Why practise makes us better?

A few days ago I started limits at Politechnique and I figured out that the best way to solve them (in order to reduce errors and solving time) is to practise as there were no tomorrow. Our body and in specific our mind learns to resolve these schemes very fast.... I thought about the famous Usain Bolt  when he won  Guiness World Record in 100m run: "I trained 5 years to run 9 seconds"