In this study we have investigated the feasibility and benefit of 6 weeks WBV in institutionalised elderly persons. The results of our study indicate that WBV might have beneficial effects on balance and mobility in elderly nursing home residents. Indeed, subjects assigned to WBV improved significantly on the timed up-and-go test and maintained their baseline level of balance (as measured by Tinetti-test), contrary to the controls who did not improve in timed up-and-go and who's balance worsened significantly (difference in change between both groups p = 0.029 for timed up-and-go, p = 0.001 and p = 0.002 for Tinetti body balance and total score respectively). It might be possible that the decrease of balance scores in the controls reflects the frail and physically unstable status of nursing home residents. The challenge for keeping equilibrium on the vibration platform in subjects of the WBV+ group could result in a possible adaptation mechanism that preserved them for further decline. Our results were in agreement with those from Bruyere et al. , who also found significant changes in timed up-and-go and Tinetti-test performance following WBV in institutionalised elderly. However, they used another type of WBV platform (Galileo, Orthometrix Inc., New-York) generating tilting oscillations; contrary to the Power-Plate, which produces vertical vibrations.
In our study, 3 subjects in the WBV+ group were lost for follow-up. One dropout was completely at random (airway infection), while two might have been related to the WBV program (one developed groin pain, the other one became afraid). This dropout rate corresponds to that reported in other WBV intervention studies [12, 25, 26] and classic intensive weight-lifting exercise  involving elderly subjects. It cannot be excluded that our results would have been different if these persons did not drop out . For the latter two dropouts, a worst rank score analysis (substitution of missing end-evaluation values by the worst rank score, a technique used when subjects are lost for follow-up due to absorbing events like death ) was carried out. In a WRS analysis, the difference in change on the timed get-up-and-go test was not significant anymore (p = 0.238). However, during our repeated attempts to convince them to participate in the end-evaluations  we observed no visible worsening in their daily functioning, and we feel that WRS analysis is less appropriate here. Therefore we have substituted the missing end-evaluations of the two latter subjects by carrying the last available observation forward as an alternative procedure . In this approach, the difference in change on the timed get-up-and-go test was attenuated and the p-value was slightly higher than the threshold-value for statistical significance (p = 0.075). LOCF assumes that the last observation of a subject who dropped out the study is an unbiased representation of what the missing value would have been had the subject been followed. It is obviously an untestable assumption, leading to distortion of the covariance structure of the data as well as the mean values, and LOCF should be interpreted with great caution .
After 6 weeks, both groups (WBV+ and control) showed significantly better leg extension performance compared to baseline (except for explosivity at 40 cm/sec for the control group, all p < 0.05). However, differences in change between both groups were not statistically significant. We observed a high variability for leg extension performance. None of the participants showed difficulties in performing the tests indicating that this variability reflected their heterogeneous condition. Important age-related variability, indeed, is a characteristic of all studies involving geriatric populations, especially when dealing with frail elderly persons. Moreover, our previous studies indicate that this test procedure is applicable in elderly persons [19, 28]. The important improvements of the controls can be explained by the fact that these persons performed exactly the same exercises as the WBV+ group, except that the platform did not vibrate. Probably the low baseline muscle performance level of the participants predetermined the possibility to obtain considerable gains in a short time. These results indicate that frail elderly are highly trainable by means of simple physical exercise (i.e., maintaining weight-bearing positions).
In our study, we assumed that if WBV is effective, adaptations in muscle function would become measurable within short time (6 weeks). Moreover, prolongation over a longer period might result in occurrence of confounding factors in these frail nursing home residents (acute disease such as influenza, cognitive decline, changes in medication use and instability of comorbidity). However, it is not excluded that higher benefit of WBV on muscle performance might be obtained after longer or more intensive training programmes. Russo et al.  described significantly improved lower limb muscle power in elderly independently living women following 6 months WBV compared to control. Their study differed from ours since they used a tilting-platform (Galileo) and their control group performed no physical exercise at all. Roelants et al.  reported significantly improved knee extensor strength following 24 weeks WBV in community-dwelling elderly women. These improvements were similar to those obtained with traditional weight-lifting strength training. Also in their study the controls did not perform any exercise at all. Moreover, their WBV program consisted in much more intensive exercises (both static and dynamic, including one-legged) with a total training duration going up to 30 minutes. In our study, only bipodal static exercises were performed, with a maximal exercise duration attaining 5 to 6 minutes WBV (10 to 15 minutes including rest periods). One of the potential mechanisms leading to muscle adaptation following WBV is by stimulating the tonic vibration reflex. Possibly, longer and/or more intensive WBV exercise sessions might result in higher motor unit activation, and thus better training effects .
Lower body flexibility (as measured by the chair sit-and-reach test) improved significantly in the WBV+ group (p < 0.05, not in WRS analysis), but not in the control group. To our knowledge, this is the first paper describing improved flexibility in elderly persons following WBV. The training-induced change in flexibility, however, was not significantly different between WBV and control group. It is assumed that vibration can improve flexibility by central mechanisms such as increase in stretch tolerance (higher pain threshold) and the stimulation of Golgi tendon organs (contraction inhibition) . Since the participants in our study firmly held the front handle of the vibration device, the vibration stimulus was also partly transmitted through the upper limbs. However, no significant changes in upper limb flexibility were observed.
The participants who completed the WBV+ program attended 96% of the exercise sessions, compared to 86% in the control group. These attendance rates correspond to those in common geriatric rehabilitation practice. It can, however, not be excluded that this small difference in compliance (10% = average of 1.8 sessions over 6 weeks) might have affected our results.
The high rate of compliance in our study (96% in the WBV+ group) supports the feasibility of WBV in frail institutionalised elderly. However, the majority (66%) of the nursing home residents at the moment of the study were excluded: 36 (36%) were not eligible and 29 (30%) presented exclusion criteria. Possibly, our inclusion and exclusion criteria were too severe, although it seems reasonable to assume that the risk for complications during WBV in subjects presenting severe levels of dependency or cognitive decline may be much higher than in the participants in our study. Therefore, the feasibility of WBV for these categories of frail institutionalised elderly remains further to be studied and it might be necessary to develop more adapted exercise programs.