Pain and weakness are prominent symptoms which occur after a delay in muscles which have been stretched during contraction (eccentric contraction). These symptoms are particularly severe when the exercise is unaccustomed and when the stretch occurs in muscles on the descending limb of the force-length relation, i.e. at long muscle lengths. It is known that sarcomeres are potentially unstable on the descending limb and it has been proposed by Morgan that uncontrolled elongation of some sarcomeres occurs during eccentric contractions on the descending limb. In this article, the evidence that this mechanism leads to the reduced force is considered. If overextended sarcomeres persist after the eccentric exercise it will cause a shift in the peak of the force-length curve. There is also evidence that in some types of muscle, excitation-contraction coupling is impaired and contributes to the muscle weakness. Cytoskeletal proteins stabilize the sarcomeric structure and may be injured either by the overextended sarcomeres or by activation of proteases. The potential of these mechanisms to contribute to the effects of muscle training and to the symptoms of muscle disease, such as muscular dystrophy, is considered.