Hip joint simulators were developed for predicting, bu attempting to duplicate in vitro physiological loads and motion, the wear rate that total hip replacements are likely to show in vivo. From a theoretical point of view, loading and motion cycles of hip joints could be closely reproduced by three rotation actuators and three force actuators. However existing devices have been designed assuming that some of these degrees of freedom are negligible, in order to reduce the complexity of the equipment. The present study singles out some preliminary indication on the design choices regarding the spatial configuration of loading and motion actuators. The aim is to define theoretically a simplified simulator but still to apply the most physiologically realistic loading cycle to the specimen. Copyright