A study in more than 15,000 people has found that physical fitness is linked with a lower likelihood of developing atrial fibrillation and stroke. Atrial fibrillation is the most common heart rhythm disorder, affecting more than 40 million people worldwide. It is estimated that one in three Europeans will develop atrial fibrillation in their lifetime. Patients with the condition have a five-fold higher risk of stroke than their peers. This study examined whether fitness was related to the likelihood of developing atrial fibrillation.