Background: Running is a popular form of exercise and inherent to many sports. Running while fatigued has been associated with decrements in performance and increased risk of acute and overuse running injuries. However, the relative contributions to running mechanics from individual muscle(s) have not been clearly established and could help further elucidate risk factors and anatomical structure foci during training. This study’s purpose was to analyze alterations in kinematic, kinetic, and ground reaction force (GRF) variables with the onset of localized fatigue. It was hypothesized that knee flexor and extensor fatigue on separate occasions would increase impact forces, joint angles, joint moments, and powers compared to pre-fatigue values. Methods: Five healthy college-aged adults (2 males, 3 females: 23.60 +/- 1.14 years; 1.71 +/- 0.13m; 67.60 +/- 14.50kg) ran at 3.61m/s prior to and following isokinetic knee flexion and extension (concentric and eccentric) efforts for a total of three, two-minute runs. Motion capture and force data were used to calculate joint motion and loading throughout each run. Data were analyzed using RM-ANOVA evaluating kinematic and kinetic changes following fatigue of knee flexors and extensors for each run. Results: Maximum braking force significantly increased from immediate post-fatigue to twominutes post-fatigue (p=0.003; η2 p=0.677). Peak vertical GRF significantly (p<0.05) decreased from pre-fatigue (2.79 +/-0.09BW) to immediate post-fatigue (2.46+/-0.10BW) of the quadriceps. Propulsive knee power significantly (p<0.05) decreased from pre-fatigue (11.32+/- 1.74Nm/kg) to immediately post-fatigue (6.93+/-0.90Nm/kg) of the quadriceps. Knee abduction moments were significantly higher (p=0.001; η2 p=0.960) for running measures following hamstring fatigue over that of quadriceps fatigue. Discussion: Quadriceps fatigue showed the greatest pre-fatigue to immediately post-fatigue changes. Vertical impact peak force and propulsive knee power decreased significantly more immediately post- quadriceps fatigue than hamstring fatigue. Horizontal braking forces exhibited similar changes for both muscles over time, having significantly increased during the fatigued run.