In several applications, such as the design and setting of prostheses, orthoses and exoskeletons, and the multibody modelling of the lower limb, it is sometimes necessary to approximate the spatial motion of human joints to a rotation about a fixed axis. The identification of these axes can be 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). Thus, the ankle requires two distinct axes to be identified in order to correctly describe the joint motion. A new technique is proposed here for the identification of the tibio-talar and talo-calcaneal rotation axes. The technique is based on a particular use of the Burmester theory and exhibits several advantages: the talus motion is not required, and only the tibia-calcaneus motion is needed; the identification of both rotation axes is simultaneous and thus the identification accuracy of both axes 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 with respect to experimental inaccuracies; it makes it possible to obtain the talus motion, even if it could not be measured during the experimental session. The technique reveals particularly useful during in vivo measurements based on skin markers or other non-invasive measures, since the talus motion cannot be obtained from skin measurements. An application example is shown by means of experimental data measured on an ankle specimen.

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