The purpose of this research is to develop and test the feasibility of an exercise intervention for persons with painful knee osteoarthritis involving the patella (kneecap). Participants in the patellofemoral osteoarthritis (PFOA) group will be treated with a 6-week supervised exercise program that targets strengthening the hip and trunk core muscles. Painfree control participants will attend one session to gather biomechanical, strength, and function data for use in comparison to the patellofemoral osteoarthritis group. Feasibility outcomes include adherence to the treatment program, recruitment, and retention. It is hypothesized that PFOA group participants will change score on a knee pain and function questionnaire by the minimum clinically important difference score for positive improvement at the end of the 6-week intervention program.

The investigators propose to develop and test the feasibility of a 6-week supervised hip and core muscle exercise intervention for persons with painful patellofemoral osteoarthritis (PFOA), a common condition for middle-aged and older adults. Currently there is limited evidence regarding effective conservative interventions for PFOA, a chronic condition that is painful and limits function and quality of life. Exercise is recommended as a first-line treatment for knee osteoarthritis, but the optimal type of exercise for PFOA is unknown. This intervention is based upon similar programs shown to improve pain and function in younger persons with patellofemoral pain, a condition that may lead to PFOA. Feasibility outcomes include adherence to the treatment program, recruitment, retention, and the impact of the intervention on participants' pain and function at 6-weeks and 6 months after finishing the program. The investigators will also compare PFOA participants to a matched pain-free volunteer control group to explore the differences in their biomechanics, muscle strength, pain report, and function at baseline. Data from this feasibility study will be used to design a future randomized controlled trial investigating the efficacy of conservative exercise interventions for PFOA. The investigators' overarching goal is to develop a conservative exercise intervention that improves the symptoms, function, and quality of life of persons with PFOA.

Inclusion Criteria:

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.

Control group: 1) age 35-70 years, 2) no known diagnosis by a physician of knee osteoarthritis, 3) pain free in the lower extremities at the time of enrollment into the study and for 1 week prior to enrollment.

Exclusion Criteria:

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.

Control group: 1) all previously listed exclusion criteria for the PFOA group, 2) knee pain produced by 2 or more of the following: descending steps, ascending steps, sit-to-stand, squatting, kneeling, prolonged knee flexion, or a history of patellar subluxation or dislocation in the past.