Subjects: Twenty recreational runners participated (mean age of 24.9 years). Methods: Kinematics and kinetics of the trunk and lower extremity were obtained at 3 conditions: level, 6° uphill, and 6° downhill, at a speed of 2.3 m/s. PFJ stress was determined using a biomechanical model that incorporates knee flexion angle and knee extensor moment as subject-specific input variables. The model output consisted of PFJ reaction force, PFJ stress, and PFJ contact area. One-way ANOVAs with repeated measures and post-hoc t-tests with a Bonferroni adjustment were used to compare outcome variables across the 3 conditions. Results: Peak PFJ stress during downhill running was significantly higher than the level and uphill conditions (P \u3c 0.001). There was not a difference in peak PFJ stress between level and uphill conditions (P = 1.000). Conclusion: The higher stress observed in downhill running was driven by an increase in PFJ reaction force as the result of elevated knee extensor moment and decreased trunk flexion angle. The similar stress level observed in level and uphill running was caused by similarities in PFJ reaction force and minimal differences in PFJ contact area between the 2 conditions. Clinical Relevance: As downhill running increases peak PFJ stress when compared to level and uphill running, alterations in running slope should be considered when treating runners with PF
