1 A Three-dimensional Multibody Model of the Human Ankle-Foot Complex T. M. Malaquias 1 , S. B. Gonçalves 1 and M. Tavares da Silva 1 1 IDMEC, Instiuto Superior Técnico, Universidade de Lisboa, Portugal, e-mail:{tiago.malaquias, sergio.goncalves, miguelsilva}@tecnico.ulisboa.pt Abstract. In this work, a three-dimensional foot and leg model is proposed, which combines six rigid bodies and five kinematic pairs to produce an articulated system of four segments (toes, mid-forefoot, rearfoot and leg) with thirteen independent degrees-of-freedom. The model is described using a multi- body dynamics formulation with natural coordinates and estimated anthropometric parameters. A mass- less link is used to model the anatomical offset between the talocrural and talocalcaneal joints, avoiding the inclusion of a specific rigid body to model the talus bone. The reliability of the model was tested through its application to the study of the kinematic and dynamic patterns of the human gait. A male subject was analyzed and his data compared with literature, enabling to attest the consistency and realism of the proposed model. Key words: Multi-segment Foot Model, Multibody Dynamics, Kinematics, Kinetics, Gait Analysis 1 Introduction One of the major issues in the analysis of human movement is modelling the ankle- foot complex. It can be described as a multi-joint structure, which is responsible for supporting and propelling the body during locomotion. Therefore, a proper model- ing of this apparatus is essential to understand its kinematics and dynamics, enabling also the study of possible dysfunctions that occur at this level. Several foot models, with a variable number of rigid bodies and degrees of freedom (DOF), have been proposed whose topological complexity varies essentially with the goal of the study [7]. Mono and two-segmental foot models are recurrent in literature [1–3], present- ing high computational efficiency, however they are inefficient to analyze all the singularities that occur at ankle-foot and tarsus level. More complex models, such as the three-segmental foot model purposed by Delp et al. [6] enable the under- standing, with more reliability, of the dynamics of the normal and pathological foot. Other foot models with a higher number of segments have also been applied only in the kinematic analysis of the foot [7]. Despite allowing the perception of other DOFs, they are overly complex, restricting their application in full-body models and in dynamics simulations, which is one of the goals of the present model.