To understand the knee cap (the patella), first it helps to look at a picture of the movement of the joint. Watch this cool animated xray.
Now to explore the patella. Try this — stretch a leg out long and relax. Now ever so gently wiggle the knee cap. Then bend the knee. The knee cap becomes unmovable, and firmly anchors the upper and lower leg together.
That silly little knee cap offers the leg both great stability and flexibility…as we bend, straighten, bend, straighten the leg…to walk, run, sit, stand, move through our day.