The role of mineral in the storage of elastic energy in turkey tendons

Mammals elastically store energy in leg and foot tendons during locomotion. In the turkey, much of the force generated by the gastrocnemius muscle is stored as elastic energy during tendon deformation and not within the muscl e. During growth, avian tendons mineralize in the portions distal to the mu scle and show increased tensile strength and modulus as a result. The purpo se of this study was to evaluate the viscoelastic behavior of turkey tendon s and self-assembled collagen fiber models to determine the molecular basis for tendon deformation. The stress-strain behavior of tendons and self-ass embled collagen fibers was broken into elastic and viscous components. The elastic component was found to be to a first approximation independent of s ource of the collagen and to depend only on the extent of cross-linking. In the absence of cross-links the elastic component of the stress was found t o be negligible for self-assembled type I collagen fibers. In the presence of cross-links the behavior approached that found for mineralized turkey te ndons. The elastic constant for turkey tendon was shown to be between 5 and 7.75 GPa while it was about 6.43 Gpa for self-assembled collagen fibers ag ed for 6 months at 22 degreesC. The viscous component for mineralized turke y tendons was about the same as that of self-assembled collagen fibers aged for 6 months, a result suggesting that addition of mineral does not alter the viscous properties of tendon. It is concluded that elastic energy stora ge in tendons involves direct stretching of the collagen triple-helix, nonh elical ends, and cross-links between the molecules and is unaffected by min eralization. Furthermore, it is hypothesized that mineralization of turkey tendons is an efficient means of preserving elastic energy storage while pr oviding for increased load-bearing ability required for locomotion of adult birds.