20 May 2009

Mr. Wizard on Knees

Emily asks...

What would chairs look like if our knees bent the other way?


Do penguins have knees?
The latter question is the easier of the patella-oriented queries, so let me answer it with the following diagram:

Now while it may not be clear in this picture, there is in fact a femur on the upper half of the lower limb. This should come as no shock as we share a common ancestor with penguins (though we'd have to go back a *really* long way to find that common ancestor.)

So yes, Emily, there is a knee. It is the joint between the femur and tibiotarsus/fibular pair.

Now as to your second question, that one's a bit of a puzzler. It's difficult to imagine a successful biped with a knee that bent in the other direction, as locomotion would be difficult at best, making it easy prey for animals with unhindered knees. However, if we assume all animals shared that odd joint, there would be no relative disadvantage to an organism with that structure. So let's assume that's the case, and somehow locomotion works with lower limbs that rotate in the same direction at both the first and second joints.

In that case, it seems that some variation of a Swedish kneeling chair would work best.

Just as the kneeling chair directs most of the downward force of the body along the length of the femur, our hypothetical reverse-kneed people would sit in a similar fashion, but the lower halves of their legs would extend out and up with a slight bend.