Age dependence of mechanical properties of rat tail tendons (hysteresis experiments).

Mechanical parameters of tail tendons have been studied depending on maturation and age using male Sprague Dawley rats at an age of 1, 2, 4, 12 and 24 months. Ultimate values, such as ultimate strain, ultimate load, tensile strength, and ultimate modulus of elasticity, were the highest at an age of 12 months. In old rats these values were slightly lower. Using tail tendons with the same diameter from the same rats hysteresis experiments have been performed at extension degrees of 2, 4, 6 and 8% respectively. Stress at the end of the loading phase was the highest at 4 months for 2 and 4% extension and at 12 months for 6 and 8% extension. Between 12 and 24 months no significant difference was found. Modulus of elasticity at the end of the loading phase showed a sharp increase due to maturation reaching the maximum at 4 months at 2 and 4% extension and at 12 month at 6 and 8% extension. Thereafter a decrease was noted as it was for ultimate modulus of elasticity. Energy input during the loading phase increased during maturation. This increase was less pronounced for energy dissipation indicated by the unloading phase. As a consequence, the ratio between energy dissipation and energy input decreases significantly during maturation, however, during aging merely a slight decrease is noted. Residual extension at the end of the unloading phase is considerably decreased during maturation. Between 12 and 24 months only minor changes of this parameter were noted. In conclusion the parameters describing the hysteresis phenomenon in rat tail tendons are considerably influenced by maturation but only to a minor degree by aging. In this phenomenon indicating plasticity no reversal in the direction of age dependent changes has been observed. The comparison of the results of the hysteresis experiments in rat tail tendons with similar experiments performed previously in rat skin shows similarities as well as significant differences between both tissues.