1. Exercise that involves stretching a muscle while active cause microscopic areas of damage, delayed onset muscle soreness and adaptation to withstand subsequent similar exercise. 2. Longer muscle lengths are associated with greater damage and recent animal experiments show that it is the length relative to optimum that determines the damage. 3. In humans, walking down stairs, taking two at a time, increases the length of the muscle during the lengthening and increases the delayed onset muscle soreness. 4. The observed pattern of damage is consistent with explanations based on sarcomere length instabilities. 5. The pattern of adaptation is consistent with the number of sarcomeres in series in a muscle being modulated by exercise, especially the range of muscle lengths over which eccentric exercise regularly occurs.