The paper presents a comparative experimental study of flexion-extension movement in normal and osteoarthritic human knee. Measurements were performed on a group of seven healthy subjects and on a group of five patients with OA knees, for which experimental data were acquired for walking cycles on treadmill. Using an electrogoniometer-based acquisition system, the data were collected during the experimental gait on a treadmill with the speed equal to 3.6 km/h. The flexion angle during the gait cycle revealed differences with respect to flexion magnitude between the OA patients and the healthy subjects group. Statistical analysis demonstrated that the knee flexion angles were significantly different for experimental measurements of the OA patients' knees and healthy knees, but, also, the healthy knees of the patients were on average less flexed than gait cycle of the healthy subjects.