Mechanisms of muscle injury after eccentric contraction.

Eccentric contractions of skeletal muscles produce injury and, ultimately, muscle strengthening. Current data suggests that the earliest events associated with injury are mechanical in nature and may be based primarily on the sarcomere strain experienced by the muscle. In this review, recent experimental data, primarily from rabbit dorsiflexor muscles, are used to provide general information regarding the factors that cause injury and means for preventing injury. Mechanical experiments reveal that excessive sarcomere strain is the primary cause of injury. We hypothesize that excessive strain permits extracellular or intracellular membrane disruption that may permit hydrolysis of structural proteins leading to the myofibrillar disruption that is commonly observed. Inflammation that occurs after injury actually further degrades the tissue, but prevention of the inflammation leads to a long-term loss in muscle function. Simple treatments such as increasing muscle oxidative capacity ("getting into shape") or cyclic stress-relaxation of tissue ("stretching out") have no measurable effect on the magnitude of muscle injury that occurs. Ultimately, an improved understanding of the damage mechanism may improve our ability to provide rehabilitative and strengthening prescriptions that have a rational scientific basis.