Competitors in one of the hardest races in the world were recently found to have a higher rate of heart arrhythmia, a major risk factor for stroke and congestive heart failure. Over a 10-year period, researchers followed 53,000 athletes who participated in Vasaloppet, an annual 56-mile cross-country ski race in central Sweden, and established that the more frequently they raced and the faster they finished, the higher the chance of arrhythmia – basically a heart beat that is too fast or slow. With the recent rise in triathlon deaths in the United States – most of them attributed to undiagnosed heart conditions – this study may prove valuable for endurance athletes in all sports. “Generally, physical activity and exercise is good for your health,” says Dr. Kasper Andersen, a cardiologist at Sweden’s Uppsala University Hospital and author of the study. “But only a small part of the population is at such a high training level [as these athletes],” and little is known about the impact that long, intense races have on the heart.