The study, built on previous research showing more strain is put on the right side of the heart during exercise, concluded that doctors were missing signs of the potentially fatal condition by conducting tests on the left side of the heart or while a patient is idle.

"You do not test a racing car while it is sitting in the garage. Similarly, you can't assess an athlete's heart until you assess it under the stress of exercise," lead researcher Andre La Gerche, head of Sports Cardiology at Melbourne's Baker IDI Heart and Diabetes Institute, said.

The study, published in the European Heart Journal, comes after a young footballer collapsed and died of cardiac arrest during a suburban match on the weekend.

Northern Saints player Mohammed Allouche, 22, became the second man to die of a suspected heart attack on the football field last month.

The risk of heart attacks during exercise was also highlighted by the death of Facebook executive Sheryl Sandberg's husband Dave Goldberg while he was running on a treadmill earlier this year.

Dr La Gerche said about 2 per cent of endurance athletes such as triathletes or marathon runners exhibited irregular heartbeats, and in a fraction of these cases the condition was life-threatening.

Australian and Belgian researchers used a number of techniques including echocardiography, which Dr La Gerche likened to an ultrasound of the heart, to monitor test subjects' heartbeat irregularities while the patient was active.

He said the tests allowed researchers to visualise the function of the heart and quantify the severity of an irregular heartbeat. "You are looking at the structure and the movement of the heart."

"Normally with exercise the function gets better. But what we found was that in athletes with serious arrhythmia (irregular heartbeat), the left hand side of the heart got better and better and the right hand side went on holiday - its function did not improve."

The heart ultrasound technology used is widespread and can be found in most hospitals and cardiology clinics, Dr La Gerche said.

Irregular heart rhythms are caused by problems with electrical impulses in the body and can be characterised by symptoms ranging in severity from a fluttering sensation in the chest to palpitations and dizziness. ECG tests are often used to determine whether someone has an irregular heartbeat.

He said if an endurance athlete has palpitations during exercise they should be assessed by a heart specialist. In a small number of cases where the heart rhythm irregularities are serious, athletes may need to give up sport and receive specific treatments. But he said such cases were uncommon.

While people who exercise regularly are at less risk of heart disease, heart attacks - while very rare - can occur during workouts because of the stress the body is putting itself through.