Whilst walking the dogs this morning, pondering the world around me. The thought struck

 

"why have we got kneecaps and not elbowcaps"

And I guess you mean the bone (Patella), and not the plastic padded protection that skaters and other extreme sports people use...

 

According to wikipedia (unfortunately, the English wiki is rather short, the Dutch one has better explanations), the patella functions to:

 

1. Optimize the leverage of the tendons. Because the patella keeps the tendons further away from the actual joint, you get a larger leverage.

2. Slowing down forward motion. (I admit I don't understand this one myself).

3. Protection of the knee joint.

 

And why not in the elbow? I guess all these things are more important in the hind legs of mammals. Hind legs are in almost all mammals far stronger, so the leverage is a major advantage especially in the hind legs.

An excellent question (meaning you may go far in this forum).

 

Let's consider the opposite ... a "kneebow", and that it might point the wrong way.

 

There is a old jackboot style where the forward edge of its top rises above the knee. Such an edge would act like a "kneebow". Consider a animal/person with "kneebows" that is creeping or on "all fours". The "kneebows" might catch on all manner of things (exposed roots, vines, etc) and hinder forward movement, especially escaping. So perhaps the question should be about why are there no "kneebows" (which is the simpler form of the two joints ... and would have evolved first???), instead of why no "elbowcaps".

Edited November 30, 201114 yr by ewmon

comprehensive answer thank you. I imagine the stress' on the arms are mostly tension (swing through the trees, carrying etc) so no need for a patella (though an elbowcap would save the pain of hiting my funnybone).

comprehensive answer thank you. I imagine the stress' on the arms are mostly tension (swing through the trees, carrying etc) so no need for a patella (though an elbowcap would save the pain of hiting my funnybone).

But hitting the funny bone has an element of humor in it, and you're more attractive when you smile, thus more likely to attract a mate. Hurting your funny bone is probably evolution being sarcastic.

 

(Ok, this wasn't a serious reply - just an attempt at being funny).