Original Research15 November 2011

Effect of 12 Months of Whole-Body Vibration Therapy on Bone Density and Structure in Postmenopausal Women

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A Randomized Trial
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    Abstract

    Background:

    Although data from studies in animals demonstrated beneficial effects of whole-body vibration (WBV) therapy on bone, clinical trials in postmenopausal women showed conflicting results.

    Objective:

    To determine whether WBV improves bone density and structure.

    Design:

    A 12-month, single-center, superiority, randomized, controlled trial with 3 parallel groups. (ClinicalTrials.gov registration number: NCT00420940)

    Setting:

    Toronto General Hospital, Ontario, Canada.

    Participants:

    202 healthy postmenopausal women with bone mineral density (BMD) T-scores between −1.0 and −2.5 who were not receiving prescription bone medications.

    Intervention:

    Participants were randomly assigned to 1 of 3 groups (1:1:1 ratio) by using a block-randomization scheme and sealed envelopes. They were asked to stand on a low-magnitude (0.3g) 90-Hz or 30-Hz WBV platform for 20 minutes daily or to serve as control participants; all participants received calcium and vitamin D.

    Measurements:

    Bone outcome assessors, who were blinded to group assignment, determined trabecular volumetric BMD and other measurements of the distal tibia and distal radius with high-resolution peripheral quantitative computed tomography and areal BMD with dual-energy x-ray absorptiometry at baseline and at 12 months.

    Results:

    12 months of WBV therapy had no significant effect on any bone outcomes compared with no WBV therapy. For the primary outcome of tibial trabecular volumetric BMD, mean change from baseline was 0.4 mg/cm3 (95% CI, −0.4 to 1.2 mg/cm3) in the 90-Hz WBV group, −0.1 mg/cm3 (CI, −1.0 to 0.8 mg/cm3) in the 30-Hz WBV group, and −0.2 mg/cm3 (CI, −1.1 to 0.6 mg/cm3) in the control group (P = 0.55). Changes in areal BMD at the femoral neck, total hip, and lumbar spine were also similar among the groups. Overall, low-magnitude WBV at both 90 and 30 Hz was well-tolerated.

    Limitations:

    Adherence to WBV ranged from 65% to 79%. Double-blinding was not possible.

    Conclusion:

    Whole-body vibration therapy at 0.3g and 90 or 30 Hz for 12 months did not alter BMD or bone structure in postmenopausal women who received calcium and vitamin D supplementation.

    Primary Funding Source:

    Physicians' Services Incorporated Foundation.

    Context
    • Whole-body vibration (WBV), which involves standing on an oscillating platform, has been hypothesized and marketed to prevent bone loss. Data regarding its efficacy are limited and conflicting.

    Contribution
    • In this 12-month trial of postmenopausal women without osteoporosis at baseline, daily WBV at either of 2 frequencies had no measurable effect on bone mineral density.

    Caution
    • Sham WBV could not be provided to the control group.

    Implication
    • Whole-body vibration does not seem beneficial for preventing bone loss in postmenopausal women who receive calcium and vitamin D supplements.

    —The Editors

    Whole-body vibration (WBV) has been introduced in the past decade as a promising new anti-osteoporotic therapy, because significant improvements in bone formation rate, bone mineral density (BMD), trabecular structure, and cortical thickness were found in animal models (1–3). Although commercially available WBV devices are marketed to and used by patients, the beneficial effects of WBV on fracture risk and BMD have not been established, and recent randomized, controlled trials (RCTs) in postmenopausal women (4–6) have shown conflicting results.

    Therapy with WBV involves standing on a motor-driven, oscillating platform that produces vertical accelerations, which are transmitted from the feet to the weight-bearing muscles and bones (7). Transmission of WBV through the body depends on the intensity of vibration, knee-joint angle, distance of the skeletal site from the oscillating plate, and possibly muscle and soft tissue dampening (8, 9). The intensity of WBV can be expressed in terms of vibration frequency (1 Hz = 1 oscillation/s), peak-to-peak displacement (in millimeters), and peak acceleration or magnitude (in units of acceleration due to gravity [g]; 1g = 9.81 m/s2) (7). Many WBV platforms are available worldwide and can be categorized on the basis of magnitude (high [≥1g] or low [<1g]) and type of vertical movement (synchronous or side-alternating) (7, 10).

    Hypothesized mechanisms by which WBV might exert osteogenic effects include changes in the flow of bone fluid caused by direct bone stimulation and transduction of mechanical signals, possibly through osteocytes and Wnt–β-catenin signaling, or indirect bone stimulation through skeletal muscle activation, possibly by means of the stretch reflex (10, 11). Therapy with WBV has also been hypothesized to be beneficial for older adults with diminished mechanical loading of the skeleton due to muscle loss and reduced mobility, because it might mimic the mechanical signals typically generated by postural muscle contractions or such low-intensity activities as walking (1, 11).

    Seven RCTs (5, 6, 12–16) investigated the effects of WBV on bone in postmenopausal women and found discrepant results. Most of these RCTs had sample sizes of less than 100 (6, 12–16) and 6 to 8 months of follow-up (5, 6, 12, 15, 16), and only 2 provided calcium and vitamin D supplements (5, 15). In addition, the discrepant results in the previous trials could have been due to use of ineffective WBV frequencies (between 12 and 40 Hz) or examination of inappropriate bone outcomes (primarily areal BMD measurements at central skeletal sites).

    To clarify whether WBV therapy has beneficial effects on bone in postmenopausal women, we conducted a 12-month RCT of daily low-magnitude (0.3g) WBV at 2 frequencies (90 and 30 Hz) versus no WBV. We sought to examine the effect of WBV on volumetric BMD and bone structure at the distal tibia and radius and areal BMD at the hip and spine in postmenopausal women who received calcium and vitamin D supplementation.

    Methods

    Our trial was a 12-month, single-center, superiority RCT with 3 parallel groups of equal size. Recruitment started in October 2006 and finished when the target sample size was achieved in November 2008. Bone outcomes were collected at baseline and 12 months from November 2006 to December 2009. Bone outcome assessors were blinded to group assignment. The research ethics board of the University Health Network, Toronto, Ontario, Canada, approved our trial.

    Setting and Participants

    Participants were recruited primarily by using posted flyers, word of mouth, and our postmenopausal health newsletter in the Greater Toronto Area in Canada. All study visits were conducted at the Postmenopausal Health Research Clinic at Toronto General Hospital. Women were eligible if they had experienced menopause 1 or more years ago and their lowest BMD T-score at the lumbar spine, femoral neck, or total hip was between −1.0 and −2.5. Our initial BMD T-score range was between −1.0 and −2.0, because clinical guidelines in effect at the onset of our trial recommended initiating prescription bone medication therapy in postmenopausal women at a BMD T-score of −2.0 or lower (17) and prescription bone medications were part of our exclusion criteria. Because initiation of prescription bone medication therapy was subsequently based on absolute fracture risk rather than BMD T-score (18), our eligibility criteria were expanded in February 2007 to include BMD T-scores down to −2.5 (19). Women who were excluded initially but became eligible in February 2007 were invited to participate.

    We excluded women with a BMD T-score greater than −1.0, because previous research (14, 20, 21) has shown that less-dense bones may have a greater response to WBV. Also excluded were women who had osteoporosis (BMD T-score ≤−2.5), fragility fracture after age 40 years, secondary causes of bone loss, other metabolic bone diseases or diseases that affect bone metabolism, active cancer in the past 5 years, a body mass of 90 kg or greater, knee or hip joint replacements, or spinal implants. We also excluded women who received hormone therapy in the past 12 months; bisphosphonates in the past 3 months or ever for 3 or more months; raloxifene or teriparatide in the past 6 months; or long-term glucocorticoid, anticoagulant, or anticonvulsant therapy and those who could not tolerate WBV for 20 minutes at screening, expected changes in their physical activity levels, or expected to travel for more than 4 consecutive weeks during the study.

    Randomization and Interventions

    One investigator used a computer-generated block-randomization scheme with a 1:1:1 allocation ratio and a block size of 12 to prepare sequentially numbered, sealed envelopes. After eligibility criteria were satisfied and baseline data were collected, these envelopes were opened sequentially by another investigator to assign eligible participants to receive 1 of 3 interventions: 0.3g, 90-Hz WBV; 0.3g, 30-Hz WBV; or no WBV (control group). Sham WBV was not provided to control participants because of limited funding; thus, participants knew whether they were in the control group. However, participants in the intervention groups did not know whether they were receiving 90- or 30-Hz WBV. The investigators who performed randomization were not involved in bone outcome assessment.

    We chose to examine low-magnitude (0.3g) WBV at 90 and 30 Hz because high-magnitude (≥1g) WBV has been shown to have deleterious effects in occupational settings and the International Organization for Standardization recommends its use for only short intervals or not at all in industries that use machinery involving vibration (ISO 2631) (8, 14, 22). Low-magnitude WBV has been examined in animal models (3), and stronger osteogenic effects were found at lower magnitude (0.3g vs. 0.6g) (23) and at higher frequencies (90 Hz vs. 45 Hz) (24). Previous RCTs of low-magnitude WBV (6, 14, 25) used a standard frequency of 30 Hz. Thus, we compared 90 Hz with 30 Hz to examine whether a higher frequency has stronger effects in postmenopausal women. Finally, we chose a dose of 20 consecutive minutes of WBV because experimental models examined 10 to 30 minutes of continuous WBV and observed significant improvements in bone density, structure, or formation rate (3); for example, 12 months of daily 20-minute WBV at 0.3g and 30 Hz was found to increase femur trabecular BMD by 30% in adult female sheep (1).

    Participants who were randomly assigned to the 90- and 30-Hz groups were each given a synchronous 0.3g-WBV platform that oscillated at 90 or 30 Hz (peak-to-peak displacement, <50 µm), respectively. At baseline, they were asked to stand on the platform for 20 minutes daily for 12 months at home; erect; with neutral posture at the neck, lumbar spine, and knees; wearing socks or barefooted; and without excessive foot or body movements. Control participants were asked not to use WBV therapy. All participants were provided calcium and vitamin D supplements at baseline and 6 months, so that their total daily intake from diet plus supplements approximated 1200 mg and 1000 IU, respectively. Calcium and vitamin D intake was estimated at baseline and at 6 and 12 months by using a recall questionnaire (26).

    Adherence

    Self-reported adherence to WBV was obtained at 6 months, and feedback was provided. Actual adherence to WBV was extracted from each platform at 12 months by using an internal digital clock recording of the date, time, and duration of every session. All platforms were designed to automatically turn off at the end of the 20-minute session, but a participant could have more than 1 session or 1 session of fewer than 20 minutes in a day. We therefore calculated the percentage of adherence to WBV on the basis of 3 measures: cumulative duration [(total minutes of WBV performed at any time during study participation) ÷ (total study days × 20 minutes) × 100], number of days [(total days during which any WBV session of any duration was performed) ÷ (total study days) × 100], and full session count [(total WBV sessions lasting 20 consecutive minutes performed at any time during study participation) ÷(total study days) × 100]. We also collected self-reported estimates (between 0% and 100%) of overall adherence to calcium and vitamin D supplementation at the end of study.

    Outcomes and Follow-up

    At baseline and at 12 months, volumetric BMD (trabecular, cortical, and total) and bone structure (cortical thickness and trabecular thickness, number, separation, and bone volume fraction) were measured at the distal tibia and distal radius with high-resolution peripheral quantitative computed tomography (HR-pQCT) by using XtremeCT (Scanco Medical, Bassersdorf, Switzerland). Areal BMD was measured at the femoral neck, total hip, and L1 to L4 lumbar spine with dual-energy x-ray absorptiometry (DXA) by using Hologic Discovery A (Hologic, Bedford, Massachusetts). Our prespecified primary outcome was trabecular volumetric BMD at the distal tibia, because trabecular bone tissue at a weight-bearing site closest to the oscillating plate was expected to have a greater response to WBV than other measurements or sites (8, 27). Trained and certified technologists assessed HR-pQCT and DXA outcomes by using standard manufacturer protocols. To assess longitudinal changes in HR-pQCT parameters, we compared the same regions between 2 time points by using limb fixation and image contours to match bone structure. The mean common region matched between HR-pQCT scans at baseline and 12 months was 96% for the distal tibia and 93% for the distal radius. For short-term reproducibility in our laboratory, the root-mean-square coefficients of variation for HR-pQCT of the distal tibia were 0.19% to 0.40% for BMD, 0.50% for cortical thickness, and 3.73% to 4.08% for trabecular structure; for HR-pQCT of the distal radius, 0.46% to 0.70% for BMD, 1.33% for cortical thickness, and 4.55% to 4.83% for trabecular structure; and for BMD with DXA, 1.0% to 1.8% (28, 29).

    Data on medical conditions, medications, and falls were collected at each study visit. Participants were also asked to inform us by telephone of any health changes they experienced during the study. Serious adverse events (defined as hospitalization, cancer, life-threatening event, or death during the study) and adverse events (defined as any untoward effects with an onset after baseline or worsening of an existing condition) were recorded by using the Common Terminology Criteria for Adverse Events from the National Cancer Institute (30). An investigator who was blinded to group assignment determined whether any of these events were related to WBV. Clinical fractures that occurred during the study were confirmed by radiography or radiologic reports and identified as fragility or nonfragility fractures. Falls were recorded on the basis of participant recall.

    Physical activity levels, body mass, and body mass index were assessed at baseline and 12 months. Activity levels were estimated from the daily activity metabolic index (kcal/d) by using the Minnesota Leisure-Time Physical Activity Questionnaire (31). Serum 25-hydroxyvitamin D concentrations were not measured at baseline but were obtained from available medical records of study participants who had a measurement at any time within 3 months of study participation.

    Statistical Analysis

    We used 2-way longitudinal data analysis with all available data included in a mixed model to examine the effect of time (baseline vs. 12-month, repeated measures) and group (90-Hz WBV vs. 30-Hz WBV vs. control) on all bone outcomes (HR-pQCT and DXA measurements), relevant nonbone outcomes (body mass and body mass index), and potential confounders (total daily intake of calcium and vitamin D and physical activity levels). To assess between-group differences in absolute change from baseline to 12 months, we specified contrasts a priori (90-Hz WBV vs. control, 30-Hz WBV vs. control, 90-Hz WBV vs. 30-Hz WBV, and combined 90-Hz and 30-Hz WBV vs. control). For sensitivity analyses, single imputations were used for missing bone changes from group means, and between-group differences in absolute changes were examined for all bone outcomes by using 1-way analysis of variance and a priori contrasts. Interactions between a priori–selected baseline variables (body mass, age, and years since menopause) and group treatment effect on absolute changes in all bone outcomes were assessed by using analysis of covariance. We hypothesized that lighter participants would experience less dampening of WBV by muscles and soft tissue (14), thus allowing for better transmission of the vibration stimulus to the skeleton, and that younger participants would have a greater response to WBV because of faster bone metabolism (32). Between-group differences in serum 25-hydroxyvitamin D concentration and adverse events were assessed by using 1-way analysis of variance and the Fisher exact chi-square test, respectively. All analyses were performed by using SAS, version 9.2 (SAS Institute, Cary, North Carolina), with a P value less than 0.05 indicating statistical significance.

    For our original sample size calculation, we used an SD of 1.26% for the 12-month change and 1.08% for the between-group difference in trabecular volumetric BMD at the distal tibia, on the basis of a case–control study of tai chi exercise (33). We estimated that 35 participants per group would be sufficient with 80% power and a P value less than 0.05. After incorporating a 20% dropout and nonadherence rate, we calculated an original sample size of 120. As our trial progressed, it became apparent that our initial dropout and nonadherence estimates were too conservative and did not represent our trial conduct. Thus, after 14 months of data collection, the actual dropout rate (15%) and percentage of participants who self-reported less than 60% adherence (70%) were incorporated into the calculation, resulting in an increased sample size of 200 participants. No interim analysis was performed on bone outcomes.

    Role of the Funding Source

    Our study was funded by a peer-reviewed grant from the Physicians' Services Incorporated Foundation. Juvent (Somerset, New Jersey) supplied the Dynamic Motion Therapy WBV platforms, and Jamieson Laboratories (Toronto, Ontario, Canada) provided the calcium and vitamin D supplements. The funding sources were not involved in the study design, conduct, analysis, interpretation of the data, preparation of the manuscript, or decision to submit the manuscript for publication.

    Results
    Participant Characteristics

    Of the 1126 potential participants interviewed by telephone, 303 attended a screening visit and gave informed consent. Of these, 202 postmenopausal women met our eligibility criteria and were randomly assigned to the 90-Hz WBV (67 participants), 30-Hz WBV (68 participants), or control (67 participants) group (Figure). Participants were primarily of European (78%) or Southeast Asian (16%) descent and had experienced menopause an average of 10 years ago (range, 1 to 43 y). The mean age was 60 years (range, 44 to 79 y), mean body mass was 63 kg (range, 43 to 88 kg), and mean total daily intake of calcium and vitamin D were 1429 mg and 817 IU, respectively. Table 1 lists baseline characteristics for each group.

    Figure. Study flow diagram.

    BMD = bone mineral density; HR-pQCT = high-resolution peripheral quantitative computed tomography; WBV = whole-body vibration.

    Table 1. Baseline Participant Characteristics

    During the course of the study, 7 participants dropped out, 2 had unevaluable tibia scans, and 4 had unevaluable radius scans (Figure). Two participants (1 each in the 90-Hz WBV and control groups) started hormone therapy but returned for follow-up assessment. The groups did not differ in 12-month changes in physical activity levels or total daily calcium or vitamin D intake (data not shown). Serum 25-hydroxyvitamin D concentrations were obtained for 180 participants (60, 61, and 59 participants in the 90-Hz WBV, 30-Hz WBV, and control groups, respectively), and the mean concentration did not differ among the groups (94 nmol/L [SD, 26], 94 nmol/L [SD, 33], 90 nmol/L [SD, 27], respectively; P = 0.67).

    Adherence to WBV was not obtained for 3 participants because the digital clock on their WBV platform malfunctioned and from 5 participants who dropped out before study completion. Most participants were either close to 100% or 0% adherent, which made the adherence distributions bimodal and heavy in the extremes; hence, medians were used. All 3 measures of adherence to WBV were similar in the 90- and 30-Hz groups. The median estimate of adherence was 79% (interquartile range [IQR], 41% to 91%) for the 90-Hz WBV group and 77% (IQR, 55% to 86%) for the 30-Hz group on the basis of cumulative duration; 70% (IQR, 33% to 82%) for the 90-Hz group and 65% (IQR, 52% to 80%) for the 30-Hz group on the basis of number of days; and 78% (IQR, 41% to 91%) for the 90-Hz group and 77% (IQR, 53% to 86%) for the 30-Hz group on the basis of full session count. Median self-reported adherence to calcium and vitamin D supplementation was also similar among the 90-Hz WBV (98%; mean, 90%), 30-Hz WBV (98%; mean, 89%), and control (96%; mean, 89%) groups.

    Bone Outcomes

    Twelve months of WBV at 90 or 30 Hz resulted in no significant change in either HR-pQCT or DXA bone outcomes compared with no WBV (Table 2). From baseline to 12 months, tibial trabecular volumetric BMD changed by 0.4 mg/cm3 (95% CI, −0.4 to 1.2 mg/cm3) in the 90-Hz WBV group, −0.1 mg/cm3 (CI, −1.0 to 0.8 mg/cm3) in the 30-Hz WBV group, and −0.2 mg/cm3 (CI, −1.1 to 0.6 mg/cm3) in the control group (P = 0.55). Results from our sensitivity analysis (data not shown) were similar to those obtained in the main analysis (Table 2).

    Table 2. Absolute Changes in All HR-pQCT and DXA Bone Outcomes From Baseline to 12 Months, Based on Longitudinal Data Analysis

    Baseline age and years since menopause were found to interact with the effect of WBV on change in several tibial trabecular structure measurements (Table 3). In older participants (age >60 years or >10 years since menopause), greater decreases in trabecular thickness and separation and greater increases in trabecular number were found in the 90- and 30-Hz WBV groups versus the control group, whereas these changes were similar for younger participants (age ≤60 years or ≤10 years since menopause) among the 3 groups.

    Table 3. Absolute Changes in Tibial Trabecular Structure From Baseline to 12 Months for Selected Subgroups

    The groups did not differ in the number of participants who experienced 1 or more clinical fractures during the study (Table 4). None of the fractures was considered a fragility fracture because they were caused by car, household, or sports accidents or involved only small bones of the feet. Four of the 8 fractures involved the foot (1 each in the 90-Hz WBV and control groups) or ankle (1 each in the 30-Hz WBV and control groups).

    Table 4. Summary of Adverse Events
    Other Outcomes

    Similar numbers of participants in the 90-Hz WBV and control groups (21 vs. 24) had at least 1 fall during the study, whereas only 12 participants fell in the 30-Hz WBV group (P = 0.063) (Table 4). Six participants in the 30-Hz WBV group spontaneously reported improvements in low back pain during the study. However, the groups did not differ in the number of participants who had back pain at some point during the study and experienced improvement, worsening, or no change (Table 4). Three participants in the WBV groups (2 in the 90-Hz group and 1 in the 30-Hz group) reported improved bowel movement (especially after a postprandial WBV session) as a beneficial effect of WBV.

    At 12 months, the groups did not differ in changes in body mass (90-Hz WBV, 0.2 kg [CI, −0.4 to 0.8 kg]; 30-Hz WBV, 0.0 kg [CI, −0.6 to 0.6 kg]; control, 0.5 kg [CI, −0.1 to 1.2 kg]; P = 0.54) or body mass index (90-Hz WBV, 0.1 kg/m2 [CI, −0.2 to 0.4 kg/m2]; 30-Hz WBV, 0.0 kg/m2 [CI, −0.3 to 0.2 kg/m2]; control, 0.2 kg/m2 [CI, 0.0 to 0.5 kg/m2]; P = 0.49).

    Adverse Events

    A few serious adverse events were reported: breast cancer (1 participant each in the 30-Hz WBV and control groups), grade 3 oligodendroglioma (1 participant in the 30-Hz WBV group), appendicitis (2 control participants), pneumonia that required hospitalization (1 participant in the 30-Hz WBV group), and pacemaker insertion (1 participant in the 30-Hz WBV group). None was believed to be related to WBV. When various adverse event categories were compared among the study groups, no statistically significant differences were observed, although our trial was not sufficiently powered to assess these differences (Table 4). Three participants discontinued WBV therapy within 2 months after starting the study because of dizziness at night (1 participant in the 30-Hz WBV group), chronic shin pain (1 participant in the 90-Hz WBV group), and chronic plantar foot pain (1 participant in the 30-Hz WBV group) that they attributed to WBV; they remained in the trial for follow-up measurements. Other mild and transient symptoms experienced during and attributed to WBV by participants included pain, numbness, or weakness at various leg sites (5 participants each in the 90- and 30-Hz WBV groups); nausea (2 participants in the 90-Hz WBV group); and exacerbation of preexisting headaches (1 participant in the 30-Hz WBV group), bladder discomfort (1 participant in the 90-Hz WBV group), inner ear sensitivity (1 participant in the 90-Hz WBV group), or neck pain (1 participant in the 90-Hz WBV group).

    Discussion

    In our 12-month RCT of 202 postmenopausal women, daily low-magnitude (0.3g) WBV at 90 or 30 Hz had no effect on volumetric BMD or bone structure at the distal tibia and distal radius or on areal BMD at the lumbar spine, total hip, or femoral neck. Our results are similar to trials that examined low-magnitude WBV at 0.3g and 30 Hz in postmenopausal women for 8 months (15-minute sessions twice weekly) (6) and 12 months (10-minute sessions twice daily) (14), and found no effect on lumbar spine, femoral neck, trochanter, proximal forearm, or whole-body areal BMD. In contrast to animal models (24), we did not find a higher WBV frequency to be more effective. Whether high-magnitude (≥1g) WBV has a different effect on bone outcomes than low-magnitude (<1g) WBV remains unclear (6). In the 6 RCTs that examined high-magnitude WBV in postmenopausal women, no significant effect was observed for lumbar spine (6, 12, 13, 16), proximal forearm (6), or whole-body (6) areal BMD or for distal tibia volumetric BMD (15). However, 2 of the 4 trials (5, 6, 12, 16) showed a statistically significant but clinically small effect on areal BMD at the hip.

    The discrepancy in observed effect of WBV on bone between trial results may be partly explained by methodological limitations of previous RCTs in postmenopausal women (4). Compared with previous RCTs, we had a larger sample size (202 vs. ≤113 participants [5, 6, 12–16]), had fewer losses to follow-up (3% vs. 5% to 53% [5, 6, 12, 14–16]), and provided calcium and vitamin D supplements. To improve the detection of efficacy of WBV, we included only women with low BMD and examined trabecular volumetric BMD at the distal tibia as our primary outcome. We also pooled the 90- and 30-Hz groups and compared them with the control group and performed a sensitivity analysis, but none of our analyses revealed a significant beneficial effect of WBV on any bone outcomes. Our results showed that WBV at either 90 or 30 Hz might have a detrimental effect on tibial trabecular structure in older postmenopausal women. However, the observed effects could have occurred by chance, because the subgroups of older participants were small and we performed multiple comparisons.

    Several participants in the 90- and 30-Hz groups reported pain and numbness of the feet, dizziness, and nausea that were possibly related to WBV. Previous investigations of prolonged exposures to occupational vibration (22, 34), such as on driving or drilling platforms, observed deleterious effects, including the vibration–white foot syndrome and motion sickness. However, WBV was well-tolerated overall in our trial, which is consistent with previous RCTs of postmenopausal women (4).

    Our study has limitations. First, median adherence to WBV ranged from 65% to 79%. Participants self-administered WBV at home and unsupervised, and we do not know whether they followed the instructions (provided at baseline and 6 months) regarding proper posture. Second, double-blinding was not possible because true WBV cannot be completely masked (14), and sham platforms that produce audible sound were not provided to the control group because of limited funding. Finally, serum 25-hydroxyvitamin D concentrations were not collected at baseline but were obtained from available medical records.

    Overall, low-magnitude WBV is not an effective therapy for preventing bone loss in postmenopausal women who are receiving calcium and vitamin D supplementation. Our recent meta-analysis of previous RCTs that examined WBV (4) showed a statistically significant effect in BMD at the hip (0.015 g/cm2), but the magnitude of this effect was not clinically significant and could have been due to variation of the test. We also found larger bone changes in children and adolescents (4); WBV may be more effective in children and adolescents with compromised bones because their skeletons are growing. Randomized, controlled trials of WBV effects on bone are under way in adolescents (35), patients with spinal cord injury (36), and institutionalized elderly persons (37).

    In conclusion, 12 months of low-magnitude (0.3g) WBV at either 90 or 30 Hz had no effect on BMD or bone structure in healthy, community-dwelling, postmenopausal women who received calcium and vitamin D supplementation, and is thus not recommended for preventing age-related bone loss in this population.

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