Stanford, Calif. -- Stanford cardiologist Euan Ashley wanted to study the hearts of endurance athletes, so he set up a mobile heart lab at the finishing line of the ultra-endurance race "Adrenalin Rush" in the Scottish Highlands and waited for the racers to come in.

At about 2 a.m. on Aug. 1, 2001, the winning team of four athletes collapsed across the finishing line after 90 continuous hours of biking, climbing, swimming, paddling and rope work with virtually no sleep. "We waited in the cold and wet and literally picked them up at the end of the race," said Ashley, who then fed the athletes donuts, tested their hearts and waited for the remaining athletes to arrive.

After testing the hearts of about 50 endurance athletes both before and after the 400-km race in Stirling, Scotland, Ashley, MD, PhD, assistant professor of cardiology at the Stanford University School of Medicine, and his colleagues found that contrary to generally held beliefs, the heart does, in fact, tire with exercise - at least it can under extreme conditions.

"I think it's amazing," said Ashley. "Your heart beats 3 billion times in a lifetime. In the absence of disease, we don't think of an 80- or 90-year-old's heart tiring."

The results of the study, to be published in the Aug.1 issue of the Journal of the American College of Cardiology, not only show that hearts can tire with exercise, but also that one particular gene variant can predict the extent to which the heart will tire.

"It was a bit of a labor of love to find this out," said Ashley, a native of Scotland who recruited scientists from Oxford and Duke, not to mention members of his own family, to help with the study. His wife and his father also waited at the finish line to take blood samples as the racers stumbled in.

"Our place became the hangout," Ashley said. "We had guys sleeping on the floor. We sent out two or three times for more donuts as finishers came in."