Journal of Aging Research but there was no diﬀerence at the spine BMD [22]. These results support walking as an eﬀective method of increasing BMD and conﬁrm that a brisk and fast walking pace may, in addition to femoral BMD, also beneﬁt calcaneus and forearm BMD. However, most of the studies have been done in women while older men have been less investigated regarding the osteogenic response to walking and running. There is a consensus that a combination of aerobic and anaerobic exercise is more eﬀective in improving bone mass than either one alone [45]. One year of brisk walking combined with gymnastic training either increased or maintained the spine BMD in postmenopausal women [25]. Results from an earlier meta-analysis showed that walking with other AEXs signiﬁcantly aﬀected the BMD at spine, but not hip [46]. Bone mass increases or decreases in response to mechanical loading depending on whether the thresholds controlling bone formation and resorption have been reached [47]. A few studies have demonstrated the importance of walking intensity on BMD preservation in postmenopausal women. Hatori et al. [8] reported that 7 months of walking with an intensity above 110% of the heart rate at its anaerobic threshold attenuated bone loss in the spine of postmenopausal women, whereas walking at an intensity below 90% of the heart rate at its anaerobic threshold had no inﬂuence on BMD. Similarly, Borer et al. [48] conﬁrmed that fast walking pace increased leg and total BMD in early postmenopausal women who were engaged in 15 weeks of walking. These studies demonstrate positive eﬀect of walking intensity on BMD at diﬀerent skeletal sites. The greater response to the higher intensity walking may be due to the elevated ground reaction forces that occur at a faster walking pace. A failure to show increase in spine BMD during a walking study by Cavanaugh and Cann [9] in 55-year-old postmenopausal women may be due to the employment of lower walking intensities. Therefore, a combination of diﬀerent kinds of AEXs may be the most eﬃcient approach to reach desired exercise intensity to enhance or maintain bone mass at diﬀerent skeletal sites in postmenopausal women. 4. Possible Mechanism by Which Aerobic Exercise Affects Bone It is generally accepted that mechanical loading on bones is probably one of the ways to induce skeleton’s structural changes and increase bone mass. Physical activity induces a mechanical load on bone tissues due to external forces and muscle contractions, the latter exerting the greater force on bones than any other weight-associated gravitational forces. To withstand the rigor of various functional activities, bone tissue rapidly accommodates changes in its microen- vironment [49]. Although the mechanism of mechanical loading eﬀects on bone is not completely understood, it is postulated that it is due to the mechanotransduction of a load. This mechanotransduction is carried out through ﬂuid ﬂow near and between osteocytes (mature cells within the mineralized bone matrix) [50]. In vivo and in vitro studies indicate that mechanical stimuli increase strain, loading 5 frequency, and ﬂuid ﬂow, all of which have an osteogenic eﬀect [51]. However, several conditions must be met to aﬀect bone positively: (1) the strain produced by loading must be of high enough magnitude to exceed the minimum eﬀective strain (or threshold) [52]; (2) the strain should be applied in an intermittent fashion [51]; (3) loading should produce a “diﬀerent from normal” strain distribution within the bone [51]. In addition, recent research implies that frequent loading on bone without rest may not allow suﬃcient time for osteocyte ﬂuid ﬂow to recover from inertial dumping between each load cycle. Therefore, ﬂuid ﬂow and subsequent osteocyte stimulation might be reduced or completely inhibited after the ﬁrst loading cycle. If so, inserting short time periods between loadings will allow for recovery from the inertial dumping eﬀect and facilitate osteocyte stimulation. This eﬀect was recently shown in vivo, in both young and older animals and in two diﬀerent species (avian and murine); by inserting 10 sec, rest period between each loading cycle greatly enhanced the osteogenic potential of the low-magnitude regimen [53]. While much emphasis is given to the resident bone cells (primarily osteocytes) and their response to the local load imposed on bone, new research implies that response to the mechanical load (exercise) may also be neuronally regulated and therefore systemic, with a resultant eﬀect on multiple bones [54]. Sample et al. [54] showed that in young rats, intense mechanical stimulation of one limb can illicit a response in other limbs and even in the entire skeleton. This newest discovery may explain why increased BMD after localized mechanical stimulation could be recorded in skeletal sites that were not directly stimulated (e.g., increased forearm BMD with walking) [23]. 5. The Effect of Whole-Body Vibration on Bone WBV is a new approach that is currently being tested for its eﬀect on BMD and bone strength [51, 55]. There are two main types of vibration devices/techniques: (a) up- and-down oscillating vibration plates and (b) reciprocating vertical displacements on the left and right sides of a fulcrum, providing the lateral oscillations. Rubin et al. [11, 51] observed an increase in bone formation in weight-bearing sites and a substantial increase in the quality and quantity of trabecular bone in sheep exposed to a low-intensity, high- frequency (20–50 Hz) mechanical stimuli [11]. It is well established that decreased estrogen due to ovariectomy in animal models decreases the bone formation rate resulting in a decrease in bone mass. In rats, Flieger et al. [56] found that low-level, high-frequency mechanical loading (50 Hz) was eﬀective in preventing bone loss shortly after ovariec- tomy. Even in aging mice, low- and high-intensity WBV signiﬁcantly increased mineralized bone surfaces [57]. This suggests that WBV may be a suitable bone-sparing therapy. Bone is known to adapt to diﬀerent loading conditions and the loading-induced strains are believed to be based on the adaptation of the bone tissue. For that reason, researchers usually administered frequencies at 15–35 Hz to obtain the maximum transmissibility of the mechanical loading