Context: Osteoarthritis (OA) is the most common chronic joint condition. Muscle dysfunction plays a critical role in the pathogenesis of knee OA. Objective: It has been suggested that the agonist-antagonist strength relationship for the knee may be better described by a functional hamstring/quadriceps (H/Q) ratio (Hconcentric/Qeccentric: the representative of knee flexion and Qeccentric/Hconcentric: the representative of knee extension). Therefore, in this study, the authors aimed to investigate this ratio and its importance for knee OA. Design: Cross-sectional study. Setting: Research clinic. Patients or Other Participant(s): Twenty healthy women and 20 women with grade 2 or grade 3 primer knee OA between the age of 50 and 80 years were included in this study. Intervention(s): Concentric and eccentric peak torque of quadriceps and hamstring muscles were evaluated for all individuals in patient and control groups with a Cybex isokinetic device. Functional H/Q ratio is calculated manually. Main Outcome Measure(s): Functional H/Q torque ratios were analyzed between the patients with OA and healthy individuals by using the isokinetic system. Results: The values of peak torque of hamstring concentric and eccentric and quadriceps concentric for the patient group were significantly lower than the control group (P < .05). No statistically important difference was found for quadriceps eccentric peak torque between 2 groups (P > .05). H/Q ratio for extension in the patient group was significantly higher than the control group (P < .05), whereas the H/Q ratio for flexion in the patient group was significantly lower than the control group (P < .05). Conclusion: This study showed the weakness of both quadriceps and hamstring muscles in patients with knee OA. The combination of functional H/Q ratio with hamstring and quadriceps muscles concentric and eccentric strength values can help to analyze the knee functions and develop preventive-therapeutic approaches for knee OA.