4-bar-linkage model, single fibre ligaments. Diagrammatic sketches of the single-degree-of-freedom mechanism in the sagittal plane as predicted by the geometrical the 4-bar-linkage model. The geometrical arrangement of the passive structures is shown in three joint positions: at 20° plantarflexion (left), neutral (central) and 10° dorsiflexion (right). The kinematics is guided by the isometric rotation of the CaFi and TiCa ligaments (solid bold). The articular surfaces (the arcs nearly in contact), the line contact, i.e. the common normal CN at the single contact point, the other ankle ligaments (buckled segments), and the instantaneous centre of rotation IC (empty circle) are also depicted.