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

Recovery from chronic strain injury (50 strains daily, five times weekly fo r 6 weeks to hyperactive soleus muscles) was followed for 3 months in femal e rats after cessation of chronic hyperactivity induced by pretreatment of the plantar flexor muscles with tetanus toxin. After 6 weeks of repeated st rains, muscle mass decreased by 62%, myofiber areas were reduced by 87%, an d noncontractile tissue expanded dramatically by 222%. Collagen content inc reased by almost ninefold (control 40 +/- 3 mug/mg, chronic injury 392 +/- 53 mug/mg), whereas the molar ratio of collagen (pyridinoline) crosslinks t o 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 m yofiber areas remained smaller by 21%, noncontractile tissue was still mark edly elevated by 18% with increased collagen content (107 +/- 15 mug/mg), a nd the molar ratio of crosslinks to collagen increased by 75% during recove ry. Thus, rat soleus muscles recovered very slowly and incompletely from ch ronic strain injuries that produced muscle fibrosis, highlighting the neces sity of devising preventative strategies for repeated strain injuries. (C) 2000 John Wiley & Sons, Inc.