Contraction-induced injury results in the degeneration and regeneration of muscle fibers. Of the three types of contractions--shortening (concentric), isometric, and lengthening (eccentric)--injury is most likely to occur and the severity of the injury is greatest during lengthening contractions. The magnitude of the injury to muscle fibers may be assessed by direct measures of cellular and ultrastructural damage; by indirect measures of changes in enzyme efflux, calcium influx, ratio of oxidized to reduced glutathione, and force development; and, in human beings, by reports of muscle soreness. The sequence of events includes an initial injury that is primarily mechanical and a secondary metabolic, or biochemical, injury that peaks 1 to 3 days after the injurious contractions. The recovery from contraction-induced injury is usually complete within 30 days. Repeated exposures to protocols of lengthening contractions result in "trained" muscles that are not injured by the protocol that previously caused injury.