Effects of repeated lengthening contractions on skeletal muscle adaptations in female rats

We examined the adaptation of plantar flexor muscles of female rats to 6 weeks (5 days/week) of lengthening contractions. After repeated lengthening contractions, a decrease in myofiber area of gastrocnemius medialis (26%) was accompanied by an increase in extracellular matrix (ECM) (42%) and collagen content (30.9%) without changes in muscle mass. Decrease in myofiber area (13%) and muscle mass of soleus (19%) was associated with increased collagen content (28%) and ECM (15%). Relative number of soleus myofibers stained for fast myosin increased by 26%. For plantaris, increases in collagen content (32.3%), percent ECM (17%), and myofiber area (6%) were recorded. We also observed (1) increases (3.3%) in the collagen content of the Achilles tendon, (2) no change in the crosslink content of any of the tissues tested, and (3) no difference in the force-frequency relationship of the plantar flexor muscles. Substantial decreases in myofiber areas with increases in muscle connective tissue by 6 weeks of repeated lengthening contractions did not appear to result in isometric force loss.

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.