Purpose: Knee osteoarthritis (OA) is the most common cause of disability in older adults, and difficulty with climbing stairs is one of the most frequently-reported functional limitations in adults with knee OA. Adequate lower body strength and functioning are important in the ability to climb stairs. However, little is known about how the frequency of climbing stairs may change over time, and to what extent lower body strength and functioning is related to such change. Understanding what trajectories of stair use frequency may exist and how lower body strength and functioning are related to these trajectories are important to predict future patterns of change and to identify potential treatment targets that may positively influence change.