Stiffness and pain occurring after eccentric exercise have been studied in human elbow flexor muscles. Increased muscle stiffness and flexion deformities of the elbow developed immediately after the exercise and were greatest 1-4 days later. Muscle tenderness and pain experienced during elbow extension developed more slowly but were both maximal at the same time as the muscle stiffness. EMG recordings at times when there was pain and flexion deformity showed the biceps to be electrically silent. This demonstrates that the pain was not due to sustained electrical activity in the muscle and the flexion was a consequence of shortening of non-contractile elements, presumably the connective tissue. It is suggested that some response to damaged connective tissue may cause increased mechanical sensitivity of muscle receptors which, in turn, gives rise to pain when the muscle is stretched or pressed.