Fibrosis and intercellular collagen connections from four weeks of muscle strains.

The effect of repeated cycles of muscle strain was studied in the soleus muscle of female rats. Muscle strains were repeated 3X/week for 1 month using two different strain protocols. Striking changes, including marked variability in fiber size, evidence of degradation and regeneration, and an expanded extracellular matrix were pronounced in the fast-stretched muscles but not in the slow-stretched muscles. However, the slow-stretched muscles did contain struts of connective tissue joining adjacent myofibers. Therefore, repeated muscle strains at high strain rates produced morphological changes similar to many myopathies, including fibrosis, whereas adaptation occurred in response to the same number of strains at slow strain rates. Such diverse tissue responses have relevance to the understanding of the mechanisms of skeletal muscle dysfunction in cumulative trauma disorders and in the design of preventive actions and treatments.