Athletes' ability to push themselves to the limit is created by the brain's need to win, a new survey has revealed.

In his study sportsmen racing against someone else managed to find an extra burst of energy that increased their performance by 1.7 per cent.
"Most sportspeople know they perform harder and better when they are competing, but until now we didn't know precisely why," the Scotsman quoted Dr Corbett as saying.
"Whenever you do exercise you're likely to think 'how much am I willing to hurt myself?' and there's usually a point which holds you back because you don't want to do yourself irreparable damage.
"But when racing someone head-to-head, the athlete's brain can manipulate this signal and keep on going," he stated.
Out of 14 cyclists participating in the study, 12 were significantly faster in the final race, when they believed they were competing against an opponent.
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The study was published in the journal Medicine, and Science in Sports and Exercise