Development of the Fitness Improvement and Tracking Tool (FITT) app has been approved for funding by the Digital Health Initiative (DHI). The project aims to develop an mHealth intervention to increase physical activity (PA) levels in people with knee osteoarthritis (OA) that is responsive to the severity and prevalence of poor sleep, fatigue, and depressed mood; prioritizes patient preferences; is scalable; and is rooted in evidence-based approaches for long-term behavior change. The Fitness Improvement and Tracking Tool (FITT) is designed with input from people with knee OA, monitors PA, provides feedback on levels of PA to users, and gathers patient-reported outcomes. The goal is to enhance FITT to incorporate a social cognitive theory (SCT) based intervention framework to increase and sustain PA in people with knee OA who have poor sleep, fatigue, and depressive symptoms. The intervention algorithm will allow messaging to be personalized to an individual’s goals and outcome expectancies; barriers to participation; and prevalence and severity of poor sleep, fatigue, and/or depressive symptoms.