Patellofemoral Osteoarthritis group: 1) age 35-70 years, 2) have been diagnosed with knee osteoarthritis in one or both knees with symptoms primarily in the anterior knee, 3) radiographic evidence of ≥ Grade 1 osteophytes or joint space narrowing at the patellofemoral joint according to the Osteoarthritis Research Society International (OARSI) grading scale (0 = none, 1 = mild/possible, 2 = moderate/definite, 3 = severe), 4) pain produced by ≥ 2 of the following: descending steps, ascending steps, sit-to-stand, squatting, kneeling, prolonged knee flexion, increased activity (e.g., hiking), morning stiffness < 30 minutes, stiffness after sitting ≥ 30 minutes, history of patellar subluxation or dislocation in the past.