In a human skeleton, the arms and legs have a pretty similar structure: one thick bone for the upper portion and two thinner bones running through the lower half of the limb. Our arms and legs also only bend in one direction. But then why do the joints in our legs have a cap, but our elbows don’t?

SciShow host Michael Aranda explains weight is the primary reason: “You don’t need elbowcaps because you don’t walk on your hands.” Our knees, on the other hand, are bearing our weight when we walk, “so they need a little extra help and protection.” The kneecap, formally known as the patella, is connected to tendons only — no bones — so in its position it works kind of like a lever, giving the quadriceps thigh muscle substantially more power as it straightens the leg.

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“The knee is the largest joint in the body and has to sustain the greatest stresses, since it supports the entire weight of the body above it,” Encyclopaedia Britannica says. It is a powerhouse among the more than 200 bones in the adult human body.

There are animals, however, that have “elbowcaps,” in a sense, such as bats.