Secret of Usain Bolt's speed unveiled

Scientists say they can explain Usain Bolt's extraordinary speed with a mathematical model.

His 100m time of 9.58 seconds during the 2009 World Championships in Berlin is the current world record.

They say their model explains the power and energy he had to expend to overcome drag caused by air resistance, made stronger by his frame of 6ft 5in.

Writing in the European Journal of Physics, the team hope to discover what makes extraordinary athletes so fast.

According to the mathematical model proposed, Bolt's time of 9.58 seconds in Berlin was achieved by reaching a speed of 12.2 metres per second, equivalent to about 27mph.