This prospective observational study will investigate the frequency of new onset atrial fibrillation (AF) and of further cardiac arrhythmias as well as of (clinically silent) cerebral lesions in endurance runners before, during and after the BMW-Berlin Marathon 2011.
Inclusion Criteria: * Age 35-65 years * at least 2 marathon runs within the last 5 years * weekly running of at least 40 km Exclusion Criteria: * Known cardiac arrhythmia or prior stroke * contraindication for MRI.