Our study shows that atrial fibrillation risk in very fit people is smaller than previously thought. voronaman/ Shutterstock Exercise is important for your heart and cardiovascular health. But for years, research has suggested that very fit people – particularly young men – have a higher risk of developing atrial fibrillation (an irregular heartbeat) later in life than less athletic people do. But our new large-scale study of more than one million young Swedish men challenges this long-held conc