Biomedical Engineering Reference
In-Depth Information
Clinical studies to expand the understanding of how bones become desensitized
to loading will be critical for refining the exercise prescription for bone health.
However, such studies will be much more challenging to carry out in humans than
in laboratory animals because of the inability to measure the acute bone formation
response to loading. In this context, additional preclinical studies are needed to
facilitate the translation of preclinical to clinical research. One important question
to address is whether bone remains sensitive to loading when the loading condi-
tions are not predictable. Most experiments in this area have used extrinsic loading
models, with force applied at a consistent rate, intensity, and direction. Although
this approach provides desirable control of the mechanical environment [ 79 ], it is
of limited clinical relevance because such conditions do not occur during exercise
in humans. Another important question is whether there are biomarkers of the
acute desensitization of bone to loading that can be measured in humans. In the
absence of such biomarkers, clinical research in this area will progress slowly.
3.2.2 Aging
It has been suggested that the skeleton loses its responsiveness to mechanical loading
with advancing age [ 4 , 24 ]. For example, 5 months of jumping exercises (50 vertical
jumps per day, 6 days/week) generated 2-3% increases in femoral neck and trochanter
BMD in premenopausal women [ 4 ], but the same intervention carried out for
11 months generated no increase in BMD in postmenopausal women not on meno-
pausal hormone therapy. The discordant responses were not explained by differences
in the peak ground-reaction forces during jumping, which tended to be higher in
postmenopausal women [ 4 ]. Although not significant, the BMD responses of post-
menopausal women to jumping were more favorable (1-2%) in women on estrogen-
based hormone therapy when compared with estrogen-deficient women. This suggests
that factors other than age, per se (e.g., reproductive status), influence the skeletal
response to mechanical loading. Preclinical research indicates that bone retains the
ability to adapt to mechanical loading into very old age (see ''Skeletal mechanore-
sponsiveness and aging-animal studies''), suggesting that there are skeletal benefits
of exercise across the life span. This is supported by clinical research demonstrating
that older women and men can increase BMD in response to exercise training [ 49 , 52 ].
Thus, the aged skeleton appears to remain responsive to mechanical loading, but the
stimulus to generate an increase in BMD may change with aging. Whether the physical
activity recommendations for bone health should be age-specific remains unclear. It is
possible that physical activity has beneficial effects on bone strength and fall risk even
in the absence of measureable benefits on BMD.
3.3 Non-Loading Factors
Meta-analyses suggest that exercise training can generate average increases in BMD
of 1-3% in humans (Table 1 ). As a general observation, such changes seem small
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