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 min (n = 8) or level treadmill walking for 30 min 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 0 D = 160.2 ± 17.4; P < 0.001) and 2-day (DHR 2 D = 177.1 ± 20.9; P = 0.002) reductions in MVC torque after DHR, with no change observed after level walking (P > 0.769). Compared with 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 and 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 postexercise. These findings suggest that novel eccentrically biased exercise induces unequal strength loss and soreness across the quadriceps muscles.NEW & NOTEWORTHY Following 60 min of downhill running, the vastus lateralis exhibited greater low-frequency force deficits than the vastus medialis. The vastus lateralis had greater soreness compared to the other quadriceps muscles at two days postexercise. EMG RMS data revealed reduced muscle activation during maximal voluntary contractions and increased activation during running. These findings suggest both muscular and neural contributions to strength deficits following unaccustomed exercise. Evidence indicates differential muscle damage in agonist muscle groups following eccentrically-biased exercise.