If we could give every individual the right amount of nourishment and exercise, not too little and not too much, we would have found the safest way to health.—Hippocrates circa 400 BC

As eloquently described by our colleagues Ruiz et al,1 numerous large observational and cohort studies have provided compelling evidence for the benefit of mild and moderate exercise on cardiovascular health and mortality. That the benefit of exercise intensity and duration continues to increase into the realms of intense competitive sport is more controversial. This is of importance given that endurance sporting events continue to increase in popularity. For example, over one million Americans competed in an endurance triathlon last year, and participation rates are currently increasing by 10–20% per annum.2 The last half-century has seen an increase in morbidity associated with a lack of habitual exercise and a simultaneous increase in the number of people performing exercise well in excess of recommendations. Very appropriately, the public health focus has been on addressing inactivity, but it may be presumptive to assume that there is no upper limit to what constitutes healthy exercise for the heart. There is accumulating evidence that strenuous endurance exercise may be associated with an increase in some cardiac arrhythmias and speculation that some of the structural changes associated with the ‘athlete's heart’ may not always be benign. Thus, there is a clear need to expand our understanding of the effects of greater doses of exercise.