Biomedical Engineering Reference
In-Depth Information
Although most of the adaptive responses were similar in males and females, some
sex differences were observed. An increase in mid-femoral cross sectional area
in mature male rats and loss of body weight in both mature and old male rats
was observed in response to exercise; female rats did not show these changes.
On the other hand, exercise increased the femoral length of mature female but not
male rats. The differences in response indicate that the sex of the animals must be
considered when studying bone adaptation under mechanical loading.
Bennell et al. [ 31 ] studied the influence of 12 weeks of treadmill running on
skeletal adaptation in female rats at two ages: 1-month old (rapid growth phase)
and 4-month old (steady growth phase). Compared to sedentary controls, running
increased bone formation indices, bone size and mechanical properties in the tibias
of both age groups. Similar findings were noted in the lumbar spine (L1-L4).
On the other hand, the tibial metaphysis and femur did not show any changes in
response to exercise. In summary, there were no age-related differences in the
effects of running on bone properties at the local level. The only difference in
response to running between the two age groups was that trained 1-month old rats
had greater gain in total body bone area and BMC than trained 4-month old rats.
Nonetheless, because both of these ages are still early in the rat lifespan, this study
addresses loading during growth rather than during aging.
Contrary to the aforementioned findings of no age-related decline in respon-
siveness to running, Umemura et al. reported that older rats are less responsive to
treadmill running than younger rats [ 32 ]. In young animals (3 and 6 months),
8 weeks of running increased the fat-free weight of the femur and tibia, length and
diameter of the femur, and length of the tibia. At a mature age (12 months), the
only significant effect of run-training was an increase in tibial weight. At older
ages (20 and 27 months), there were no differences in these parameters between
run-trained and control animals, indicating that the older animals did not respond
to treadmill running. The authors noted that because they imposed the same
running speed (30 m/min) on all age groups, ''it is considered that the intensity
was too high for the old rats''. We note that the outcomes in this study were limited
to relatively simple measures of bone geometry and mass; it is possible that other
measures (e.g., local bone structure, bone mechanical properties) may have been
more sensitive to running.
Jarvinen et al. observed differences in adaptive mechanism between young and
mature rats in response to treadmill running [ 33 ]. After 14 weeks of treadmill
running, both young (1 month) and mature (7 months) animals had significantly
higher BMC, BMD and breaking load at the femoral neck. However, only the
young animals had a significant increase in femoral neck cross-sectional area
(+25%) compared to a non-significant change (+10%) in mature rats. By contrast,
young rats had a smaller increase (+11%) in BMD than mature rats (+23%). Run
training did not have any influence on the length of the femur for either age group.
It was concluded that growing animals respond primarily by changes in bone
size (increased area) whereas mature animals respond primarily by changes in
bone density. We note that this conclusion is not strongly supported by the data,
as the increases with running did not differ significantly between age groups.
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