters in a Sit-to-Stand Movement
We measured ground reaction force (GRF) parameters in a sit-to-stand movement to evaluate participants' lower extremity muscle function more directly.26–28 Participants sat in a chair of standard height (40 cm) with legs shoulder-width apart, trunk stretched vertically in a straight line, and ankles held at a 90° angle on the force plate with data collected on a computer with installed data analysis software (TKK5809; Takei Scientific Instruments Co Ltd, Niigata, Japan). They stood up from the chair as fast as possible, with arms folded, rested for approximately 2 seconds, and then sat down again. They performed 3 trials in succession, with an interval of 2 seconds between 2 trials. The software provided a curve of vertical GRF during the sit-to-stand movement at 100 Hz. On the basis of previous research,26–28 we collected data on 2 GRF parameters: the peak reaction force per body weight (F/w; kgf·kg−1), which reflects the maximal downward force pushing the body upwards; and the maximal rate of force development per body weight (RFD/w; kgf·s−1·kg−1), which is an index of the capacity for rapid muscle force production. Maximal rate of force development per body weight is defined as the steepest gradient of the force-time curve over a 90-millisecond period. We used the highest values of F/w and RFD/w for analysis.
We conducted the sit-and-reach test by using a digital flexibility testing device (TKK5112; Takei Scientific Instruments Co Ltd, Niigata, Japan) to evaluate participants' trunk flexibility. They maintained a long sitting position with legs fully extended in front of them and feet together. The participants then stretched forward slowly as far as possible with hands and arms outstretched. We recorded the best of 2 trials in centimeters.
Functional reach estimates a participant's flexibility and dynamic balance.29,30 Participants stood with arms outstretched in front and reached forward beyond their arm's length as far as possible without taking their heels off the floor. The longer reach of 2 trials was recorded in centimeters.
Descriptive statistics are reported as mean and SD. We calculated intraclass correlation coefficients (1, 1) for each variable by using data from all 18 participants to detect within- and between-run reproducibility and interpreted the results as poor (<0.40), moderate (0.40-0.75), or excellent (>0.75).31 Two-way analysis of variance was used to determine whether differences existed between values (3 levels for the time factor: pretest, immediate posttest, and 30-minute posttest) for the AT and Con interventions (intervention factor, 2 levels), and the post hoc t test with Bonferroni correction was conducted. We calculated Cohen d effect sizes20 between mean scores from 2 interventions: a Cohen d of 0.20, 0.50, and 0.80 indicate small, moderate, and large effects, respectively.20 For each variable, the intervention effect was expressed as the relative change between the pretest and the immediate and 30-minute posttests. Statistical significance was set at P < .05 in all analyses. Data were analyzed by using PASW Statistics 17.0 (SPSS Inc, Chicago, Illinois).
All 18 participants completed all the study protocol without any adverse reaction, side effect, or injury. We pooled the data of men and women in our analyses because no gender difference was shown in response to AT or Con intervention. Table 1 presents the mean, SD, and intraclass correlation coefficient of each variable for tests 1-1, 1-2, 2-1, and 2-2, which were calculated from all 18 participants' data. All parameters showed moderate to excellent within- and between-run reproducibility. However, the mean values of 3 parameters (TUG, functional reach, and f/w) showed significant changes from day 1 (test 1-2) to day 2 (test 2-2).
Table 2 shows the mean and SD at the pretest, immediate posttest, and 30-minute posttest for each measured parameter. Table 3 shows the results of 2-way repeated-measures analysis of variance, and Figure 3 shows the relative change in each variable. Only the TUG test had a significant time × intervention interaction. Comparing the mean values for the pretest and the immediate and 30-minute posttests revealed a significant decrease in the amount of time it took to complete the TUG after the AT intervention but not after the Con intervention. Significant time effects were observed in the sit-and-reach and functional reach tests and with the f/w. Compared with pretest values, the sit-and-reach results increased significantly at the 30-minute posttest, and the functional reach results increased significantly at the immediate posttest. Furthermore, in the graphs in Figure 3, the improvement rates after the AT intervention rise higher than the improvement rates after the Con intervention at the immediate and 30-minute posttests, and Cohen d effect sizes between mean scores from the 2 interventions ranged from 0.22 to 0.53. No statistical significance was observed in RFD/w.
This study had 2 significant findings. First, short-term AT provided more improvement in functional mobility, as evaluated by the TUG test, than the training without vibratory stimulus, and this effect lasted 30 minutes. Second, short-term AT tended to enhance participants' flexibility, as evaluated by the sit-and-reach and functional reach tests.
Our finding of improved functional mobility, which is affected by lower extremity muscle function, is partially supported by previous research, using the triple-plane VV device (Power Plate) with younger participants. Cormie et al10 reported that applying a 2.5-mm (low) amplitude at 30 Hz significantly increased a counter movement jump (CMJ) height immediately after a single 30-second bout of AT. Adams et al12 also found an approximately 1.5% increment increase of CMJ peak power 1 minute after a single 30-second bout of AT (low amplitude at 40 Hz) and anticipated that short-term gains of muscle function might be attributable to neuromuscular facilitation.12 While it should be noted that comparisons are affected by differences in the intervention protocols, the outcomes, and the participants' ages, the results of those reports10,12 and of the present study suggest that short-term AT at low (2-4 mm) amplitude may enhance lower extremity function regardless of age in adults.
In addition, it is notable that the improved TUG performance was maintained for 30 minutes after the short-term AT intervention. This finding conflicts with previous research that examined short-term responses to AT in younger adults. For example, Bedient et al11 and Adams et al12 reported that CMJ performance peaked 1 minute after an AT intervention (30-50 Hz, low amplitude, 30 seconds) and declined to baseline level 10 minutes postintervention. Cormie et al10 also found a significant increase in CMJ performance immediately after a single AT bout (30 Hz, low amplitude, 30 seconds), but this increase disappeared within 5 minutes. Participants of those studies, however, engaged in only a single 30-second bout of AT in a half-squatting position, while participants in this performed six 30-second bouts of AT in half-squatting and hamstrings-stretching positions to substantially stimulate neuromuscular functions. Although our protocol might provide a more persistent effect from short-term AT, these comparisons between younger and older adults do not sufficiently explain the inconsistency in the sustained effect.
Although it is difficult to compare our results pertaining to short-term AT with effects from long-term AT, in a 6-week intervention study7 with older adults residing in a nursing home, the usual-paced TUG test results improved only in the static AT training group and not in the Con group (the same training on the platform without vibration) despite significant increases in leg extension maximal force and power. The TUG test itself is representative of various activities of daily living: rising from a chair, walking, turning, and sitting down. To perform these complex tasks smoothly, participants need sufficient lower extremity muscle function plus postural control and coordination, which are also enhanced by short-term or long-term WBV training.8,9 In the present study, the GRF parameters in a sit-to-stand movement, which more directly evaluate participants' lower extremity muscle function, increased only slightly immediately after the short-term AT intervention, indicating that improvement of the TUG test results were also affected by other factors, not just by enhancement of lower extremity muscle function. Discussion of the rigorous physiological mechanisms for this improvement is a matter for a future debate.
The second major finding of our study was that short-term AT tended to enhance flexibility compared with the training without vibration. While there is limited information about the short-term effects of WBV training on flexibility compared with muscle function, a few reports on younger adults lend some support to our results. Jacobs and Burns13 reported that the increase of sit-and-reach scores after WBV treatment (Galileo device, 0-26 Hz, 6 minutes) (+4.7 cm) was statistically greater than after cycling ergometry treatment (+0.8 cm). Gerodimos et al14 also found a significant increase in sit-and-reach scores after a single 6-minute WBV bout (Galileo device, 15-30 Hz, 4-8 mm amplitude) that persisted for at least 15 minutes. However, those results are difficult to compare with our study's findings because of the differences between the Power Plate and Galileo devices, as previously mentioned. In the earlier-referenced, previous 6-week intervention research7 on the long-term effect of AT, the chair sit-and-reach scores also improved only in the static AT training group and not in the Con group. It was proposed by van den Tillaar32 that increased range of motion after short-term and long-term WBV training results from enhanced local blood flow, inhibiting activation of the antagonist muscle through Ia-inhibitory neurons and the increase of the pain threshold. While these favorable changes might have been shown in participants in the present study, the underlying physiological mechanisms remain to be solved.
This study has several limitations. First, we used the same vibration and parameters (frequency, amplitude, duration, and interval) for each intervention; hence, we cannot discuss the optimal protocol for enhancing functional mobility and flexibility. For instance, Feland et al33 reported that a 60-second hamstrings-stretching program for 6 weeks produced a greater rate of gains in knee-extension range of motion than a 30-second program in older adults. In this study, we did not observe a significant interaction in the measures of flexibility because the protocol may not have been optimal. Second, we could not establish a strict examiner-blinding procedure because of the limited resources with respect to facility space and the number of examiners. Therefore, there was potential for examiner bias. Third, the mean values of several parameters changed significantly from day 1 to day 2. These changes might be due to the learning curve factor despite our attempt to diminish that by conducting tests 0, 1-1, and 2-1. However, we anticipated the possibility of this learning curve factor and established a crossover design to take this into account. Because of this crossover design, both interventions would be equally affected by this learning factor. Fourth, because participants in our study performed the 4 tests in succession immediately after the intervention, measurements for an individual test varied from about 5 seconds to 3 minutes postintervention. Finally, our findings may not be generalized because the participants were healthy volunteers who already engaged in regular exercise at least 1 day a week and they were not a random sample of the general, older population. Furthermore, in this study on Japanese community-dwelling older adults, the mean time to complete the TUG test (<5.0 seconds) was less than that in previous studies (approximately 6.0-8.0 seconds).24,25
Short-term WBV training by using the triple-plane VV device (ie, AT) elicited a significantly larger improvement of functional mobility. The effect on flexibility was similar with and without vibration stimulus, but there was a greater tendency to improve with WBV training in healthy, older adults. In addition, these short-term effects were maintained for about 30 minutes. Engaging in short-term AT as a warm-up may help older adults perform activities of daily living or certain other exercises more smoothly and steadily.
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