Muscle ultrastructure and contractile properties were examined before and after a single bout of resistance exercise (8 sets of 8 repetitions at 80% of 1 repetition maximum). Eight untrained males performed the concentric (Con) phase of arm-curl exercise with one arm and the eccentric (Ecc) phase with the other arm. Needle biopsies were obtained from biceps brachii before exercise (Base), immediately postexercise from each arm (post-Con and post-Ecc), and 48 h postexercise from each arm (48 h-Con and 48 h-Ecc). Electron microscopy was used to quantify the presence of disrupted fibers in each sample. Analysis of variance revealed a greater (P < or = 0.05) proportion of disrupted fibers in post-Con, post-Ecc, 48 h-Con, and 48 h-Ecc samples compared with Base. Significantly more fibers were disrupted in post-Ecc (82%) and 48 h-Ecc (80%) samples compared with post-Con (33%) and 48 h-Con (37%), respectively. Voluntary and evoked strength measurements recovered to Base values within 24 h in the Con arm but remained depressed (P < or = 0.05) for 72-96 h in the Ecc arm. These data indicate that both the raising and lowering phases of weightlifting produced myofibrillar disruption, with the greatest disruption occurring during the lowering phase.

Pain, stiffness, and indicators of muscle damage occur at different times after eccentric muscle action. After a single bout of maximal resisted lengthening of the elbow flexors, elbow position, pain perception, and indicators of cellular damage were measured. Immediately postexercise, a significant decrease in resting muscle length was observed that continued to 48 h. At this time, an increase in perceived muscle soreness was noted (P less than 0.05), and a biopsy of the biceps brachii revealed mast cell degranulation, separations of the extracellular matrix from myofibers, and increased plasma constituents in the extracellular space. It is proposed that myofiber disruption allows intracellular proteins to escape and extracellular proteins and ions to enter, causing swelling, whereas the disrupted extracellular matrix initiates the inflammatory response, which includes the release of mast cell granules seen at 48 h postexercise. Thus the delayed sensation of pain (soreness) after repeated eccentric muscle actions probably results from inflammation in response to extracellular matrix disruption.

No abstract

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.