OBJECTIVE The purpose of this investigation is to determine the functional performance of the mobile bearing total knee replacement prosthesis as compared to the fixed bearing type total knee replacement prosthesis. DESIGN Kinematics, kinetics, and electromyography data were gained from 10 patients with mobile bearing and 10 patients with a fixed bearing posterior stabilized Insall Burstein II total knee replacement during ascending and descending stairs. A control group of 10 normal subjects, matched by sex and age, was also analysed. BACKGROUND No significant biomechanical differences in patients with different total knee replacement designs have been reported from level-walking studies. Slightly better performance of posterior retaining with respect to cruciate sacrificing total knee replacement designs have been claimed from stair climbing studies. Only one study has been conducted regarding mobile versus fixed bearing total knee replacement assessed by gait analysis. This study did not show any biomechanical differences between the two groups. METHODS Motion analysis was used to quantify the knee kinematics, kinetics, and electromyography (right and left longissimus dorsi, gluteus medius, rectus femoris, biceps femoris, semitendinosus, gastrocnemius and tibialis anterior muscles) during stair ascent and descent. RESULTS The mobile bearing group demonstrated a reduced knee extensor moment during stair climbing and descending, and a reduced knee adductor moment during stair climbing. When ascending stairs, most of the mobile bearing patients show a peak knee flexion and a peak knee flexion moment at the late stance phase during the double support period. This kinematic and kinetic pattern is absent in normal subject. Both mobile bearing and fixed bearing groups showed abnormal electromyography patterns in both descending and ascending. CONCLUSIONS During stair climbing, the mobile bearing design demonstrates a different kinematic pattern to the fixed bearing total knee replacement. Lower limb compensatory mechanisms seemed to be adopted particularly by the mobile bearing patients during ascending stairs. RELEVANCE Total knee replacement patient with mobile bearing design can feel excessive femoro-tibial motion during daily living activities such as stair climbing and descending. Proprioceptive control of this tibio-femoral translation is needed as demonstrated by the lower limb compensatory mechanism. This data suggest that antero-posterior constraint structures (ligamentous or mechanical) are important to obtain reproducible knee kinematics.