Impact of repetition number on muscle performance and histological response

Purpose: We investigated the effects of increasing stretch-shortening contraction (SSC) repetition number in vivo and the resulting changes in functional performance and quantitative morphometry in rat skeletal muscle. Methods: Functional testing was performed on the ankle dorsiflexor muscles of Sprague-Dawley rats, which were randomly exposed to 30 SSC, 70 SSC, 150 SSC, or 15 isometric contractions of equal duration. Changes in functional performance and muscle morphometry were assessed at 48 h after exposure. Stereology was used to quantify the volume density of degenerative myofibers and normal myofibers in the tibialis anterior muscle from each group, as well as measures of inflammation and swelling and changes in the interstitial space. Results: At 48 h there was a significant decline in isometric force for the 70- and 150-SSC groups (P < 0.05 and P < 0.05, respectively). Stereological measures indicated significant decreases in the percentage of volume density of normal myofibers in the 70- and 150-SSC groups (P < 0.05). Measures for percentage of volume density of degenerative myofibers and inflammation were increased (P < 0.0001 and P < 0.05, respectively) in the 70- and 150-SSC groups. Moreover, a significant increase in the percentage of volume density of degenerative myofibers in the 150-SSC group compared with the 70-SSC group was observed (P < 0.05). Conclusion: These data strongly suggest that exposure to increasing SSC repetitions results in increased functional decrements and morphometric indices of myofiber degeneration and inflammation, and that there is an apparent threshold (repetition number) at which this occurs.