Rotation axis identification at the human tibio-talar and talo-calcaneal joints by the Burmester theory

A correct identification of the human joint rotation axes is important in several applications, such as prosthesis and exoskeleton design, and multibody modelling of the human body. This identification is particularly difficult at the ankle joint, where two different articulations are observed, namely the tibio-talar joint (connecting the tibia and talus) and the talo-calcaneal joint (connecting the talus and calcaneus). The motion of both these joints could be approximated to some extent with a rotation about a single axis; thus, the ankle requires the identification of two distinct axes in order to correctly describe the joint motion. A new method is proposed in this study for the identification of the tibio-talar and talo-calcaneal rotation axes that does not require the measure of the talus motion. This feature is particularly important for in vivo measurements, since non-invasive experimental techniques do not allow the talus motion to be obtained, due to the inaccessibility of this bone. The method also exhibits other advantages: the rotation axes are identified simultaneously and thus their identification accuracy is independent; the method is not based on optimization techniques and thus it does not require the definition of an objective function; it is robust; it makes it possible to obtain the talus motion. The theoretical bases of the method are presented and a case study is used to assess the potentiality of the identification procedure.