Several potential advantages exist for cementless tibial fixation including preservation of bone stock and increased longevity of fixation. However, clinical results have been variable, with reports of extensive radiolucent lines, rapid early migration, and aseptic loosening. The primary stability of an implant depends on the micromotion of the bone-implant interface, which depends on the kinematics and kinetics of the replaced joint. Finite element analysis was used to examine the micromotion for different activities (walking, stair ascent, stair descent, stand-to-sit, and deep knee bend) for three commercially available tibial tray designs. Similar trends were observed for all three designs across the range of activities. Stair ascent and descent generated the highest micromotions, closely followed by level gait. Across these activities, the mean peak (maximum) micromotions measured across the entire resected surface ranged from 64 to 78 (186-239)μm for PFC Sigma, 61-72 (199-251)μm for LCS Complete Duofix, and 92-106 (229-264)μm for LCS Complete. The peak micromotions did not necessarily occur at the peak loads. For instance, the peak micromotions for level walking occurred when there were low axial forces, but moderate varus-valgus moments. This highlights the need to examine the whole gait cycle to properly determine the initial stability of tibial tray designs. By exploring a range of activities and interrogating the entire resected surface, it is possible to differentiate between the relative performance of different implant designs.