The material presented has been designed to offer an approach to the diagnosis and treatment of chronic tendinitis in the runner. Although there is a paucity of experimental data dealing with the behavior of the muscle-tendon unit during lengthening contractions as compared with the volume that exists concerning shortening contractions, results indicate that force increases with length from both eccentric and concentric contractions. The literature also suggests that eccentric contraction is a means by which muscles can maximize their force production while minimizing time delays and energy expenditure. Using this information in our clinical experience, we have developed an exercise program to treat chronic tendinitis. Pain, the cardinal sign of tendinitis, is used as both a way to classify the severity of the tendinitis and a yardstick for the progress of treatment.