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).
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.
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.  and Torvinen et al. ) 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:©2011The American College of Sports Medicine
WBV; EXERCISE; POSTMENOPAUSAL WOMEN; BMD; STRENGTH; POWER