Eli5: Why do knees have kneecaps, but elbows dont have elbowcaps?
In: Biology
Many animals actually do have elbowcaps, including many lizards, frogs, birds. Elbowcaps are rare in mammals, but bats sometimes have them. In humans, the bit of bone at the very tip of your elbow is called an “olecranon”. The olecranon is usually attached to a bone called the “ulna”, that runs from the elbow up your forearm to the wrist on the pinkie side. However, in some people, it actually isn’t part of the ulna, and is a free floating elbow cap. When this happens in humans, it is called “patella cubiti”.
The reason that humans usually have a knee cap but not an elbow cap isn’t totally clear. Both the olecranon and the kneecap serve to increase the leverage with which your muscle. It may be that having this be a free-floating bone (as in the typical human knee) rather than attached to another bone (as in the typical human elbow) is better for exerting high amounts of force, as one needs to do in order to walk and run. Human arms don’t need to be nearly as strong as legs, and so don’t need a free-floating elbow cap.
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