Downhill Running Differentially Reduces Stimulated Contraction Torque in Human Quadriceps Muscles

Unaccustomed exercise can lead to focal myofiber damage and substantial reductions in maximal joint torque, ranging from 20% to 50%. However, the uniformity of strength loss across individual muscles within agonist muscle groups remains unclear. The primary aim of this study was to test the hypothesis that individual quadriceps muscles (QMs) experience differential strength loss and soreness following downhill running (DHR). Fifteen healthy, sedentary, or recreationally active male subjects aged 18-35 engaged in either DHR on a treadmill for 60 minutes (n = 8) or level treadmill walking for 30 minutes as a control (n = 7). Measurements were taken before, immediately following, and 2 days after exercise, assessing maximal voluntary contraction (MVC) torque of QMs, torque produced by individual muscles through electrical stimulation, soreness of individual QMs, QMs electromyography root mean square (RMS) during level running and MVCs. Results revealed significant immediate (DHR Pre = 214.5 ± 19.2; DHR 0D = 160.2 ± 17.4; p < 0.001) and 2-day (DHR 2D = 177.1 ± 20.9; p = 0.002) reductions in MVC torque after DHR, with no change observed after level walking (p > 0.769). Compared to the vastus medialis (VM), evidence suggests that the vastus lateralis (VL) exhibited greater 10-Hz stimulated torque deficits (p = 0.028) and soreness (p < 0.001) after DHR. Reduced muscle activation (R² = 0.444; p < 0.001) and stimulated (10 & 40 Hz) torque depression (R² = 0.253-0.278; p < 0.01) accounted for the greatest variability in MVC torque. The DHR group showed greater (p < 0.001) quadriceps muscle activation (RMS) during level running than the control group immediately post-exercise. These findings suggest that novel eccentrically biased exercise induces unequal strength loss and soreness across the quadriceps muscles.