sophisticated kind of four-bar mechanism because the cruciate ligaments are kept taut by the rolling action of the bones. In order for the cruciate ligaments to be kept under the right tension, the four-bar mechanism must produce a motion which is exactly compatible with the curved profile of the bones. When a mechanical engineer looks at the anatomy of the human leg, the four-bar mechanism in the knee stands out as one of the most important and impressive mechanisms. Despite this fact, the four-bar