Why do we have kneecaps on our legs but no “armcaps” on our arms?
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Your elbow acts as an arm cap. Your knees and elbows have all of your tendons running over them. Their placement allows for the tendons to apply a torque force that let’s you straighten your arms and legs.
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Your elbow is basically a kneecap for your arms. Your ulna (forearm bone) has a curved end that basically looks like a knee cap when it attaches to your humerus (upepr arm bone). Your leg bones dont have this curved end so thats why we have the extra patella bone. Both serve the same purpose: it lets tendons stretch across them so you can convert a stretching linear force into a hinging motion.
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Kneecaps allow to have a lot more torque when the leg is fully folded.
This is great because when you are squatting at the point of sitting on the floor, you need a lot of strength in your knee to be able to stand up with just your legs. You may notice it’s pretty hard with just one leg, this requires a lot of torque. The kneecaps are crucial in allowing for this.
So why not having this on the elbow? Well, those have a cost: they make the articulation less precise as there are more moving components. So elbows have fused the cap with the forearm, that’s why you forearm is pointy. It’s not as effective, and you will have really little force when your elbow is fully bent, but in exchange you get higher rigidity and precision on your arms. It’s likely that without this rigidity we would be much worse in term of accuracy.
But in general it’s hard to find a definite reason for not having something. Sometimes it’s just nature haven’t yet stumbled on the solution and the current mechanism is good enough.
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There are good answers in the thread, so I’m just going to share [this interesting video about how kneecaps work.](https://youtu.be/XnYO4TnpTCo)
Our forearms, the area between the elbow and the wrist, need to be able to rotate, like when we are turning a door knob. Another way to think of this motion is pronation (making your palm face downwards) vs supination (making your palm face upwards). An “armcap” or elbow cap would get in the way of pronation/supination.
The feet and shins do not need to rotate in this way, we are unable to make our toes point behind us for example, therefore it makes sense to have a more stable and rigid joint where the knee cap can act as a fulcrum or lever for more powerful movement.
Your elbows are hooked to make it easy to hang from trees. Your knee have bones in the tendons to make them stronger when they they stretch around your bending knee.
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