Background: Quantification of the in vivo position of the medial condyle throughout flexion is important for knee replacement design, and understanding knee pathology. The influence of consciousness, muscle action, and activity type on condyle translation was examined in patients who had undergone medial unicompartmental knee replacement (UKR) using lateral video fluoroscopy. Methods: The position of the centre of the femoral component relative to the tibial component was measured for 9 patients under different conditions. The following activities were assessed; passive flexion and extension when anaesthetised, passive flexion and extension when conscious, active flexion, extension and step-up. Results: The position of the centre of the femoral component relative to the tibial component was highly patient dependent. The greatest average translation range (14.9 mm) was observed in anaesthetised patients, and the condyle was significantly more anterior near to extension. Furthermore, when conscious but being moved passively, the femoral condyle translated a greater range (8.9 mm) than when moving actively (5.2 mm). When ascending stairs, the femoral condyle was more posterior at 20-30 degrees of flexion than during flexion/extension. Conclusions: The similarity between these results and published data suggest that knee kinematics following mobile-bearing UKR is relatively normal. The results show that in the normal knee and after UKR, knee kinematics is variable and is influenced by the patient, consciousness, muscle action, and activity type. Clinical relevance: It is therefore essential that all these factors are considered during knee replacement design, if the aim is to achieve more normal knee kinematics.