OBJECTIVES: To identify baseline characteristics that modified the effect of a 12-week quadriceps-focused (QE) versus hip muscle-focused (HE) exercise program on symptoms and physical function, through changes at baseline in the Anterior Knee Pain Scale (AKPS) in patients with PFP. DESIGN: A secondary analysis of a 26-week randomized trial involving 200 participants with patellofemoral pain. METHODS: Participants were randomly assigned to a QE or HE program with a duration of 12 weeks. The primary outcome was change from baseline in the AKPS at week 12 and week 26. Subgroups were predefined and based on baseline information: presence of low back, hip, ankle, or bilateral knee pain; body mass index (BMI); sex; age; education; occupation; hypermobility; quadriceps strength; dynamic knee alignment; midfoot mobility; exercise self-efficacy; pain self-efficacy; pain catastrophizing; neuropathic pain; pain duration; and pain severity. RESULTS: Participants with pain catastrophizing seemed to benefit from HE with a subgroup difference in treatment effect of 8.3 AKPS points at week 12 (95% CI 1.6 to 15.0). At week 26, participants with a baseline BMI above 25 seemed to benefit from HE with a subgroup difference in treatment effect of 11.1 (95% CI 4.8 to 17.4), and participants with severe knee pain at baseline seemed to benefit from QE with a subgroup difference of -9.1 (95% CI -15.7 to -2.6). CONCLUSION: Hip-focused exercises may provide more benefits than quadriceps-focused exercises among patients with patellofemoral pain and pain catastrophizing or overweight. Quadriceps-focused exercises may provide more benefits than hip-focused exercises for patients with severe knee pain.

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