Recovery from 6 weeks of repeated strain injury to rat soleus muscles.

Recovery from chronic strain injury (50 strains daily, five times weekly for 6 weeks to hyperactive soleus muscles) was followed for 3 months in female rats after cessation of chronic hyperactivity induced by pretreatment of the plantar flexor muscles with tetanus toxin. After 6 weeks of repeated strains, muscle mass decreased by 62%, myofiber areas were reduced by 87%, and noncontractile tissue expanded dramatically by 222%. Collagen content increased by almost ninefold (control 40 +/- 3 microg/mg, chronic injury 392 +/- 53 microg/mg), whereas the molar ratio of collagen (pyridinoline) crosslinks to collagen remained the same (control 0.20 +/- 0.01, chronic injury 0.16 +/- 0.01). After 3 months of ambulation, muscle mass returned to normal but myofiber areas remained smaller by 21%, noncontractile tissue was still markedly elevated by 18% with increased collagen content (107 +/- 15 microg/mg), and the molar ratio of crosslinks to collagen increased by 75% during recovery. Thus, rat soleus muscles recovered very slowly and incompletely from chronic strain injuries that produced muscle fibrosis, highlighting the necessity of devising preventative strategies for repeated strain injuries.