Biomedical Engineering Reference
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peak strains, consistent with Frost's mechanostat hypothesis. Importantly, the
effects of jump training on bone hypertrophy were not limited by age.
Honda et al. also observed that jump training has skeletal benefits that accrue
(and are preserved) independent of age [
36
]. In both young (3 month) and mature
(10 month) rats, 8 weeks of jump training increased tibial BMC and BMD
[determined by dual-energy X-ray absorptiometry (DXA) every 4 weeks] and
strength (determined by three-point bending) compared to sedentary controls. This
bone accrual was maintained throughout the de-conditioning duration of 24 weeks,
again irrespective of age. It was concluded that aging does not influence the ability
of skeleton to respond to exercise or the maintenance of skeletal benefits induced
by jump training. Tibial parameters measured ex vivo (BMC, ultimate force,
cortical area, medullary area, endosteal perimeter, periosteal perimeter, fat-free
weight, maximum and minimum moment of inertia) were greater in the exercise
group at the end of the de-conditioning period compared to the sedentary group.
Exercise effects were not different between age groups, except for periosteal
perimeter which was increased more in the mature rats than in the young. Thus,
there was no loss of responsiveness with aging.
Buhl et al. [
35
] studied the influence of squat-like training (a similar loading
modality as jumping) on young (4 months), mature (12 months) and old (22 months)
male rats. While the above studies found that older animals are as responsive to jump
training as younger animals, Buhl et al. concluded that aged bones actually display
higher responsiveness to loading. Training had no influence on mechanical proper-
ties of the femur (determined by three-point bending) or on femoral cross-sectional
area, cortical area and moment of inertia in any age group. In addition, loading did not
influence mineral apposition rate and bone formation rate at the tibial diaphysis.
However, the tibial metaphysis of older animals subjected to loading had signifi-
cantly lower trabecular spacing and greater BV/TV and trabecular number
(non-significant) than the control animals. Training also increased the medullary area
of the femur in older animals. Based on these observations it was concluded that the
training, though it failed to benefit young and mature bones, was marginally bene-
ficial to old bone.
3.3 Summary: Bone Responses to Intrinsic
Mechanical Loading
We found only eight studies of intrinsic mechanical loading that directly compared
different ages, and only five of these include a true old age group (Table
2
). Of
these eight studies, the majority found no effect of age. Therefore, they support the
view that the ability of the skeleton to adapt to altered mechanical loading is not
compromised by aging. The only study that found a clear, negative influence of
age was the running protocol of Umemura et al. [
32
], wherein young rats had
favorable bone responses to treadmill running while older rats did not respond.
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