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To compare hamstring to quadriceps muscle coactivity during level walking, stair ascent, and stair decent between individuals with and without knee osteoarthritis.In a cross-sectional study, subjects with grade II knee osteoarthritis (n = 26), healthy age- and gender-matched (n = 20) and healthy, young adults (n = 20) performed three activities of daily living. During the stance phase of these activities surface electromyography was measured. Two coactivity ratios were computed, the biceps femoris to vastus lateralis ratio and the ratio of the biceps femoris EMG activity relative to the EMG activity measured during contraction- and velocity-specific maximal voluntary biceps femoris contraction, i.e., biceps femoris to maximal biceps femoris activity.Subjects with knee osteoarthritis had significantly higher coactivity than age-matched healthy adults and young adults and healthy adults had more coactivity than young adults regardless the type of coactivity ratio. The biceps femoris to vastus lateralis ratio yielded 25% higher coactivity value than the biceps femoris to maximal biceps femoris ratio (P < 0.0001). The EMG activity of the vastus lateralis relative to maximal vastus lateralis EMG activity was 92% in subjects with knee osteoarthritis, 57% in age-matched controls, and 47% in young adults (P < 0.0001).Patients with knee osteoarthritis revealed increased hamstring muscle activation while executing activities of daily living. Altered muscle activation at the knee may interfere with normal load distribution in the knee and facilitate disease progression. Therapeutic interventions should focus not only on quadriceps strengthening but also on improving muscle balance at the knee.