Animation 2: Four-bar linkage sagittal place model of the knee, passive flexion

Animation 2 shows the knee joint represented as a four-bar linkage with the two bones and isometric fibres of the two cruciate ligaments acting as the four links. It is a model of passive flexion/extension. The instantaneous centre of the joint, through which the flexion axis passes, lies at the intersection of the two isometric fibres and moves backwards on the tibia during flexion and forwards during extension as the two ligament fibres rotate relative to each other and relative to the bones.