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T he knee is the most common area of pain in runners, with the patellofemoral joint frequently the joint of dysfunction (10). In adolescents, patellofemoral pain syndrome is the leading cause of chronic knee pain (2). Atrophy of the m t u s medialis oblique muscle is frequently associated with patellofemoral pain syndrome, possibly as a result of the interaction between mechanical and neuromuscular factors (8,12,17). This atrophy is thought to result in extensor mechanism dysfunction, decreased muscle strength, and imbalance in the control between medial and lateral portions of the quadriceps femoris muscle (21). Traditionally, quadriceps strengthening exercises have been used in the rehabilitation of patellofemoral pain syndrome, and more recently, "closed chain" quadriceps strengthening, or quadriceps strengthening in a weight-bearing position, has become an accepted method of treatment (14).Backward running incorporates the "closed chainn concept of quadriceps exercise. Backward running has been documented as increasing quadriceps strength and power (1 1,20). In addition, at self-selected running speeds during the stance phase of backward running, the quadriceps are active for a longer period of stance than during forward running (7). It appears that backward running may be indicated in the rehabilitation of patellofemoral pain syndrome, but the magnitude of the

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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.