Originally published at: http://boingboing.net/2017/01/31/why-we-have-kneecaps.html
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Why don’t we have elbow-caps then?
I suppose because even in mammalian quadripeds, the major locomotive force comes from the quadricep (or equivalent thereof), and the forelegs are basically just to keep us from falling face-first into the dirt.
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"Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.”
Or, at least, your foot.
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Might just as well be an accident. Knees and elbows evolved independently and nature happened to come up with two different working solutions. Once they had diverged it’s hard for evolution to jump from one type to the other.
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It’s also noteworthy that this is a little less leverage than it is gearing. Such arrangements multiply movement at the cost of leverage and control. An elbow built the same way as a knee would not enjoy it’s current level of fine motor control.
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I got lost in there for a moment.
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The main strength of the leg is a lengthening push (quad). The main strength of the arm is clenching pull (bicep). An elbow cap wouldn’t help with a pull.
Presumably, that pull strength (lifting things, climbing trees, gripping parents/babies) is evolutionarily more important than the push strength (punching).
Or rather, it is more important that the legs have a good push. Even in other mammals (most which have knee patellas), the leap is a push initiated from the hind legs.
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What a nice, simple and elegant demonstration. Saving this for my students.
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[quote=“SamSam, post:8, topic:94046, full:true”]Or rather, it is more important that the legs have a good push. Even in other mammals (most which have knee patellas), the leap is a push initiated from the hind legs.
[/quote]Ha ha ha ha, I was just looking this up while you were editing. I think it definitely says more about the lack of necessity for fore-leg push due to (according to my quick googling) not many quadipeds having elbow-caps. That is: we never needed them, even before we had hands.
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Our legs (and butts, hehe) are also home to the strongest muscles. So it makes sense to prioritize them.
And they have to carry a a shitload of weight on just two legs, as opposed to quadrupeds.
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