We investigated localization of damage within the knee extensors (KEs) and plantar flexors (PFs) induced by downhill running (DR) by using transverse relaxation time (T2)-weighted magnetic resonance imaging (MRI). Fourteen young adults performed 45-min DR (-15% slope) at their maximal tolerable velocity. At pre-and 24, 48, and 72 h post-exercise, T2-MRI was scanned and T2 values for each muscle composing KEs and PFs at proximal, middle, and distal sites were calculated. Maximal isometric torque and rate of torque development (RTD: 0-30, 0-50, 0-100, 0-200 ms) were also measured. Maximal torque significantly decreased in KEs (14-17%) and PFs (6-8%) at 24-48 h post-exercise, with greater reductions for KEs. RTD in all phases, except for 0-200 ms in PFs, significantly decreased in KEs (11-42%) and PFs (13-23%) at least at one time point post-exercise. T2 significantly increased at several sites (3-5%) in both muscle groups at 24 and/or 48 h post-exercise. Among the T2-increased sites, the peak effect size (Cohen's d) regarding T2 change was pronounced at proximal (1.05) and middle (1.64) vastus intermedius compared to the other sites (0.72-0.77). These results suggest that DR induces damage in both KEs and PFs, and especially affects proximal-middle sites of the vastus intermedius.
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