Influence of physical exercise on aging rats. III. Life-long exercise modifies the aging changes of the mechanical properties of limb muscle tendons.

We have previously shown that long-term regular physical exercise has a systemic influence on the rat by slowing the aging of its connective tissues, measured as thermal stability and biomechanical properties of tail tendons. This paper analyses whether the properties of limb muscle tendons are influenced not only by the aging process and the systemic effects of exercise but also from direct mechanical stimuli from long-term physical exercise. Male Sprague-Dawley rats were trained in a treadmill from the age of 5 to 23 months. The effects of training on muscle tendons were analyzed with respect to biomechanical properties. Also, the viscoelastic activation energies for interactions between collagen and the proteoglycan gel as well as between collagen fibrils were measured. Finally the asymptotes from the creep curves were calculated in order to estimate the magnitude of the viscoelastic creep. The effects of aging were analyzed with respect to the same parameters by comparing the group of 23-month-old sedentary rats with a 5-month-old baseline group. The biomechanical parameters did not change significantly with physical exercise. Neither did the activation energies change, but the asymptotes of the creep curves decreased, showing that there was less viscoelastic creep. Aging rendered the tendons significantly stronger and stiffer, increased the energy-absorbing capacity and decreased the strain values. The activation energies did not change with aging, but the high creep curve asymptote for the flexor tendons decreased. We conclude that aging rendered both types of tendons stiffer, and decreased their strain values at breaking point. Aging also increased the stress value, the energy absorption and the dry weight for the flexor tendon. Further, while physical exercise has a systemic delaying effect on age changes in connective tissues, in tendons subjected to substantial mechanical loads this effect as measured with biomechanical methods is counteracted by the optimization process elicited by the same physical exercise.