Total knee replacement (TKR) is commonly used to reduce pain and improve function in patients with advanced, symptomatic knee osteoarthritis (OA). While more than 80% of patients undergoing TKR report improved pain and functional status, growing evidence suggests that post-TKR physical activity (PA) does not surpass pre-TKR levels. Given the substantial time and financial investment associated with TKR and the rich evidence supporting health benefits of PA, the effectiveness and cost-effectiveness of TKR could be meaningfully enhanced if TKR recipients would be more physically active. The period following acute TKR rehabilitation provides a unique window of opportunity for these patients to transform their lifestyles. This proposal addresses whether introducing behavioral strategies that incorporate both intrinsic and extrinsic motivators can help TKR recipients engage in sustained, meaningful PA. Telephonic active coaching with motivational interviewing (TAC(MI)) uses regular conversations to resolve ambivalence and identify means of overcoming barriers to PA. Financial incentives (FI) address the temporal delay between completing healthy behaviors and receiving long-term health benefits by offering immediate rewards contingent on activity and performance. With these behavioral, psychological and economic principles in mind, we propose KArAT (Knee Arthroplasty Activity Trial). The long-term objective of this research is to determine the most effective and cost-effective behavioral interventions post-TKR to help patients develop a sustained commitment to becoming physically active. We plan to conduct a three-arm parallel RCT to establish the efficacy of personalized intervention built on the principles of behavioral science and behavioral economics in improving PA among patients who have undergone TKR. The three arms will include: Arm 1: Usual Care (UC); Arm 2: Attention Control (AC); Arm 3: Telephonic Active Coaching (Motivational Interviewing) + Financial Incentives (TAC(MI)+FI). In general, RCTs focus on either treatment-specific effect or total treatment effect. In this application, we propose a design that will permit us to estimate both. By including a ‘usual care’ arm, we will be able to estimate the overall effect of the intervention, which is relevant to estimating the value of the intervention and understanding the impact on clinical practice. The primary outcome will be the proportion of individuals engaging in at least 150 minutes per week of moderate-to-vigorous physical activity (MVPA) at ≥3 METs by the end of the six-month intervention. Change in average daily step count from pre-TKR to the end of the six-month intervention; change in weekly minutes of MVPA; reduction in sedentary time; and sustainability of efficacy at 12, 18 and 24 months post-TKR will be secondary outcomes. The results from KArAT will help clinicians, patients and policymakers make evidence-based decisions about improving PA after TKR, one of the most common ort...