Biomedical Engineering Reference
In-Depth Information
However, in considering the applicability of exercise, a training program with moderate
exercise intensity would be expected to show a higher compliance and therefore be more
appropriate for the general population. As shown in Table 2, animals trained at a relatively
moderate intensity (20-24m/min), which corresponds to 70%
‧
O
2
max,
76
also had lower
body masses and slightly lower (~5% lower) total BMD (p = 0.04), but were not found to
have enhanced structural bone strength.
55
The authors' previous study used body mass as a
covariate to equalize raw data, which then revealed a comprehensively stronger bone tissue
either in structural or tissue-level biomaterial properties.
52
With lower body mass, the data
of the exercise group would be adjusted to a higher level, and the effects of ERT seemed to
become “good” for animal bones. However, it would be a more relevant and natural study if
animals were fed ad libitum and data were not adjusted
In aspects of biological efficiency, an athlete at her/his optimal physiological status will not
necessarily be absolutely higher in every physiological parameter. Therefore, a smaller
muscle mass or skeleton size seems to be a benefit, rather than a weakness, for a distance
runner or an endurance athlete. With such a smaller bone size, moderate ERT rats did not
show absolutely enhanced structural bending load values but, interestingly, they showed
better energy absorption capacity in long bone tissue that ERT rats were found to have a
four-fold increase in energy absorption after long bone tissue reached the yield-point (post-
yield energy).
55
As mentioned in previous studies,
41
post-yield behaviors are highly
correlated to tissue-level changes (e.g. collagen fiber orientation).
5. Effects on tissue-level (material) properties
In our previous studies, we used mathematical methods to estimate tissue-level biomaterial
properties. Through calculating long bone's cross-sectional moment of inertia, we
normalized load-displacement data to stress and strain. Under such conditions, ERT rats'
worse structural material properties disappeared. Additionally, exercise and control groups
showed no differences in yield stress, yield toughness or elastic modulus (Young's
modulus),
55
suggesting that endurance training is not harmful for bone material properties.
ERT's benefits on the post-yield biomaterial behaviors seemed to be more size-independent
and associated with tissue-level (e.g. bone matrix, collagen) changes. Because measuring
post-yield mechanical properties using beam bending theory is only valid in the pre-yield
regime,
75
reporting post-yield stress, strain or toughness is inappropriate. Therefore, we
discussed this tissue-level adaptation base on the post-yield parameters measured from
load-displacement data. As shown in our two recent studies, either moderate ERT or
endurance swimming training benefits bone tissue more in terms of energy absorption
capacity,
55,77
especially in post-yield energy. Similar results of enhanced post-yield behavior
were provided by another ERT animal study, which showed a short-term treadmill running
(21 days) enhanced tibia post-yield deformation in mice.
78
Moreover, such effects on post-
yield behavior changes seem to apply not only to endurance training. After a short-term (5
days) freefall landing exercise, Wistar rats revealed an increased post-yield energy
absorption in ulnae.
79
Such an enhanced absorption capability is more likely due to tissue-
level (e.g. bone matrix, collagen orientation etc) changes rather than structural adaptation.
As mentioned in previous studies, tissue-level properties can be divided into the inorganic
mineral phase (e.g., hydroxyapatite), which determines tissue stiffness and strength,
80, 81
and the organic bone matrix, which plays a key role in energy absorption,
82, 83
It has been
suggested that the networks of collagen, one of the major components of bone matrix, could
affect the energy dissipation between the yield point and fracture point in bone tissue.
84-86
