OBJECTIVE: The study objective was to quantify the association between daily physical activity measured by accelerometer and 1-year changes in symptoms among people with knee osteoarthritis. METHODS: Participants from the Osteoarthritis Initiative had knee radiographs and physical activity assessed using GT1M ActiGraph (Pensacola, FL) uniaxial accelerometers at the 48-month visit. Physical activity was calculated and categorized as tertiles of average daily minutes in light and moderate-to-vigorous activity. Outcomes were 1-year change in symptoms measured by Western Ontario and McMaster Universities scales, including pain, stiffness, and physical function. Adjusted multivariable linear models estimated the relationship between tertiles of light or moderate-to-vigorous physical activity and changes in knee symptoms. RESULTS: Among 1059 participants (55% were women; mean age, 66 +/- 9 years), greater time in light activity was associated with a trend toward declined physical function (P = .01). Greater time in daily moderate-to-vigorous activity also was associated with declined physical function (P = .01) and increased pain (P = .08). None of these average changes in symptoms reached minimally important clinical differences. However, greater daily time in both activities was associated with a higher probability of worsening symptoms among persons with Kellgren-Lawrence grade 4 osteoarthritis. CONCLUSIONS: Objectively measured daily activity was not associated with 1-year symptom improvements among community-dwelling adults with knee osteoarthritis. In those with advanced disease (Kellgren-Lawrence grade 4), greater daily minutes in physical activity were associated with worsening symptoms. How best to implement exercise regimens in persons with advanced knee osteoarthritis to reduce the deleterious impact on symptoms needs to be explored.