The purpose of this study was to analyze changes in the levels of matrix metalloproteinase-9 (MMP-9), tissue inhibitor of metalloproteinase-1 (TIMP-1), and transforming growth factor-β1 (TGF-β1) in response to strength recovery after eccentric exercise in humans. In this study, 16 healthy males participated and were divided into a faster recovery (FR) group (n = 8, >80% of baseline) and a slower recovery (SR) group (n = 8, <40% of baseline) on the basis of their recovery of maximal isometric strength (MIS) 96 h after eccentric exercise. For both groups, measurements were taken of muscle soreness, creatine kinase (CK) activity, and MMP-9, TIMP-1, and TGF-β1 levels during the 24- to 96-h period after eccentric muscle contraction of their non-dominant elbow flexor. Muscle soreness (p < 0.001), CK activity (p < 0.01), and TGF-β1 level (p < 0.01) were significantly lower in the FR group compared with SR group, whereas no significant differences in MMP-9 and TIMP-1 levels were found between the two groups (p > 0.05). These results suggest that scar tissue formation caused by the pro-fibrotic activity of growth factors such as TGF-β1 is a potential cause of delay in strength recovery after exercise-induced muscle injury.
Keywords: eccentric exercise; muscle injury; scar tissue; strength recovery; transforming growth factor-β1.