Survivor rates of unicompartmental knee arthroplasty (UKA) vary, and reasons for this phenomenon are not clear. Biomechanically, knee joint kinematics affect physical function and long-term survivorship. Therefore, the goal of the first study was to compare, the in-vivo tibio-femoral kinematics displayed during a step-up task stair ascent of lateral (LAT) and medial (MED) UKA users. The purpose of the second study was to compare the biomechanics of LAT- and MED-UKA users to control (CON) participants for stair ascent. Fourteen MED-UKA and eight LAT-UKA (N = 22), participants were recruited in the first study. Twenty-six UKA participants (17 MED- and 9 LAT-UKA) participated in the second study, and 26 healthy matched participants were recruited. UKA participants received a CT scan of the operative knee, and the knee kinematics during step-up motion was measured using videofluoroscopy system in the first study. For Study 2, all participants performed stair ascent, and kinematics and kinetics were calculated. One-way ANOVA tests were used to examine differences between MED- and LAT-UKA groups in the first study. Paired t-tests were used to examine differences between medial and lateral condyles in each UKA group in the first study, and were used to examine differences between MED/LAT-UKA and MED/LAT-CON groups in the second study.In general, the predictions that LAT-UKA would display different tibio-femoral kinematics than MED-UKA during step rising and that both UKA groups would display differences for kinematics and kinetics compared to their control groups for stair ascent were not supported. This was likely due partly to small sample size and inter-individual difference for some variables. However, both MED- and LAT-UKA individuals displayed knee kinematics that were mostly typical when compared to values of healthy knees from the literature (Study 1) and their respective control groups values (Study 2).Based on findings of both studies, both MED- and LAT-UKA appear to indicate that both UKA groups demonstrated typical knee biomechanics. Furthermore, current results might suggest that MED-UKA individuals show some slightly better biomechanical outcomes than LAT-UKA individuals.