Emmy and Sarah I agree with both of you, running or any endurance sport will have an effect on the heart structure and so lots of us will have a larger left ventricle and thickened walls, much as most of us will have bradycardia in terms of resting heart rates. But all of us are also capable of suffering arrythmias and sometimes for no detectable reason. Getting checked out if you suffer from certain symptoms can at least detect anything obvious.
Micah seems likely to have suffered an arrythmia and unlike Fabrice Muanba the footballer, he was miles from anybody with a defibrillator when he collapsed.
Because of the high profile the recent deaths and collapses in professional sport has given to cardiac health and screening, all Team GB athletes have been tested for the Olympics. Out of about 550 athletes two were found to have a condition, Wolff Parkinson White syndrome which can cause sudden cardiac death and were told they would not be included in their squads without undergoing catheter ablation to eliminate the problem. Obviously no names were released and I am not sure if they have still been included in their sports.