INJURY TO SKELETAL-MUSCLE FIBERS DURING CONTRACTIONS - CONDITIONS OF OCCURRENCE AND PREVENTION

Contraction-induced injury results in the degeneration and regeneratio n of muscle fibers of the three types of contractions-shortening (conc entric), 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 asse ssed by direct measures of cellular and ultrastructural damage, by ind irect measures of changes in enzyme efflux, calcium influx, ratio of o xidized to reduced glutathione, and force development, and, in human b eings, by reports of muscle soreness The sequence of events includes a n initial injury that is primarily mechanical and a secondary metaboli c, or biochemical, injury that peaks 1 to 3 days after the injurious c ontractions. The recovery from contraction-induced injury is usually c omplete 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.