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.
Other answer is right from an evolutionary view, but I read the question a bit differently, so my answer is a bit different.
We have kneecaps and not elbowcaps because we have kneecaps and we don’t have elbowcaps.
A “cap” in this case is a distinct bone, not just the hardness on a joint. You knee joint is made up of a femur, a tibia, a fibula, and a patella. The patella is a separate bone the forms the hard part of your knee.
The elbow is structured differently. Your arm has a humerus, a radius, and an ulna. The radius and ulna form your lower arm, the humerus is your upper arm. The elbow is a spur on the ulna.
So we don’t call it an elbowcap because we don’t have a distinct bone there.
Said a little differently, your lower body benefits more from structures that assist in stability (for load-bearing activities like standing and walking) while your upper body benefits more from structures that help with mobility/range of motion. Kneecaps have two main jobs: to protect the ligaments and tendons around your knee joint and to assist in extension (straightening your leg). Since your upper body joints are not as consistently load-bearing, and elbow extension is easier to achieve than knee extension (smaller = easier to move freely), your body wouldn’t benefit from ‘elbow caps’ even if the term sounds wonderful and hilarious.
Hope that helps!
The kneecap enhances the leverage the quadriceps muscle has on the lower leg by acting as a simple lever on which the quadriceps tendon rides. This allows the quadriceps muscle to perform the powerful movements the leg requires.
Simply put, the elbow doesn’t have the same requirements for massive muscular power to be exerted across the joint. The triceps has enough leverage the way it’s configured to meet the needs of our upper limbs without another structure to enhance it.
What does it look like if someone is missing a kneecap? Can someone live without a kneecap? When I imagine missing a kneecap, my brain melts. I don’t think I want to live missing a kneecap.
Edit: prior to reading this post, I read another post about why repeating words makes the word feel and sound weird.
Now kneecap doesn’t sound right.
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