Biomedical Engineering Reference
In-Depth Information
previous animal studies. Briefly, the results of animal studies using rodents as subjects
showed gender differences, which might affect their further applicability in human subjects.
As mentioned above, intensity-trained runners usually have a lower body mass and often
have equal or even lower bone mass than non-athletes. Therefore, an animal model of ERT
would be expected to reveal the same phenomena. Animal studies reviewed in the present
article were selected according to training type (typically endurance treadmill training) and
the age of animals (growing or young adult rats).
3.1 Rodents adapted to endurance exercise showed gender differences
3.1.1 Male rodent studies
Table 2 summarizes the outcomes of studies using male rats as subjects. The studies were
reviewed and classified into two categories. The first category includes those using diet
control or adjustment to achieve equivalent BW gains between exercise and control
groups.
42-47
These studies demonstrated that trained animals have a higher BMD.
43-45
Tissue
mechanical properties were not available in every study, and only one of them shows a
higher load-withstanding capacity in the femoral diaphysis.
43
However, it must be
mentioned that diet prohibition for the purpose of equalizing body weights among groups
might cause an additional negative effect on tissue mechanical properties. Diet prohibition
impairs the tissue levels (intrinsic) and mechanical properties of bone, suggesting that
dietary manipulation of a control group might not be appropriate.
48, 49
The second category of studies included animals fed
ad libitum
. In these studies, the exercise
groups revealed significantly less BW gain after a programmed ERT.
50-55
With lower BW,
exercise trained animals showed no difference or lower BMD values as compared to the
sedentary control group. As in human subjects, male rats undergoing intense ERT exhibit
lower BW gain and no benefits to bone health when considering BMD or BMC as a
predictor. However, the higher load-withstanding and energy-absorption capacity of the
bones in training rats introduced new research into how endurance exercise benefits bone
quality (see section 4).
3.1.2 Female rodent studies
Compared with the treadmill training results for male rats, those for female rats are
inconsistent with human subjects, and the data are somewhat controversial. Table 3
summarizes the results of ERT in growing or young adult female rats. Most studies indicate
that female growing or young adult rats exhibit no change in BW after a period of ERT.
56-64
One study even reported increased BW in female rats after training.
58
Of the studies
performing BMD analysis in female rats, many report positive effects from endurance
running not only in site-specific increments but also in whole bone. Although densitometric
measurements demonstrate this advantage, female rats rarely show improved
biomechanical properties and may even reveal adverse effects after an intense training
program (see Table 3). Therefore, female rats acclimate to ERT differently than do male rats.
In human beings, distance runners are also expected to exhibit gender differences in
physiological response to similar ERT. However, it seems inappropriate to use the gender
difference found in rats to explain the one found in humans, since a period of programmed
ERT would commonly reduce BW either in women or men. Thus, the phenomena observed
in female rats may not be applicable to female humans. Based on the idea that animal
models should mimic the phenomena shown in human subjects, studies using female rats
may not be applicable to female humans, since female rats and women have been shown to
respond differently to ERT. Possible reasons are discussed in the next section.
