A British Heart Foundation team found exercise in mice triggers molecular changes in the part of the heart that generates its natural beating rhythm.
This may explain why elite athletes have low resting heart rates and a higher risk heart rhythm disturbances, they told Nature Communications.
However, the benefits of exercising still outweigh any risks, experts say.
Super-fit
Endurance athletes are generally very fit.
Yet, paradoxically, they are more likely to have heart rhythm disturbances, known as arrhythmias, especially as they get older - although the risk is still small.
Experts have suspected that this is because long-term training for extreme endurance events such as marathons and triathlons slows the heartbeat down.
While normal adults have resting heart rates between 60-100 beats per minute, hearts of endurance athletes can beat only 30 times per minute or even less at night time when there can be long pauses between heart beats.
Cyclists Sir Chris Hoy and Miguel Indurain reportedly had resting heart rates of 30 and 28 beats per minute.
The heart rate is set by the heart's pacemaker, which is controlled by the nervous system.
And so it was assumed that the low heart rate of athletes was a result of the autonomic nervous system going into overdrive.
Read more here: http://www.bbc.com/news/health-27389257
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