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relationships between applied force and peak periosteal strain measured at the
mid-diaphysis, leading the authors to perform strain-matched and load-matched
comparisons. Old mice were not responsive when loaded at a force (5.9 N) that
produced a strain magnitude of 1200 le periosteal, while young mice were
responsive when loaded at a force (11.5 N) that produced the same strain. By
contrast, when old mice were loaded at 11.5 N (2200 le) they had a positive
anabolic response that was similar in magnitude to the young mice at the diaphysis
(cortical) but less than the young mice at the metaphysis (trabecular). Notably,
trabecular BV/TV was increased in both age groups, contrary to our findings of
trabecular bone loss with loading [
57
,
58
]. (Ongoing work in our lab suggests that
the difference in trabecular responses is attributed to the waveform/cycle number
differences between our protocol versus the protocol used by Lynch et al.) These
authors concluded that loading was anabolic for cortical and trabecular bone in
adult mice, albeit to a lesser degree than in young, growing mice.
4.5 High-Frequency Low-Magnitude Vibration
Loading modalities using high-frequency, low-magnitude vibration have gained
increasing attention [
61
]. It is believed that this kind of loading mimics the muscle
forces induced on bone during postural activities such as standing. Huang et al.
[
62
] reported that the 30-50 Hz component of postural muscle activity is altered
with aging, perhaps contributing to age-related bone loss. It was hypothesized that
extrinsic vibrational loading might compensate for this decline, and either stim-
ulate bone formation or prevent bone loss. One common method to apply this type
of loading is to place the animal (or human subject) on a platform that oscillates
vertically and subjects the animal to whole-body vibration (WBV). The loading
magnitude and frequency of the platform can be precisely controlled. However if
the animals (esp. rodents) are allowed to move freely over the platform, consistent
strains at a particular skeletal site cannot be guaranteed.
WBV loading has been reported to be mildly anabolic in young rodents [
63
,
64
],
but it has not been widely studied in aged animals. We examined the influence of
WBV on bone in mature (7 months) and aged (22 months) male, BALB/c mice. As
reviewed above, these mice exhibit anabolic cortical responses to low-frequency,
high-magnitude tibial compression [
57
]. In contrast, we observed that WBV had
minimal influence on cortical bone at tibial mid-diaphysis or trabecular bone at the
proximal tibia for both mature and old mice [
65
]. Loading increased lower leg BMC
(determined using DXA) in adult mice but not in old mice. However, since most
outcomes (microCT, dynamic histomorphometry) did not show any loading
induced changes, we concluded that both the mature and old mice are unresponsive
to WBV.
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