Why do we have kneecaps on our legs but no “armcaps” on our arms?
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Since I’ve seen a lot of ELI10 answers, I’ll have a go myself.
First, we have to understand that arms and legs have different purposes.
Arms are generally for picking up things and reaching. Arms benefit more from extra range than they would from extra power.
Legs are the other way around, since their main purpose is supporting your body weight. Legs benefit more from extra power than they would from extra mobility, in general.
the kneecap, or the Patella, came into being through evolution to let our legs function more efficiently, in two ways.
The first way, is by providing something less prone to breaking tearing, which coincidentally also slides easier. This way, knees last way longer than they would without a kneecap, as the tendon that goes over top of the knee would wear out way faster because of all the friction.
The second way is a bit more physics based. The kneecap puts distance between the knee joint, and the part where the power is relayed through. By putting distance between the rotating point, and the point where force is applied, it becomes easier on the muscles to uphold your body weight, even though you didn’t have to increase your muscle mass to do so.
Throughout evolution, a kneecap was deemed favourable, so it was passed down and evolved into the kneecap we know and love to bump into things today.
In retrospect, it also says a bit more about the preferred characteristics of an arm. The absence of an “elbow cap” reduces power output, but allows the arm to reach a higher speed (speed and power are usually a trade-off pair within biomechanics).
Apparently, the ability to move arms swiftly was more favourable throughout evolution than it was to generate more power.
TL;DR: Arms have different functions than legs, and therefore different preferred characteristics. Evolution shaped the respective limbs for lighter and heavier tasks.
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