Biomedical Engineering Reference
In-Depth Information
hormone in growing male rats may account for the significantly lower protein mass gain
and BW gain. In female rats, however, estrogens would suppress body mass, food
consumption and fat deposition.
70, 71
Progesterone, on the other hand, has been verified to
increase body fat and body mass.
72, 73
Moreover, a previous study suggests that regular
treadmill training results in extended periods of progesterone secretion, which was
associated with significant weight gain.
74
Women may respond to ERT with similar
regulation in progesterone. However, the up-regulation of progesterone may be more
pronounced in rats than in women, since female rats reveal no decrease in BW even under
vigorous ERT.
60
3.2 Studies of male rats mimic human practice
An analysis of gender differences observed in the above animal studies reveals that ERT
increases BMD and BMC in female rats but not in male rats. However, the physiological
response (
e.g.
BW gain) of female rats to ERT differs from that in female humans. Given that
animal studies are intended to clarify the mechanisms of biological phenomena in humans,
female rats may not be a suitable model for investigating the effects of ERT on developing or
young adult bone. On the other hand, the changes in BW and densitometric parameters
associated with ERT in male rats were similar to those in humans, indicating that male rats
are a suitable model for investigating the effects of endurance running.
4. Effects of endurance training on bone biomaterial properties
Aside from BMD and BMC, biomaterial related analysis will provide more valuable
information to predict the capacity that bone tissue can withstand extra mechanical loading
generated by daily physical activity or accidents (e.g. fall), and thus, prevent bone from
loading-induced damage.
Generally, biomaterial properties of bone tissue can be analyzed at a structural level and a
tissue level. Structure biomaterial properties are size-dependent, that is, tissues bigger in
size tend to be stronger than smaller ones. Conversely, tissue-level properties are analyzed
under size-independent conditions using mathematic methods (e.g. normalized tissue size
by cross-sectional moment of inertia) or mechanical methods (e.g. a specimen with
consistent size is sectioned from a whole tissue).
75
4.1 Effects on structural (whole bone) properties
Structural properties are calculated from original biomechanical testing raw data without
any normalization. Related parameters are load (Nt), displacement (mm), energy (mini
joule, mJ) and stiffness (Nt/mm) etc. As shown in previous studies, results of rodents'
whole tissue biomechanical properties after a period of endurance running training were
controversial. Some studies show that exercise groups were higher in load-withstanding
capacity,
51-53
while others revealed a higher energy absorption capacity.
55
One possible
explanation for these discrepancies may be differences in training protocol. Animals trained
at a higher intensity tended to show higher bone strength (e.g. higher bending load or
moment),
51, 53
suggesting that higher mechanical loading generated by intensively running
may benefit to bone strength. Moreover, the specific testing conditions would also affect
testing results; for instance, Nordsletten and colleagues measured bone strength
in vivo
, at
which time bone strength may be affected by muscle strength gains achieved through
training.
53
