6 produced by the vibrating plate. While the adaptation to the mechanical loading is most likely the limiting factor, elderly subjects have been shown to beneﬁt from simulated mechan- ical loading. Early-postmenopausal women who stood on a vertical plate of low magnitude (0.3 g) and high frequency (30 Hz), twice/day (10 minutes each) for a year, showed no diﬀerence in BMD compared with controls [13]. However, evaluating those with ≥80% compliance, controls lost 2.13% of bone mass in the femoral neck, whereas treatment group gained 0.04% over one year. When the analysis focused on the lower weight (<65 kg women, a known risk factor for osteoporosis) and those who were compliant, the beneﬁt of treatment became signiﬁcant and demonstrated a 3% and >2% positive diﬀerence at the spine and femoral neck BMD, respectively. Any potential beneﬁt of WBV strongly depends on com- pliance and vibration stimulus that can be varied in multiple ways (including type, magnitude, frequency, and duration), and diﬀerent types of vibration loading are likely to result in diﬀerent eﬀects on BMD [58]. Low-level mechanical stimuli may be more eﬀective in lighter than in heavier women, particularly for hip BMD. A recent meta-analysis (although published only as an abstract) also showed that low-intensity WBV eﬀectively attenuated postmenopausal BMD decline in hip but not in spine [59]. Another study in postmenopausal women where intervention was employed on a vertical plate, higher magnitude (2.28–5.09 g), and high-frequency (35– 40 Hz) for 6 months showed a signiﬁcant 1.51% net increase in total hip BMD but not in total body or spine BMD [35]. A 5-month study examining an older man (79 years) employing multiple vibration intensities and frequencies to evaluate BMD showed detectable increase in spine, femoral neck, trochanter, and forearm BMD [36]. Though this study demonstrated positive results on BMD, only one participant was examined, therefore, the results cannot be representative of the larger group. Gusi et al. [19] investigated whether WBV is more eﬀective than walking for maintaining or increasing BMD. They compared the eﬀect of walking and WBV using a reciprocating plate, low amplitude (3 mm), and medium fre- quency (12.6 Hz), for 8 months on BMD in postmenopausal women. After 8 months, femoral neck BMD was signiﬁcantly (4.3%) higher in the WBV group than in the walking group. The diﬀerence in BMD at spine and other sites of the hip was not signiﬁcant between two groups. The results suggest that the vibration could be an easier approach to increase BMD at the femoral neck than walking and could be applied to provide a surrogate for suppressed bone loss of hip after menopause. However, at the present time, it is too early to make a conclusion due to insuﬃcient research in elderly adults and varied vibration protocols. Compliance to WBV in elderly population is another critical concern. 6. Possible Mechanism by Which Low-Intensity Whole-Body Vibration Affects Bone Possible mechanism by which WBV aﬀects bone may be based on the same principle as AEXs that activate the Journal of Aging Research osteoblasts while reducing the activity of the osteoclasts. The strain, magnitude, and frequency are essential factors for the eﬀect of WBV. WBV also increases the sensitivity of musculoskeletal systems to adapt to the mechanical stimu- lation. Controlled loading study has shown that high-strain magnitude and high-strain rate are the most osteogenic [60]. It has been hypothesized that mechanical stimulation recruits additional osteoblasts and increases the percentage of mineralizing surfaces therefore, increasing the rate of bone formation and decreasing the rate of bone resorption [60]. It has been suggested that the high-frequency vibration may have played an important role in the osteogenic eﬀect. There is a general perception that signals must be large enough to elicit a positive inﬂuence on bone mass and morphology. However, the high-frequency stimulation may be capable of inﬂuencing skeletal architecture by distributing uniform stresses on bones [61]. Rubin et al. [62, 63] hypothesized that this inﬂuence may be achieved directly by mechanical strain, or indirectly through ampliﬁcation of the signal by intramedullary pressure or ﬂuid ﬂow in bone tissue. The mechanism behind the frequency-dependent adaptive response of bone to stimuli might be the stochas- tic resonance. Stochastic resonance is a phenomenon in which mechanical noise (broad-band frequency of vibration) enhances the response of a nonlinear system to a weak signal by boosting it over a threshold. The stochastic threshold may be modiﬁed through a system such as neuromus- cular feedback ampliﬁed by the low-level signal [64] or by stimulating skeletal muscle pump activity, resulting in signiﬁcant eﬀects on circulatory ﬂows and ﬂuid ﬂow through the bone tissue. In addition, previous study has shown that stochastic resonance can enhance mechanosensitivity of diﬀerent mechanoreceptors in the body, for example, muscle spindles [65]. These ﬁndings indicate that vibration stimulation employs multiple ways to inﬂuence bone mass and structure. 7. Conclusions In summary, evaluation of the published literature provides evidence of the eﬀectiveness of AEX and WBV in increasing or at least maintaining bone mass in the elderly. The mechanism could be due to increasing the circulation of ﬂuid and activating the osteoblasts while reducing the activity of the osteoclasts via mechanical stimulation. The osteogenic eﬀects of both AEX and WBV could be site speciﬁc (to the spine or hip), depending on the exercise load and the type of exercises. The beneﬁcial eﬀects on bone can be maintained for a longer time if the exercise continues— although the exercise may not maintain the age-related reduction of BMD in elderly. In addition, the risk of injuries or falls could be high and the compliance to AEX particularly in elderly low. Walking is an inexpensive, practical exercise associated with low injury rates and demonstrates high acceptability by elderly. For these reasons, walking could be an appropriate approach to prevent osteoporosis and maintain bone mass. However, there is evidence that the osteogenic eﬀect of load bearing may decline with aging