Inclusion Criteria for the main study group:  At least 65 years of age Self-assessed ability to use the study devices Ability to give informed consent Volunteering for the study  Inclusion criteria for the active endurance athlete group:  Minimum of 10 years of active training of endurance sport (e.g. long distance running, skiing, cycling, triathlon…) and currently an average amount of training of at least 10 hours a week. Age: ≥ 50 years Self-assessed ability to use the study devices Ability to give informed consent Volunteering for the study  Exclusion Criteria for both groups:  Cardiac pacemaker Earlier diagnosis of atrial fibrillation Inability to give informed consent Denial