Yet the startling reality is that athletes are more than twice as likely as the general population to drop dead without warning before they reach the not so ripe age of 35. No prior symptoms, no gradual withering away. One minute they're scoring goals or breaking records and the next they're gone.

Last year David Kimani, six-time national champion cross country and track runner at the University of Alabama left his new wife and team-mates stunned when he fell off his chair in the campus dining hall and could not be revived. With a Nike sponsorship almost in his back pocket and plans to race in Athens, Kimani seemed to be in his prime. Instead he was dead at 25.

Kimani is not the only high profile athlete to die suddenly of cardiac arrest, which accounts for the vast majority of sudden deaths in young athletes. Cameroon soccer star Marc-Vivian Foe collapsed at the end of the semifinal for the Confederation Cup last year. Another famous victim of sudden cardiac death was Olympic figure skating gold medallist Sergei Grinkov, who died during an ordinary practise session at age 28. None of these men had a known history of heart problems, and being elite athletes they were regularly monitored by a host of top physicians.

So what went wrong?

According to associate professor Chris Semsarian, a leading molecular cardiologist at the University of Sydney's Centenary Institute, at least 80 per cent of sudden cardiac deaths in young athletes are caused by genetic abnormalities dubbed "killer genes".

Most – about two-thirds – are structural abnormalities. Hypertrophic cardiomyopathy, where the heart muscle is abnormally thickened, is the most common structural cause of sudden cardiac death in people under 35. Athletes' hearts are often naturally thicker than the average person's, but a cardiologist can tell the difference between a heart that is thickened by hypertrophy and one is thickened by sport, Semsarian says.

In the other third of cases the heart can look completely normal, which can make determining the cause of death more difficult – as was the case for Kimani, whose doctors were baffled when he died. Semsarian says these 'unsolved mystery' cases are likely due to electrical abnormalities.

There's no evidence to suggest that athletes are more likely to carry the defected gene than anyone else. It's just that intensive exercise can trigger sudden death if there's already an underlying defect, which is why competitive sports people are more likely to die of cardiac arrest than others.

What happens is that vigorous exercise causes surges of adrenalin that can disturb the rhythm of the heart in affected people.

About 70 per cent of sudden deaths in young people occur during or immediately after exercise, Semsarian says.

An important Italian study published in 2003 added support for the idea that competitive sport only posed a danger factor in people with an underlying defective condition. Researchers compared the rates of sudden death among 1.4 million people aged 12 to 35 in the Veneto region of Italy between 1979 and 1999. They found 300 cases of sudden death, about one death per 100,000 people per year.

Among athletes the figure rose to 2.3 cases of sudden death reported per 100,000 people per year, but in these cases the higher numbers of sudden deaths were strongly linked to underlying heart disease.

So for most people exercise poses no harm.

"After reading all the data I went out and went running," says Dr Roberta Williams, a cardiologist specialising in non-evasive sudden death at The University of Southern California.

"I think the real issue is when people who have symptoms go out and exercise. That's when you get into problems."

But what's frightening is that by and large people carrying the defective genes experience no symptoms whatsoever, and only 20 per cent have transient symptoms, says Dr Barry Maron, an expert on sudden cardiac death at the Minneapolis Heart Institute Foundation in Minnesota.

That's not to say there are no predisposing factors. The defective gene is an inherited one, and dominant at that. So if you're a carrier there's a 50 per cent chance you'll pass it on. That means that anyone with a family history of sudden death or heart disease should get screened. About 30 per cent of people who suffer sudden cardiac deaths have a family history of it, Semsarian says.

Statistically men are more likely to suffer sudden cardiac death than women: nine to one, in fact, Maron says, and the average age is a mere 17.

Cardiologists say while cardiac death is rare, it may be more prevalent than we realise.

In Sydney alone there are 50 sudden cardiac deaths each year among people under 35, nearly one a week, Semsarian says. And that figure is likely an underestimation.

"There are more who die that we're not aware of. There's a big part of the population that we're missing," Semsarian says.

"For instance there might be a drowning, but when you dig a little deeper you find the person was a perfect swimmer, so the underlying cause could have been a cardiac abnormality."

Cardiac arrest could also explain seemingly mysterious motor vehicle accidents – did the person really just lose concentration, or was it a case of cardiac arrest at the wheel?

As well, between 10 and 15 per cent of sudden infant death cases have cardiac genetic abnormalities in their families. Given that there is a one in two chance of passing the dominant killer gene along it's likely at least some SIDS cases actually died of cardiac arrest.

Part of the problem is that there are so many nuances it can be difficult for an untrained eye to spot the signs of cardiac arrest.

"There are a lot of subtleties that people might miss unless they're a cardiac pathologist," Williams says. "Being able to determine the exact cause of death is not easy. There are some abnormalities that only a sophisticated cardiologist would recognise."

And so far there has not been a national registry of cases in the United States or Australia, again making it difficult to determine the frequency with which these cases occur.

Cardiologists differ in their opinions of the best way to decrease the number of sudden cardiac deaths.

"The best thing we can do is take a careful history, and perhaps an ECG if you have the funding, and do a physical examination by someone who knows what to look for.

"Someone who can tell the difference between the soft murmur that many people have and a murmur that's suspicious, or can tell when the heart sounds aren't exactly right," Williams says.

The country that comes the closest to that standard is Italy, where an extensive pre-screening program has been in place for 30 years.

All prospective athletes at all levels go through a detailed examination before they are allowed to participate in their desired sports, Maron says.

Not only are athletes screened for their history through a clinical examination, but they are also given a physical exam and an electrocardiogram, something some cardiologists see as beneficial and others see as a waste of time for use in the general population.

If there are any abnormal results more detailed evaluation follows.

Just how well this system is working depends on who you ask.

More cases are certainly being detected than elsewhere, but whether the results have been substantial enough to justify the enormous expenses is something of a debate.

"It hasn't proven to be cost-effective, and it still misses people with the disease." Semsarian says.

"So at this stage screening should be reserved for individuals with a family history of heart problems or if they have problems themselves, such as symptoms."

And there's always the question of whether even rigorous screening that finds problems will be enough to stop keen athletes.

Two recent events suggest otherwise.

Doctors discovered Brazilian soccer player Serginho had a heart condition during routine tests at the beginning of this year. The 30-year-old defender died during the 59th minute of a game on October 29.

And in an equally eerie case, Latvian ice hockey player Sergei Zholtok died from heart failure on the ice mid-game on November 4 in Belarus, more than a year after he was diagnosed with having an irregular heartbeat. He had missed seven games before being cleared to play again.

American Heart Association

http://www.americanheart.org

The Centenary Institute

http://www.centenary.usyd.edu.au

National Heart Foundation of Australia

http://www.heartfoundation.com.au