The growing popularity of extreme sports such as marathon running and Ironman triathlons may be putting competitors at risk of permanent damage to their hearts, researchers have warned.

A study by researchers from the University of Melbourne, Australia, of 40 elite athletes training for an endurance event found their hearts changed shape during the race, increasing in size but decreasing in pumping power. There were signs of excessive stretching of heart muscle cells.

The hearts recovered in most of the athletes after one week, but in five, who had been training and competing longer than the others, there were signs of scarring suggesting the damage was permanent.

Experts stressed that the risk was small and vastly outweighed by the benefits of exercise.