Mechanical properties of human tendon and their age dependence.

There are no previously published data on changes in the mechanical behaviors of human tendon from maturation in the second decade to senectitude in the seventh decade or thereafter. In this study, 44 tendons from individuals ranging in age from 16 to 88 yr were subjected to an extensive series of mechanical tests which included preconditioning, extensions at strain rates of 100 percent/s, 1 percent/s, and 0.01 percent/s, and stress relaxation with cyclic and constant extensions. Pairs of extensions at 1 percent/s were run throughout the protocol to evaluate the repeatability of tissue response. It was found that these responses changed little for any single sample within a pair of such tests; however, throughout the protocol, the peak stresses and moduli decreased. Extensions at different rates revealed a definite rate dependency of tendon responses with sample modulus being directly related to extension rate and slightly less hysteresis at 1 percent/s than at 100 percent/s or 0.01 percent/s. The load relaxation in samples subjected to either cyclic or constant extensions was generally best described by a linear function of the logarithm of time. The rate of relaxation with constant extension varied little with extension magnitude. The rate of relaxation in the cyclic tests was greater at 10 Hz than at 0.1 Hz. The results indicate that subject age has no effect on tendon modulus and a very small effect on hysteresis and relaxation. Extensive information on subject history was not available in this study for correlation with mechanical responses so that an age effect may have been masked by other variables, possibly health, diet, disease, or exercise.