Muamba's collapse possibly triggered by exercise

Although doctors aren't yet sure what caused soccer player Fabrice Muamba's heart to stop beating 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.

Dr. William McKenna, director of inherited cardiac diseases at University College London, says "exercise could be a trigger for a cardiac event."