Although doctors aren’t yet sure what caused football player Fabrice Muamba’s heart to stop beating on Saturday, some experts say vigorous exercise may have been the trigger.

The 23-year-old Bolton midfielder collapsed face-first on the field during a match against Tottenham, with no one else around him.

Paramedics immediately began trying to revive Muamba, but his heart didn’t start beating on its own until about two hours later, after he arrived at a London hospital.

“Exercise could be a trigger for a cardiac event,” said William McKenna, director of inherited cardiac diseases at University College London. “If you have a known problem and push the system to the limit, it may fall apart.”

Other experts said it was likely Muamba’s problem was a pre-existing one that hadn’t been detected before.

“In someone his age, genetic abnormalities are the most common cause,” said Douglas Zipes, a distinguished professor at the Krannert Institute of Cardiology at Indiana University.

Zipes said he wouldn’t expect people under 35 to have any serious heart problems without a genetic cause.

He said there were a number of genetic heart abnormalities that could have caused Muamba’s heart to stop, and that athletes with such problems were potentially more at risk.

Those abnormalities can cause the heart muscle walls to become too thick, causing the heart to become overworked in its attempt to pump enough blood around the body.

“Athletes under the stress of a game have a lot of adrenalin in their bodies,” Zipes said. “That can interact with an underlying congenital problem and cause a cardiac arrest.”

In recent years, several elite football players have collapsed on the pitch, including Antonio Puerta, a 22-year-old Sevilla midfielder who lost consciousness and fell near his own goal during a Spanish league match in 2007.

Doctors treated him on the field and he walked off, but then had a heart attack in the locker room and another in the emergency room of a Seville hospital. He died three days later.

In the United States, fewer than 100 young athletes die from sudden cardiac arrest every year.

Muamba is under sedation, which McKenna said was likely to prevent any brain swelling that may have occurred.

“They’re doing all the right things and are probably monitoring his vital organs, which may not have received enough blood during the cardiac arrest,” he said.

McKenna said it was possible Muamba would make a full recovery but that it was too soon to tell.

“The medics got to him very quickly, but we just don’t know if the blood flow to his vital organs was adequate,” he said.

The American Heart Association recommends a thorough physical exam and detailed family and personal medical history for athletes, but not an automatic electrocardiogram, or EKG, which measures a heart’s electrical activity.

The idea is to look for red flags – such as fainting episodes, a heart murmur or whether a relative died young of a heart problem – that would prompt the doctor to order further cardiac testing.

In contrast, the European Society of Cardiology and the International Olympic Committee recommends the addition of EKGs to pre-sports check-ups. The tests cost about $US25 ($A23.63) to $US100.