Exercise-induced muscle injury and overtraining.

Objectives - The aim of this paper was to review the main features of exercise-induced muscle damage, The specific events that initiate exercise-induced muscle fiber injury and its diagnosis are discussed. A second aim of this review is to determine the relationship between muscle damage and overtraining. Topics - Prolonged heavy exercise is associated with muscle fiber injury. The damage varies with the intensity, the duration and the type of exercise. It has been well established that muscles are injured to a greater extent when exercice involves lengthening contractions (stretch of active muscle). When cell death occurs, the necrotic tissue is removed by phagocytic cells, satellite cells are activated, migrate to the injured area, become myoblasts and further muscle fibers. Several muscle-injury markers have been proposed for determining muscle damage, but all presently available parameters have limitations. Muscle enzyme release do not clearly reflect structural damage as estimated by histological analysis, Changes in these biological markers of muscle-injury (such as serum creatine kinase activity, CK, lactate dehydrogenase, LDH, or the malondialdehyde concentration in the plasma, MDA) are frequently examined during overtraining. Serum CK activity or plasma MDA concentrations may fail to increase during overtraining and changes in these biochemical parameters can only reflect a muscle overload. However, after prolonged exercise, pro-inflammatory cytokines are produced locally in the skeletal muscles. The production of pro-inflammatory cytokines is able to contribute to a decline in immune function and to an increased susceptibility to specific infections. Interestingly, immunity can also be adversely affected during overtraining. Conclusions - Exercise-induced muscle fiber injury results from a mechanical overload applied to skeletal muscle. However, the effects of such damage on immune function can constitute also one of the clinical features of overtraining. (C) 2001 Editions scientifiques et medicales Elsevier SAS.