Photo: © 2010 AAP/SIMON GRIMMETT

They say endurance sports, like Abbott's recent 14-hour Ironman challenge, can cause excessive fatigue, poor sleep and loss of libido.

"With over-training you release hormones like cortisol that do the opposite - they break down muscle tissue and break down the enzymes that are important in performance," associate professor Mike Hamlin told the Herald Sun.

"A lot of athletes don't recognise it - they think, 'I didn't do very well today; I must go harder', when what they should be doing is pulling back a bit."