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Medicine & Science in Sports & Exercise:
doi: 10.1249/MSS.0b013e318202f3d3
Applied Sciences

Effects of Whole-Body Vibration Training on Different Devices on Bone Mineral Density

VON STENGEL, SIMON; KEMMLER, WOLFGANG; BEBENEK, MICHAEL; ENGELKE, KLAUS; KALENDER, WILLI A.

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Author Information

Institute of Medical Physics University of Erlangen-Nueremberg, GERMANY

Address for correspondence: Simon von Stengel, Ph.D., Institute of Medical Physics, University of Erlangen, Henkestr. 91, 91052 Erlangen, Germany; E-mail: simon.von.stengel@imp.uni-erlangen.de.

Submitted for publication June 2010.

Accepted for publication October 2010.

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Abstract

Purpose: Whole-body vibration (WBV) is a new nonpharmacological approach to counteract osteoporosis. However, the specific vibration protocol to most effectively reduce osteoporotic risk has not been reported. In the ELVIS II (Erlangen Longitudinal Vibration Study II) trial, we determined the effect of different WBV devices on bone mineral density (BMD) and neuromuscular performance.

Methods: A total of 108 postmenopausal women (65.8 ± 3.5 yr) were randomly allocated to 1) rotational vibration training (RVT), i.e., 12.5 Hz, 12 mm, three sessions per week, for 15 min, including dynamic squat exercises; 2) vertical vibration training (VVT), i.e., 35 Hz, 1.7 mm, as above; and 3) a wellness control group (CG), i.e., two blocks of 10 low-intensity gymnastics sessions. BMD was measured at the hip and lumbar spine at baseline and after 12 months of training using dual-energy x-ray absorptiomety. Maximum isometric leg extension strength and leg power were determined using force plates.

Results: A BMD gain at the lumbar spine was observed in both vibration VT groups (RVT = +0.7% ± 2.2%, VVT = +0.5% ± 2.0%), which was significant compared with the CG value (−0.4% ± 2.0%) for RVT (P = 0.04) and borderline nonsignificant for VVT (P = 0.08). In the neck region, no significant treatment effect occurred. Neck BMD values tended to increase in both VT groups (RVT = +0.3% ± 2.7%, VVT = +1.1% ± 3.4%) and remained stable in CG (−0.0% ± 2.1%).

Both VT groups gained maximum leg strength (RVT = +27% ± 22%, VVT = +24% ± 34%) compared with CG (+6% ± 20%, P = 0.000), whereas power measurements did not reach the level of significance (P = 0.1).

Conclusions: WBV training is effective for reducing the risk for osteoporosis by increasing lumbar BMD and leg strength.

In industrialized nations, a sedentary lifestyle combined with increasing life expectancy will increase the prevalence of osteoporosis in the next decades (6). Whole-body vibration (WBV) training is a new promising approach for counteracting osteoporotic risk factors in the elderly population. Several, but not all, human studies with the elderly demonstrated that WBV positively affects bone mineral density (BMD) (7,22,24,35), falls (37), or fall-related parameters such as maximum strength (14,15,20,27,28,35), muscular power (14,18,26,27), gait quality (3,5), and balance (3,5,15,35). There are notable methodological differences between the studies with respect to the type of vibration device, vibration protocol, training protocol, and primary end points of the studies. Thus, "vibration studies" are difficult to compare, and the optimum settings for inducing effects on the skeletal or neuromuscular system are still unknown. Besides frequency and amplitude (which, together, determine acceleration), the construction determines the mechanical loading characteristics of vibration plates. Principally, two different devices are marketed: 1) Plates that rotate around a fulcrum and produce alternating forces to the left or right foot. Here, the peak-to-peak displacement (1-12 mm) depends on the distance of the feet from the axis of rotation. In clinical use, the frequency varies between 10 and 30 Hz. 2) Devices in which the whole plate vibrates vertically with a certain amplitude producing forces on both feet simultaneously. These devices usually are operated at frequencies in the range of 20-40 Hz. Two categories of vertically vibrating plates can be distinguished: 2a) plates that produce peak-to-peak displacements of 1.5-4 mm, resulting in accelerations that are several times greater than the acceleration of gravity; and 2b) low-intensity plates using extremely low-magnitude mechanical signals (∼0.5 mm; <1 g).

In the ELVIS II (Erlangen Longitudinal Vibration Study II) trial, a randomized controlled 12-month interventional study, we determined the effect of WBV training using two different high-intensity WBV devices (rotational vs vertical) and different vibration protocols on fracture risk parameters. The vibration plate-specific settings, which differed in frequency and amplitude, were selected to result in the same acceleration. The purposes of this study were to determine the effect of WBV on BMD and fall-related neuromuscular performance and to identify effective vibration protocols by directly comparing two different devices. This work helps to develop recommendations and guidelines for clinical WBV use.

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MATERIALS AND METHODS

The ELVIS II is a semiblind randomized controlled intervention trial with 108 women. The study was approved by the ethics committee of the University of Erlangen (Ethik Antrag 3693) and by the Federal Radiation Protection Agency (22462/2-2007-064). All study participants gave written informed consent before the study began. The trial is fully registered, and additional design details beside those presented below can be obtained from www.clinicaltrials.gov.

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Study Subjects

Mailings lists of postmenopausal women aged 60 to 75 yr and living independently in the community of Erlangen (Germany) were obtained from the database of the Siemens Health Insurance Company (Siemens Betriebskrankenkasse, Erlangen, Germany). Women were informed by mail about the study design, its aim, and the exclusion criteria in January and February 2008. Exclusion criteria were 1) diseases or medication affecting bone metabolism, 2) diseases or medication affecting neuromuscular performance, 3) implants of the lower extremity or spine, 4) eye diseases affecting the retina, and 5) low physical capacity (<50 W). Women who responded (n = 323) were personally invited if no exclusion criteria were present and informed orally about the study. A total of 108 postmenopausal women (mean age = 65.8 ± 3.5 yr) were enrolled in the study and randomized via a computer-generated age-stratified randomization list to 1) WBV training on rotationally vibrating devices (RVT), 2) WBV training on vertically vibrating devices (VVT), or 3) a wellness control group (CG; Fig. 1).

FIGURE 1-Flow of par...
FIGURE 1-Flow of par...
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We tried to blind the study at the participant level by the implementation of "sham exercise" for the control group. Participants were not informed about the study hypothesis. Training and control groups were trained separately to prevent contact between the cohorts. In addition, measurements were performed by blinded research assistants.

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Intervention

The intervention started in May 2008 and ended in April 2009. The training was carried out in the Institute of Medical Physics (University Erlangen, Germany). On the basis of a nutrition assessment by questionnaire, each participant received supplemental calcium (up to 1200 mg) and vitamin D (up to 800 IU). Apart from the intervention, subjects were requested to maintain their habitual lifestyle.

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Exercise Program

Both VT groups were requested to perform three weekly WBV training sessions of 15 min each, consisting of exercises carried out in a standing position. For guidance, a DVD player mounted above each plate played a video of the training program. In the first three sessions, certified instructors introduced the participants to the operation of the devices and how to accurately carry out the exercises. Afterward, women performed the video-animated WBV training on their own. Every 6 wk of the 12-month training period, an instructor controlled if the exercises were still being executed properly.

The 15-min training program consisted of seven one- or two-legged dynamic leg strengthening exercises, performed on the plates in standing position, each lasting about 90 s, resulting in 10 min of vibration exposure on the plates: 1) two-legged squat with static phases of 4 s at three positions, 2) two-legged dynamic squats including heel rises, 3) leg abduction, 4) one-legged squats, 5) one-legged squat including hip flexion of the contralateral side, 6) repetition of exercise 1, and 7) repetition of exercise 2. The strengthening intervals were intermitted by an active break of 40 s containing relaxation or stretching exercise standing beside the plate with one leg positioned on the vibration plate. The VVT group (plate Vibrafit, Solms, Germany) vibrated at a frequency of 35 Hz (1.7 mm peak-to-peak displacement) and the RVT group (plate Qionic, Burtenbach, Germany) at 12.5 Hz in the most distant position from the axis of rotation (12 mm peak to peak). Both protocols resulted in similar accelerations that were arithmetically about eight times the earth's gravitational force (8g). For reducing transmission of vibration through the body, participants were advised always to stand with bent joints with the body focus moved to the forefoot. The women wore flat-soled shoes during WBV. Compliance was monitored by training logs in which the participants had to sign each conducted session.

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Wellness CG

The CG performed two blocks of 10 weekly sessions containing light physical exercises and relaxation exercises. The low-intensity program focused on well-being and was designed not to affect the primary study end points (for details, see Kemmler et al. [11]).

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Measurements
Bone densitometry.

The primary end point bone density was determined at the lumbar spine (L1-L4) and the femoral neck at baseline and after 1 yr by dual-energy x-ray absorptiomety (DXA, QDR 4500A, discovery upgrade; Hologic, Bedford, MA) according to the manufacturer's protocol. Coefficients of variation for patient measurements for this device are 1.3% for the lumbar spine and 1.4% for the femoral neck region (31).

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Anthropometric data.

Body height was measured on a stadiometer. Weight was assessed using a digital scale (TBF-305; Tanita, Tokyo, Japan). Percent body fat was determined by a total-body DXA scan.

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Neuromuscular Performance
Isometric maximum strength.

Maximum isometric leg extension strength was measured on a self-constructed leg press with an integrated force-measuring plate (mtd-Systems, Neuburg v. Wald, Germany). The knee angle was set at 100°. Adjustment of the leg press was recorded and replicated during retest for optimum reproducibility. Measurements were performed following Tusker's protocol (33). Participants were instructed to push with maximum effort for 5 s. Instructions were standardized without further motivation. Two maximum voluntary efforts were conducted. A rest period of 1 min was guaranteed between the two trials. The better attempt was taken for analysis. In case of divergence of >10%, a third trial was performed to identify the outlier that was neglected. The CV of the strength measurements was <5%.

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Power.

Leg power was determined by a counter movement jump (CMJ) on a force-measuring plate (see above). Force-time curves were recorded. Two maximum jumps were performed again following Tusker's procedure. The CV for the jump measurements was <7%.

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Questionnaire.

A questionnaire was used at baseline to assess osteoporotic risk factors, diseases, and medications. Possible confounding factors affecting the main end point BMD (medication changes, diseases, major lifestyle changes) and vibration-related adverse effects were asked for in interviews after 6 and 12 months. Pain at lumbar spine and big joints was determined by the questionnaire at baseline and during follow-up (for details, see Kemmler et al. [10]). The scale ranged from 0 to 7. Pain intensity zero reflects no pain at all, and seven reflects the worst pain imaginable.

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Statistical Analysis

The sample size calculation was based on BMD of the spine as primary end point. We anticipated a balanced study with BMD differences of 1.5% between treatment and control (SD = 2.1%) after 1 yr. A sample size of 32 participants in each group resulted in a power of 0.80 with a P of 0.05. Assuming a dropout rate of 10%-20%, our goal was to recruit 36 subjects per group.

Unless stated otherwise, all values are reported as means and SD. The Kolmogorov-Smirnov test was used to check for normal distribution. Homogeneity of variance was determined using the Levine F-test. All variables were normally distributed. For a comparison of the baseline characteristics of the three groups, a one-way ANOVA was used. The intervention effects on bone parameters were also tested using a one-way ANOVA to compare the delta values (baseline − follow-up) of the groups. Post hoc Bonferroni was calculated for parameters with significant group difference. Percentage changes are given in the text for clarity. Effect sizes (ES) based on the absolute difference (±SD) between baseline and follow-up were calculated using the Cohen d (small, d = 0.2; medium, d = 0.5, large, d = 0.8).

Data were analyzed following an intention-to-treat (ITT) approach, i.e., including all subjects as randomized irrespective of compliance. We did not impute missing values. All tests were two-tailed, and a 5% probability level was considered significant. We used SPSS 17.0 (SPSS, Inc., Chicago, IL) for statistical analysis.

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RESULTS

Figure 1 shows the flow of participants through the trial. A total of 108 women were included at study start. Three subjects of the RVT group, two of the VVT group, and two of the CG group withdrew from the study for personal reasons; four women withdrew from the RVT group because of health-related reasons that were not associated with the vibration training. Following the ITT procedure, all participants were invited for follow-up measurements. Three women of RVT were not able (gastric carcinoma, intestinal operation, lumbar disc herniation), and one was not willing (knee operation) to conduct the follow-up measurements. Furthermore, three women of the RVT, two of the VVT, and one of the CG group were unwilling to conduct the final measurements. Follow-up measurements were performed on 34 subjects of the VVT, 29 of the RVT group, and 35 of the CG. Two women of the CG group started taking bone medication (bisphosphonates) after the study started because of the low BMD values detected at baseline and were excluded from analysis, thus resulting in 33 subjects in the CG group who were analyzed.

There was no significant difference in the training attendance of both VT groups (VVT = 2.2 ± 0.7 (73%); RVT = 2.0 ± 0.9 (68%) sessions per person per week). Training attendance of the CG group was 14.2 ± 5.0 (71%) of 20 sessions. No adverse effects related to the vibration stimulus were stated in the vibration group.

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Baseline Characteristics

At baseline anthropometric parameters, maximum strength and osteoporotic risk factors were not significantly different between the groups (Table 1). Likewise, no differences could be detected for osteodensitometric baseline parameters (Table 2).

Table 1
Table 1
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Table 2
Table 2
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Treatment Effects
Anthropometric parameters.

No significant differences between groups were detected for changes of weight (VVT = −0.83 ± 2.36 kg, RVT = 0.38 ± 1.69 kg, CG = 0.18 ± 2.49 kg) or body fat (VVT = −0.45% ± 1.62%, RVT = 0.11% ± 1.26%, CG = 0.06% ± 1.74%). However, weight loss was more pronounced in the VVT group than in the other groups (four women lost >5 kg of body weight), leading to significant intragroup changes according to a paired t-test.

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Bone densitometry.

Both VT groups gained lumbar spine BMD (VVT = 0.5% ± 2.0%, RVT = 0.7% ± 2.2%) versus a loss in the CG (−0.4% ± 2.0%). According to the ANOVA, there was a significant difference between the groups (P = 0.026). Post hoc Bonferroni testing showed a significant difference between RVT and CG groups (difference = 1.1%, confidence interval = 0.44%-2.57%, P = 0.04) and a borderline nonsignificant difference between VVT and CG groups (0.9%, confidence interval = −0.08% to 2.34%, P = 0.08). Table 2 lists the DXA values of the lumbar spine and femoral neck at baseline and follow-up and intragroup changes. Table 3 gives the multiple between-group differences as calculated from the post hoc Bonferroni test.

Table 3
Table 3
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Because of the weight loss of 0.8 kg in the VVT group, we performed a two-way ANOVA with weight as covariable as a post hoc procedure to eliminate weight changes as confounding factor, which resulted in significant differences of lumbar BMD in VVT compared with CG (P = 0.027).

In the femoral neck region, in both VT groups, a gain was observed (VVT = 1.1% ± 3.4%, RVT = 0.3% ± 2.7%), whereas BMD remained stable in the CG (−0.0% ± 2.1%). In this region, the between-group difference was not significant (P = 0.173). However, unlike the LS region, in the femoral neck region, the relative BMD gain (compared with CG) in VVT group (1.1%, −0.41% to 2.92%, P = 0.205) was more pronounced than in the RVT group (0.3%, −1.15% to 2.31%, P = 1.0).

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Maximum Strength

Table 4 lists the maximum leg strength and power (jumping height) at baseline and follow-up and intragroup changes with P value for between-group differences. Maximum isometric leg strength significantly (P = 0.000) increased in both VT groups (VVT = 24.4% ± 33.6%, RVT = 26.6% ± 21.5%) compared with CG (6.2% ± 19.7%). According to a post hoc analysis, the difference between VVT and CG and also the difference between RVT and CG group were significant. Table 5 gives the multiple comparisons as calculated from the post hoc Bonferroni test.

Table 4
Table 4
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Table 5
Table 5
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Both VT groups gained jumping height (VVT = 2.8% ± 14.8%, RVT = 3.3% ± 13.3%), whereas jumping height decreased in CG (−2.7% ± 12.1%). However, between-group differences were not significant (P = 0.101; Table 4).

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Questionnaire

Apart from the cases of bone medication described above, no other relevant changes in confounding factors (disease, medication, lifestyle) were detected from the questionnaire after 6 and 12 months. A reduction of pain was observed in both training groups, which was significant (P = 0.006) for the big joints region in both training groups compared with the CG group (RVT (baseline/follow-up) = 3.1/1.9, VVT = 1.7/0.7, CG = 1.6/2.1) and borderline nonsignificant (P = 0.087) for low back pain (RVT = 3.1/2.0, VVT = 1.6/0.8, CG = 1.9/1.8). In subjects who experienced low back pain or pain of the big joints at study start, the reduction of pain intensity was distinct (about 50% in both VT groups). However, significance (P = 0.007) was reached in the big joint region only, whereas lumbar spine region was borderline nonsignificant (P = 0.052) because of the smaller sample size.

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DISCUSSION

In this study, we directly compared the effect of the two different vibration protocols on osteoporotic risk factors. A significant increase in lumbar BMD was observed in the group who trained on the rotational vibration devices, whereas the effect in the vertically vibrating group was borderline nonsignificant. No significant changes were found for hip BMD in either group. Further, WBV increased maximum leg strength by about 25% in both VT groups.

Owing to the sensitivity of bone to its mechanical environment, exercise is recognized as an effective nonpharmacological approach for influencing bone mass and morphology. In experimental studies using different loading models, a dose-response relationship for several mechanical parameters was determined. In these studies, strain magnitude (25,32), rate (16), and frequency (23) had an osteogenic effect. There is evidence of an interdependence between these mechanical loading parameters. Results of animal studies demonstrated that, for example, extremely low-intensity strains become osteogenic when applied at high frequencies and that WBV is highly potent for positively affecting bone, even when applied at extremely low intensities (23).

However, the results of the few existing clinical trials in humans are rather heterogeneous, and effects are less pronounced than in animal studies. Large methodological differences with respect to vibration and training protocols might contribute to these discrepancies. This especially refers to the kind of vibratory loading (rotational vs vertical), the amplitude, the frequency, and the resulting acceleration. Because results of clinical studies are sparse and barely comparable among themselves, only little knowledge can be obtained on the optimization of vibration protocols for musculoskeletal adaptation. The most effective vibration devices and protocols for clinical use to counteract osteoporosis have still to be identified.

Following a pragmatic approach, we compared two marketed vibration devices with different loading types (vertical vs rotational). Furthermore, the applied vibration distinctly varied in amplitude and frequency but produced the same acceleration.

With respect to bone, there was a significant difference between RVT and CG groups for lumbar BMD only. However, the effects in the VVT group were just borderline nonsignificant (P = 0.08), and a post hoc analysis with weight changes as covariable resulted in significant results in VVT. Thus, with respect to lumbar BMD, no evidence-based conclusion can be drawn about the superiority of one plate or protocol, respectively.

Unlike with lumbar BMD, in the femoral neck, there was a nonsignificant trend in favor of the VVT group. The relatively lower effect in the hip region compared with the lumbar spine region that was observed especially for the RVT group could be explained by the mechanical characteristic, which results from the seesaw plate construction. On rotational devices, the loading frequency for the pelvis region upward is twice as high as for each single leg, which could result in stronger bone stimulation in the spine compared with that in the hip region. This is also supported by an animal study of Judex et al. (9), which showed that the loading frequency was more critical than the strain magnitude or rates. In this study, 90-Hz signals were more effective at inducing bone adaptation than 45-Hz signals despite the significantly lower strain rate and magnitude at 90 Hz compared with 45 Hz. However, because of comparable higher frequencies in this animal study, insignificant results in the present study, and the general question about the transferability of results of animal studies on humans, these explanations remain rather speculative.

Two other studies with postmenopausal women examined the effect of WBV training on rotational devices on BMD. In one study, Gusi et al. (7) reported a significant gain at the femoral neck (+4.3%; DXA) compared with a walking control group using a vibration protocol with the same frequency as used in the present study (12.5 Hz, 8 months, three times per week, 6 min, 3 mm). However, unlike our study, BMD of the lumbar spine was unaltered, and effects were seen in the hip region only. In a study by Russo et al. (27) using the same devices at a higher frequency (28 Hz, 6 months, two times per week, 6 min), no effects on bone parameters at the tibia were detected using peripheral quantitative computed tomography (pQCT).

Three WBV studies carried out training of postmenopausal women on high-intensity vertical devices. Verschueren et al. (35) reported a significant increase in DXA-BMD at the hip of +1.5% (but not at the lumbar spine) after 6 months of WBV training (three times per week, 20 min, 35-40 Hz, 2 mm) compared with a control group and a group performing ordinary strength training. In a recent study, Verschueren et al. (34) found no additive effect of adjuvant WBV training (three times per week, 15 min, 30-40 Hz, 1.6-2.2 mm) in two groups receiving a different dose of vitamin D supplementation after 6 months. In another study using vertically oscillating plates (30 Hz, 5 mm), a significant difference for DXA BMD at the lumbar spine (+6.2%) and hip (+4.9%) was observed in favor of the training group compared with a sedentary control group after 6 months of WBV training (five times per week, 10 min) (22). In the recent ELVIS I study, we investigated whether the effect of an intense conventional training program (two 1-h sessions per week) was enhanced by superimposed vibrations (15 min, 25-35 Hz, 1.7 mm). The conventional program showed an effect on lumbar BMD (but not at the hip region), but the vibration stimuli did not enhance these effects (37).

There is one study with postmenopausal women that used low-magnitude, low-intensity vertical devices (12 months, daily, 20 min, 30 Hz, 5.5 μm) (24). This study failed to show vibration-induced effects at the hip and spine (DXA) using an ITT analysis. Post hoc tests demonstrated that light women (<65 kg), who were in the highest compliance quartile, exhibited a statistically significant benefit on BMD at the hip region of the vibratory treatment.

In summary, about half of the studies with postmenopausal women showed significant results, suggesting an effect of WBV on bone in humans. Different types of vibration devices and different vibration protocols resulted in changes of BMD. In contrast to other studies where BMD changes were determined at the femur only, in the present study we observed effects in the lumbar region only. In line with the other study results, in the present study no vibration device or protocol showed superiority.

The largest vibration effects reported so far were obtained for a vertical plate by Ruan et al. (22). The vibration protocol arithmetically produced accelerations of about 18g. Participants were advised to stand vertically while the body focus was moved on their heels during vibration exposure, leading to high axial stress along the spine up to the head. Vibration transmission through the body is known to be more pronounced in vertical than in rotational vibration devices (2). Further, the transmission of acceleration depends on the joint flexion angles and is highest in the upright standing position (2), whereas in the semisquat position, acceleration is reduced up to 10 times at the knee and hip (17). In this context, the safety aspect of vibration training has to be discussed. Although so far no study reported vibration-related adverse effects, the knowledge about the effect of intense vibration on aged cartilage tissue, for example, is scarce. Although study protocols varied widely with respect to vibration type, amplitude, acceleration, duration, standing position (i.e., knee angle, heels vs forefoot), and subjects, nothing is known about a potential threshold beyond which WBV may overload bone or other tissues causing adverse effects (2,12). In accordance with other studies, no vibration-related adverse effects were determined in the present study. On the contrary, the pain score in the lumbar spine and big joints improved in both training groups compared with the CG group. A positive effect of WBV on low back pain was reported by Ruan et al. (22). In a study by Rittweger et al. (19), WBV was as effective in reducing LBP as back strengthening exercise. In another study by Iwamoto et al. (8), bisphosphonate therapy plus WBV was more effective in reducing LBP in osteoporotic postmenopausal women than medication alone. In the light of these promising results, the pain-reducing effect of WBV should be one important topic of future studies.

Because of the association with fall frequency, low maximum leg strength is an important risk factor for osteoporotic fractures. In line with our results, other WBV studies with postmenopausal women (21,28,35), elderly men (4), or mixed geriatric collectives (3,15) reported increases in leg strength, whereas positive results did not depend on plate construction. In our study, gains in maximum leg strength were more pronounced (25% in both VT groups) than in other studies (15% in the average). This could be related to the specific (e.g., one-legged squats) exercises performed on the plates, which might have been more intense than exercises that were performed in most other studies.

One cross-sectional study compared the acute effects of rotational versus vertical vibration (both 30 Hz, 4 mm) on EMG activity. Both vibration types significantly increased neuromuscular activity (EMG) of the leg muscles. However, average responses of the leg extensors were significantly greater in rotational vibration, whereas the responses of the tibial anterior muscle were greater during vertical vibration (1). Two studies of Torvinen et al., which used almost an identical study design but different vibration plates, also allow conclusions about the short-term effect of vertical versus rotational plates. In these studies, a 4-min WBV training including light exercises was conducted on (a) vertically (2 mm, 25-40 Hz) (30) or (b) rotational plates (15-30 Hz, ca. 8 mm) (29). The mean power frequency of the EMG signal was reduced in all measured muscles (soleus, gastrocnemius, vastus lateralis) with time in the study using the rotational device and in vastus lateralis and gluteus medius but not in soleus and gastrocnemius in the study using the vertical device, indicating regional differences of muscle fatigue. Positive short-term effects (2 and 60 min after vibration) on muscle performance and body balance could only be observed using the rotational plate. However, the correlation of short-term effects after a single vibration exposure with long-term adaptations as the result of regular WBV training is arguable; thus, this discussion is not further extended here.

In line with the results of the above-described longitudinal studies, in the present study no vibration type showed superiority in increasing neuromuscular performance. Our program showed effects comparable to specific resistance training regimen (13). Self-rated exercise intensity as assessed by the Borg scale was only 10 on average (very light to fairly light) in both VT groups compared with 11 (fairly light) in the CG group. Thus, WBV might be an alternative especially for people unwilling or unable to perform strenuous conventional exercise.

Our study possesses several strengths. Semiblinding was applied to participants. Participants were not informed about the hypothesis and were unaware of whether they belonged to the real or to the control group, which carried out "sham exercise." Furthermore, measurements were performed by a blinded observer. The compliance was relatively high (70%), and the dropout rate was low (10% in total).

Our study has also several limitations. Like the majority of other comparable studies, exercises were conducted on vibration plates in the present study, whereas the CG group did not perform identical exercises. Thus, the study design does not allow a separation of the effects induced by the vibration stimulus and the conventional exercises performed on the plates. There are only two vibration studies including elderly subjects focusing on neuromuscular capacity (3,36) and two studies focusing on BMD (24,37), which performed identical treatments in the training and control groups. Concerning strength, power, or BMD development, there was a trend in favor of the VT groups in all four studies, but between-group differences were insignificant. In the study of Ruan et al. (22), upright standing on the plates without performing exercises resulted in significant BMD gains compared to an inactive CG. In line with the results of animal studies, this indicates an effect of the vibration stimulus itself on bone. With respect to the muscle, the reflectory enhanced response during WBV, which was demonstrated in numerous studies (e.g., Abercromby et al. [1] and Torvinen et al. [29]) might increase the effect of the less strenuous exercises performed on the plates and be a key to the observed long-term functional and structural neuromuscular adaptations after WBV. However, the role of the reflexive muscle contraction provoked by WBV, and its potential to induce strength and power gains is still not totally understood.

Because both vibration types demonstrated to be effective on BMD and neuromuscular performance in previous studies, we did not expect differences between both vibration protocols. After this assumption, in the power analysis we assumed a BMD difference of 1.5% between treatment and control. Thus, the study was underpowered to detect possible small differences between the two vibration modalities.

In summary, WBV was effective for reducing fracture risk factors by increasing BMD at the lumbar spine and maximum muscular strength, but there were no significant differences between the two vibration types. The high compliance showed that the video-animated program was attractive and feasible. Given the feasibility, low demands on manpower, and high time flexibility, a video-based WBV training might have a high potential for large-scale implementation in different institutions. The differential effect of different devices or vibration protocols should be determined in further studies to identify critical variables and most effective programs in humans.

Trial registry name: Effects of whole-body vibration on bone and fall related parameters: ID: NCT00292916; www.clinicaltrials.gov.

This work was supported by the Elsbeth Bonhoff Foundation, Germany. None of the authors has a conflict of interest.

The authors thank the help of Opfermann (Wiehl, Germany) who supplied Ca and vitamin D (CALCIGEN). The authors also thank mtd-Systems (Neuburg v. Wald, Germany), which supplied the force plates.

The results of the present study do not constitute endorsement by the American College of Sports Medicine.

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Keywords:

WBV; EXERCISE; POSTMENOPAUSAL WOMEN; BMD; STRENGTH; POWER

©2011The American College of Sports Medicine

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