Biomedical Engineering Reference
In-Depth Information
Fig. 4.8 Homeostatic response of ligaments and tendons to physical activity and in vivo loading
(reproduced, with permission, from [ 61 ])
specimens; however, ultimate elongation was not found to be significantly different
between groups. No significant difference was found in the cross-sectional area or
wet or dry weight between groups. Similarly, no statistically significant difference
was found between the stress-strain relationships of exercised and control
specimens. However, despite the lack of change in the stress-strain curve, a large
increase in the ultimate load was found, while tensile failure at the tendon insertion
to bone occurred. These data suggest that exercise has a positive effect on the
strength of bony insertions.
The differences in the effects of exercise on the extensor compared to flexor
tendons is likely due to their anatomical location and function. The flexor tendon
has an inherent high strength and resiliency, and thus is unlikely to hypertrophy.
Additionally, it is constrained by and must glide within a sheath, so there is little
room for additional growth. In contrast, the extensor tendon does not experience
spatial limitations and has a different biochemical composition that may be favor-
able for increased collagen production in response to stress [ 20 ].
Based on the results of studies of joint immobilization and exercise, a highly
nonlinear curve describes the relationship between load and activity level and the
biomechanical properties of ligaments and tendons (Fig. 4.8 ). The left region of the
curve represents a rapid reduction in tissue properties and mass caused by immobi-
lization; in contrast, the right region of the curve shows only a slight increase in
mechanical properties following exercise. Thus, despite the striking penalty found
by immobilization after injury, long-term exercise regimens seem to provide only
moderate improvement in tendon properties [ 11 ]. Further, this concept has led to
the approach of using controlled passive joint motion as a therapy for
ligament injuries [ 21 ].
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