The majority of adults with arthritis in the United States do not meet the federal physical activity guidelines of 150-300 minutes/week of moderate-to-vigorous intensity physical activity (MVPA). Adults with arthritis face numerous barriers to engaging in regular MVPA and seek specific recommendations for activities from healthcare providers. A major gap remains in identifying the minimal dose of activity necessary to see clinical improvements in arthritis-related outcomes. Lower doses of activity could lead to improvements in arthritis- related symptoms and may be a more feasible, initial goal for adults with arthritis who have difficulty engaging in activity at the recommended levels. Further, shorter bouts of MVPA could result in fewer short-term negative effects on arthritis-related symptoms as compared to higher daily durations of activity. Therefore, the purpose of this study is to identify whether a dose-response relationship exists between objectively-measured MVPA and arthritis-attributable outcomes in adults with arthritis. The first aim is to examine the effect of 3 doses of MVPA (45 min/week, 90 min/week, and 150 min/week) on arthritis-attributable outcomes (e.g., physical function, health-related quality of life, pain, and depression symptoms) in adults with arthritis at 6 and 12 months. The second aim is to examine the between-group differences in momentary levels of fatigue, pain, depressive symptoms, confidence, and happiness after engaging in MVPA, using ecological momentary assessment at 6 months. Inactive adults with arthritis (n=285) will receive a 6-month adaptive physical activity intervention. Participants will be randomized to receive one of 3 doses of MVPA: (1) 45 min/week, (2), 90 min/week, or (3) 150 min/week. MVPA goals will begin lower and gradually increase towards the 45, 90, or 150 min/week goal every 2 weeks. Participants will be encouraged to spread the MVPA out over at least 3 days. To help participants achieve goals, they will be provided with the Arthritis Foundation’s Walk With Ease Guidebook, behavioral lessons, monthly behavioral telephone coaching, and a Fitbit. If a participant achieves less than 80% of his/her goal for a given week, or does not track his/her activity for the week, the participant will be stepped up to receive additional intervention components (e.g., text message/email, biweekly or weekly coaching calls). Objectively-assessed MVPA and arthritis-attributable outcomes will be obtained at the baseline, 6-, and 12-month assessments. Momentary arthritis-attributable outcomes will be obtained through ecological momentary assessment at 6 months. Results from this randomized controlled trial will provide insight on whether adults with arthritis can experience benefits from engaging in less physical activity than what is currently recommended. Additionally, gaining a better understanding of how long and short durations of activity influence momentary symptoms will provide clinical and public heal...