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Athletes are more likely to need pacemakers in old age, according to a new study.

Researchers at The University of Manchester identified the reason why top sportsmen who complete in endurance events like marathons, triathlons, and iron man challenges are significantly more likely to have arrhythmias or heart rhythm disturbances.

The latest findings reveal that exercise training triggers molecular changes in the heart's pacemaker. Researchers said that the latest finding debunks the theory that an increased activity of the autonomic nervous system causes this specific reaction to endurance training.

Normal, healthy adults have resting heart rates between 60 to 100 beats per minute. However, the typical endurance athlete has a heart rate of 30 or less times a minute. What's more, there tends to be long pauses between heartbeats for athletes.

"The heart rate is set by the heart's pacemaker, but this is controlled by the nervous system. The 'vagal' nerves lower the heart rate and therefore it was assumed the low heart rate of athletes is the result of over activity of the vagal nerves," first author Dr Alicia D'Souza of The University of Manchester said in a news release.

"But our research shows this is not the case. Actually the heart's pacemaker changes in response to training and in particular there is a decrease in an important pacemaker protein, known as HCN4, and this is responsible for the low heart rate," D'Souza added.

D'Souza and her team believe that these molecular changes in the cardiac structure responsible for generating heart rhythm, or the sinus node, might help them understand the more frequent occurrence of heart rhythm disturbances or even loss of consciousness in athletes.

"This is important because although normally a low resting heart rate of an athlete does not cause problems, elderly athletes with a lifelong training history are more likely to need an artificial electronic pacemaker fitted," lead researcher Professor Mark Boyett said in a news release.

"Although endurance exercise training can have harmful effects on the heart, it is more than outweighed by the beneficial effects," he noted.

"This study shows the heart's electrical wiring changes in mice that exercise for long periods, and these changes in heart rhythm are sustained afterwards," researcher Professor Jeremy Pearson, Associate Medical Director at the British Heart Foundation, said in a statement.

"If the findings are reproduced in humans they could have implications for heart health in older athletes. But much more research is needed before we could draw that conclusion," he concluded.

The findings are published in the journal Nature Communications.