Project Summary/Abstract The number of individuals undergoing total knee replacement (TKR) each year continues to rise. While TKR is effective for improving pain and function, subsequent improvements in physical activity are not common. As a result, patients spend most of their day engaged in sedentary behavior, which may put them at higher risk of experiencing poor function and disability, as well as lower the overall success of the surgical treatment. Intervening on sedentary time, rather than physical activity, may be a more feasible first-step approach for modifying activity-related behaviors in this population. Therefore, the purpose of this innovative clinical trial is to use TKR as a teachable moment for implementing a sedentary reduction intervention. We propose to use a just-in-time mobile health (mHealth) intervention to reduce sedentary time among TKR patients. Patients (n=92) scheduled for TKR will be recruited to participate prior to surgery and then randomized at 4 weeks after surgery to either (1) NEAT!2 or (2) Control. NEAT!2 participants will use the NEAT!2 (Non-Exercise Activity Thermogenesis version 2) smartphone app until 3 months after surgery. The NEAT!2 app is designed to provide a vibration and/or audible tone to interrupt prolonged bouts of sitting detected from the smartphone’s internal accelerometer. Further, NEAT!2 has been adapted based on TKA patient input and program preference. NEAT!2 participants will receive biweekly coaching calls between 4 and 12 weeks after surgery. Control participants will receive an education control app and receive non-intervention calls to assess general recovery. Both groups will receive monthly maintenance calls between 3 and 6 months. Data collection will occur pre-operatively, as well as at the end of treatment (3 months after TKR) and the maintenance assessment (6 months after TKR). The primary aim is to examine the effects of the NEAT!2 intervention on sedentary time at end of treatment (3 months) and maintenance (6 months). Secondary aims include examining the effects of the NEAT!2 intervention on physical function, physical activity, and pain as well as examining the dose response relationship between adherence to NEAT!2 and changes in outcomes. The proposed study will examine the innovative approach of decreasing sedentary time prior to targeting physical activity in adults after TKR and determine if a remotely-delivered, mHealth sedentary reduction intervention can decrease sedentary time. The study is innovative for integrating a just-in-time technology-based approach to reduce sedentary time during a “teachable moment” for TKR patients. The results of this study will build on our previous and current work to improve health behaviors after TKA and help to identify potentially effective and scalable strategies to improve long-term physical activity behaviors and physical function in patients after TKR.