That is a good question. Having joints in our limbs is very important for allowing our bodies to move in complex ways, while retaining power (strength). Imagine what it would be like to be without knees – it would be very difficult to move about in complex environments. For example, try climbing a tree, or jumping from a modest height, without beinding your knees … on second thought – dangerous experiment, don’t try that!
Today, particularly in developed countries, we live in very controlled conditions where our environment is easy to navigate. But throughout our evolutionary history (I’ll assume the theory of evolution holds) being able to move quickly through complex environments was essential, and so provided a competitive advantage. Even now, being able to perform complex physical task (like football perhaps), could provide an evolutionary advantage – why are footballers’ wives attracted to footballers …?
Having said this, I do I think that knees are rather poorly designed. They are very prone to arthritis as we age, and injury while we’re young. The standard argument for this is that for most of our evolutionary history we had 4 legs; with a switch to standing upright (so we now have 2 arms and 2 legs) the forces on the “legs” effectively doubled, and now have to cope with loads that they were not designed for. I’m not sure that I buy this argument, though, because elephants have knees and have very large forces to cope with. So, I think it has more to do with the knees having evolved to efficiently develop to provide an acceptable level of performance over the reproductive lifespan of the organism. So, we’re not over-engineered!
That’s quite a challenging question to answer Saffron!
Knees are clearly there to aid flexibility and allow us to move across different environments effectively. If you look throughout the animal kingdom, you can see where this applies; Elephants have evolved on flat areas such as desserts and plains so have very limited knee movement (have you ever seen an elephant jump?!), whereas monkeys and other climbing animals have a lot of movement in their knees to aid them climbing. Flamingos are another great example – their knees bend the other way. Why? Because they evolved around flat environments.
One interesting question to ask is why we have knee caps? Most animals don’t have knee caps, and when you think about it they don’t serve any major purpose – even when we kneel we rest on the top section of the leg, not the knee itself… Something to think about!
Of course, there is another answer to this question. The real reason we have knees is because if we didn’t, the song ‘heads, shoulders, knees and toes’ wouldn’t make sense…
We have knees so that we can move in different directions and they help support the weight of the body. It’s a very complex and complicated joint made up of 4 different small bones, several ligaments and cartilage which wears away as we get older. If we don’t have knees we can’t move easily and twisting movements would be very difficult
I’m not an expert in the human body, but from an engineering perspective, knees act as a wonderful shock absorber like the ones in your car. When we run or jump, they allow the muscles in our legs to manage the reactive forces from when we hit the ground so we don’t damage our pelvis or spine.