In reply to steelbru:
"Animals that run fast.... all have very small feet ( relative to humans ), would they run faster if they had longer feet, and if so why have they not evolved with them bigger"?
You have a bit of a misunderstanding there about what constitutes a Cheetah or greyhound foot. The bits on the ground (the Paw) are just the ends of their fingers and toes. That's not some sort of backwards second knee up above the ground you know! This stance is called digigrade and is faster, the flat footed thing we do is called plantigrade and is slower but more stable.