Patellofemoral Osteoarthritis (PFOA) group: 1) the presence of other conditions that may cause knee pain (e.g., patellar tendonitis, rheumatoid arthritis), 2) neurologic or musculoskeletal conditions that may alter lower extremity kinematics (e.g., multiple sclerosis, hip arthritis/arthroplasty), 3) history of a fracture of the knee, 4) pregnancy, 5) knee injections within the past 3 months, 6) inability to understand English.