Exercise-induced injury to skeletal muscle.

Strenuous or unaccustomed exercise can cause injury to skeletal muscle. This paper reviews our understanding of the mechanisms of exercise-induced injury. Measurements of exercise-induced injury have included muscle soreness, increased serum levels of intracellular enzymes, increased lysosomal enzyme activities, structural changes in muscle fibers, and prolonged decreases in force development that cannot be attributed to fatigue. Injury can be induced by exercise of small muscle groups, which suggests that it involves processes localized in skeletal muscles. Exercise of relatively short duration can result in injury, which indicates that long durations of exercise and associated metabolic changes are not necessary for injury to occur. Exercise that involves lengthening contractions results in greater evidence of muscle injury than exercise involving isometric or shortening contractions. Lengthening contractions are associated with higher levels of force and lower metabolic costs per muscle fiber than isometric or shortening contractions. These results suggest that changes in muscle metabolism are not responsible for exercise-induced injury to skeletal muscle. Exercise-induced injury is more likely the result of mechanical disruption of muscle fibers.