Veterans face a high prevalence of knee osteoarthritis (OA), but current diagnostic methods often miss early stages when interventions are more effective. This project will evaluate smartphone-based motion capture via OpenCap to measure joint mechanics in knee OA patients during functional activities, comparing its performance to a conventional motion capture system, patient-reported symptoms, and knee joint structure. The findings will have the potential to enable clinicians to trial OpenCap in its current form, provide insights into tracking joint health, and guide refinements to advance toward earlier diagnosis of knee OA by complementing symptom assessments with measures of joint mechanics.