It's rare that someone dies running a marathon or half marathon. Extraordinarily rare when three people die. Yet that's what happened in Detroit, Sunday, Oct. 18, 2009.

Within 16 minutes, three men collapsed and died while running the half marathon in Detroit. All had trained to do the event and according to news reports all were relatively healthy.

Emergency medical staff were on hand quickly for each man, but none could be saved.

It's a tragedy but unfortunately it happens. People who sign up for any organized race have to sign a waiver acknowledging that they understand the risk they are taking in participating in an extreme event.

No sane person would compete in a half marathon or marathon without doing the proper training. It is hard on the body and you need to be prepared for it.

A whole lot of exercise doesn't necessarily mean that you're immune from heart troubles.

Studies have suggested that you're at higher risk than the general population of suffering a heart attack while you're running a marathon, but the rest of the time, your risk is much lower. A study out of the University of Manitoba also suggests that long-distance races can cause damage to your heart but it's healed within a week, with no long-term effects.

But the evidence is not clear-cut. A three-year old study suggests that for runners over the age of 50, it might be difficult to tell the difference between the positive effects of training on the heart and the onset of cardiovascular disease.

You have to know your limits. But the heart is a complicated and tricky little organ. By the time you realize something's seriously wrong, you may already be knocking on the pearly gates.

But there's risk in any activity probably more in a life of non-activity.

The Detroit deaths prompted the Sudden Cardiac Arrest Association in the United States to issue a warning: we need to be more aware of the prevalence of sudden cardiac arrest.

The association estimates that 300,000 Americans die every year from sudden cardiac arrest. That's one person every two minutes. Clearly, not all of them are running marathons.

Some of them are at home watching television or outside office buildings, taking a smoke break.

Yes, three runners died in Detroit doing something that gave them great joy on Sunday.

What didn't make the headlines was that on the same day, in Columbus, Ohio and Toronto, 30,000 people crossed the finish lines for marathons and half marathon and while maybe not extending their lives, sure improved the quality of it.