STAUBER, W.T., V.K. FRITZ, D.W. VOGELBACH, and B. DAHLMANN. Characterization of muscles injured by forced lengthening. Med. Sci. Sports Exerc, Vol. 20, No. 4, pp. 345-353, 1988. Myofiber injury-repair was studied in rat soleus muscles to elucidate the role of infiltrating cells in the injury-repair process. Muscle injury was induced by forced muscle lengthening with the contralateral muscle serving as a control. The muscles were removed for histologic, histochemical and immunohistochemical procedures at varying periods (12-120 h) post-injury. All injured muscles were severely damaged with many cells present in the interstitial spaces between myofibers. Normal appearing myofibers demonstrated elevated lysosomal proteolytic activity, but no evidence of increased activity, indicative of phagocytic cells, was found in or between damaged myofibers. The esterase stain for macrophages and immunohistochemical techniques for mast cells also provided no support for either cell type predominating in the damaged area, although mast cell degranulation could be observed in the pericapillary regions. In contrast, the use of a specific antisera for a multicatalytic protease uniquely defined most of these cells as myogenic in origin. They appeared to be most numerous between the torn ends of a myofiber. Surprisingly, the remainder of the cells appeared to be of lymphoid origin.