ABOUT THE KNEE: The knee is made up of three bones designed to produce smooth, stable motion: the shinbone (tibia), the thighbone (femur) and the kneecap (patella). The bones are enclosed in the joint capsule, which is lined with a tissue that produces a thick liquid to keep the joint lubricated and nourished. The knee is kept in alignment by ligaments and tendons. The anterior cruciate ligament (ACL) is one of two major stabilizing ligaments of the knee joint. The other one starts at the back of the knee -- the posterior cruciate ligament. These two ligaments cross each other at the center of the knee. There is another set of stabilizing ligaments on either side of the knee as well, which stabilize the joint when the knee moves from side to side.
HOW WE WALK: Walking is different from a running gait because only one foot at a time lifts off the ground. During forward motion, the leg that leaves the ground swings forward from the hip, like a pendulum. Then the leg strikes the ground with the heel and rolls through the toe in a motion similar to an inverted pendulum. The motion of the two legs is coordinated so that one foot or the other is always in contact with the ground -- a so-called 'double pendulum' strategy. The process of walking recovers about 60% of the energy expended thanks to the pendulum dynamics and the ground reaction force. (The legs act as long levers that transfer ground reaction force to the spine.)
The Human Factors and Ergonomics Society contributed to the information contained in the TV portion of this report.