Birkebeiner-II is clinical a follow-up study of the Birkebeiner Studies (The Birkebeiner Ageing Study and the Birkebeiner Atrial Fibrillation Study) at Diakonhjemmet Hospital. Data from the Birkebeiner Studies demonstrated that participation in endurance sports is associated with increased risk of atrial fibrillation in men. Birkebeiner-II aims to assess biomarkers associated with AF in veteran endurance athletes.