The purpose of this study was to compare the electromyographic (EMG) activity of the quadriceps femoris (QF) muscles of women with osteoarthrosis (OA) of the knee with the EMG activity of the QF muscles of healthy women. To this end, a total of 14 subjects attended a single test session during which full-wave rectified surface electromyograms were recorded from the rectus femoris (RF), vastus lateralis (VL) and vastus medialis (VM) muscles during maximum voluntary isometric quadriceps contractions performed at knee angles of 30 degrees and 60 degrees flexion. A comparison of the resultant normalized peak EMG ratios using independent t-tests demonstrated significantly higher composite QF muscle activation for the osteoarthrotic subjects (p < 0.05). A higher RF activation also differentiated the two groups. The findings raise the possibility of a direct correlation between abnormal activation of the QF and joint status. They also suggest that patients with knee OA may benefit functionally from rehabilitation designed to improve QF coordination and endurance, and not merely strength.