However, Hull [4] pointed out that to produce valid results, the axes in each body should be related to the functional axes of the motion. If not, crosstalk will occur, meaning that there will be displacements determined for the motion which are a result of incorrectly placed axes, rather than being valid motions themselves. As an example, the axis of exion–extension occurs about a line joining the centers of the circles of the lateral and medial condyles; the axis of internal–external rotation is about a vertical axis through the medial side of the tibia. Axes in the femur and tibia which are displaced from these functional axes will result in displacements of the tibia and femur which are misleading. For example, if the vertical tibial axis is cen­tral in the tibia, it will result in AP displacements on the medial side, whereas in a normal knee, the medial contact is close to being constant. This requirement, that axes xed in the femur and tibia should align with the functional axes, can be applied to the healthy intact knee, but even then, the functional axes are not constant through the motion, but will change during the exion range. Furthermore after the implantation of an articial knee, the functional axis in the tibia may well be in the center for a standard type of total knee, but on the medial side for a medial pivot type. Hull pointed out these aspects in his discussion. That said, studies of three­dimensional motion involving 6 parameters of motion, 3 displacements, and 3 rota­tions should attempt to dene the axes in the locations indicated by Hull, whether using the engineering denitions, or the Grood–Suntay system.