It is well known that long-term effects of exercise reduce cardiovascular morbidity and mortality. Until recently, mostly positive consequences of physical training for the heart have been described, even though it is known that short-term risk of sudden coronary events is increased during vigorous exercise Citation[1,2]. In recent years possible negative cardiac effects of long-lasting endurance competitions and long-term endurance training have been described among participants in triathlon, bicycling, marathon, 24-h run and orienteering Citation[3,4]. Most of the studies have evaluated the effects of extreme endurance events on ventricular function and rhythm.
Lone atrial fibrillation
Recently, we and others have demonstrated that long-term endurance training may result in lone atrial fibrillation (LAF) in many middle-aged and old male athletes such as cross-country skiers, marathon runners, former professional bicyclists and orienteers Citation[5,6]. In our follow-up study on high-performance male participants in endurance cross-country ski competitions after 28–30 years we found a prevalence of LAF of 18% in athletes aged 58 years at follow-up. This is at least ten-times higher than expected in age-matched controls Citation[5]. Others have not found this negative consequence of training for the heart among younger athletes Citation[3,7]. It is suggested that exercise-induced atrial fibrillation may be an even greater problem in the future, because more and more middle-aged and older men (and to a lesser degree women) are participating in long-lasting endurance competitions and are practicing endurance training for many hours per week for decades. From the official result lists from the most popular long-distance cross-country race in Norway (the 54 km long Birkebeiner race) we found that as many as almost 8000 individuals (6629 men and 1364 women) aged 40 years or older participated in the race in 2010. In 1970 only 450 men over the age of 40 years took part in the competition, while women were not allowed to participate at that time.
LAF & atrial remodeling
In one study we found that more than 80% of healthy middle-aged and old current or former cross-country skiers without cardiovascular or pulmonary diseases had enlarged atrial dimensions. There was no difference in atrial or ventricular dimensions between two different age groups of 58 and 75 years (no cardiac reverse remodeling with ageing), despite reduced training time in the oldest age group Citation[8]. Atrial remodeling with enlarged dimensions seems to be a consequence of endurance training and competing, because of the high volume overload (increased preload) and cardiac output needed for long periods. Slow heart rate owing to pronounced parasympathetic tone and increased stroke volumes, long atrioventricular conduction time and enlarged atrial dimensions seem to be risk factors for developing atrial fibrillation after many years of endurance training Citation[5].
We still do not know the dose–response curve for the increased risk of atrial fibrillation among long-term endurance athletes. Many questions still remain unanswered: for how many hours and for how many years do we have to train to have an increased risk? Do women have the same risk as men, or are they more protected because of smaller cardiac dimensions or other unknown reasons? Do we have to practice endurance training at high intensities to have the increased risk for atrial fibrillation or other negative effects of cardiac remodeling? Are there individual genetic factors predisposing athletes to atrial fibrillation?
Recommendations
All recommendations in cardiac primary and secondary prevention programs encourage people to live an active life with daily physical activity. There is in our opinion no need to change the normal activity level recommendations among the general population. On the contrary, all physicians should encourage inactive and moderately active men and women at any age to increase physical activity, because physical inactivity is an increasing major problem in most countries today. Encouraging men and women to perform moderate-to-hard physical activity most days of the week is good medicine; it is safe and gives health benefits. However, we have to ask whether it is time to give warnings against long-term intensive endurance exercise after, for example, the age of 50 years owing to the increased risk of atrial fibrillation, and warnings about ultra-endurance competitions at any age owing to other possible negative effects for the heart. We still do not have the answers, but challenge sports cardiologists and physicians in sports medicine and sciences to discuss these questions in future recommendations for participating in leisure time and professional physical training and activities.
The future
The available evidence regarding the negative effects of long-term endurance sports for the heart is still contradictory. Most of the studies available are small case–control studies or longitudinal cohort studies including a limited number of mostly male participants. The lack of studies in sports cardiology in women is notable. We need more longitudinal studies in large populations of male and female long-term endurance athletes compared with control groups of an inactive population and a less active population to answer the remaining questions. Until then we have to take care of the athletes who have developed a negative cardiac consequence of long-term physical activity by giving them the best available treatment today, either by detraining and regular follow-up, medical treatment or pulmonary vein ablation Citation[9].
Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.
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