OBJECTIVE:To determine if the electromyographic onset of vastus lateralis and kinematic knee joint motion in individuals with knee osteoarthritis (OA) differs from that of asymptomatic persons, during the task of stair stepping. DESIGN:Cross-sectional. SETTING:University laboratory in Australia. PARTICIPANTS:Twenty-five participants with symptomatic knee OA and 33 asymptomatic controls. INTERVENTIONS:Not applicable. MAIN OUTCOME MEASURES:Surface electromyography to determine the onset of vastus lateralis activity. Two-dimensional kinematic analysis of sagittal plane knee motion during stance phase of stair ascent and descent. RESULTS:Participants with knee OA showed delayed onset of vastus lateralis activity during stair descent (P<.05) but not ascent. Both groups displayed a similar total range of knee motion. However, during stair descent, participants with OA had less knee flexion during early stance (P<.05) than controls. CONCLUSION:Individuals with knee OA display altered quadriceps function and knee joint kinematics during stair descent. These impairments may have implications for force attenuation across the knee joint and warrant future investigation.