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