Early events in stretch-induced muscle damage

Unaccustomed exercise involving stretch of active muscle at long length cau ses an immediate loss of tension-generating capacity, a shift of optimum le ngth, and changes in excitation-contraction coupling. Eventually, fiber dam age may be observed, resulting in pain and tenderness. The subject of this review is the early stage in this process, particularly the cause of the im mediate drop in tension. There is strong evidence pointing to sarcomere len gth instabilities and nonuniformities as important contributors to these ch anges. The evidence includes the influence of initial length, electron micr oscopy of rapidly fixed active fibers, the shift in optimum length in singl e fibers, and the effects of training on sacomere numbers. Experiments usin g Ca2+-sensitive dyes clearly show changes in excitiation-contraction coupl ing, but cross-species comparisons indicate that these are not always able to explain the consequences seen. We conclude that sarcomere length instabi lities provide the most comprehensive explanation of the early consequences of eccentric exercise.