This study evaluates cardiac structure and function differences between healthy endurance trained athletes with atrial fibrillation and healthy age matched endurance trained athletes without atrial fibrillation. It is hypothesized that despite having similar structural adaptations of the heart (due to endurance training), athletes with atrial fibrillation display will impaired heart functional measures compared to endurance athletes without atrial fibrillation.

Regular physical activity, including vigorous exercise, lowers cardiovascular risk factors, including the risk of developing Atrial Fibrillation (AF), one of the most commonly diagnosed types of arrhythmias. Paradoxically, despite the cardioprotective effects of exercise, middle aged endurance athletes are at 5 fold risk of developing AF compared to active individuals. Long standing endurance training induces significant changes to the heart- collectively known as the 'athlete's heart.' While these changes may be beneficial for performance, they may be conducive in promoting the risk of AF in this cohort. The aim of this study is to compare cardiac structure and function in endurance athletes diagnosed with AF (n=17) and in healthy aged matched endurance athletes (n=17). Male subjects between ages 45 and 65 years old with a long-standing history of endurance training and competition will be recruited. Heart structure and function will be compared at rest and during submaximal exercise. Participants will also complete fitness assessments and questionnaires that characterize their lifetime involvement in exercise training. This study will address the gap in the literature regarding the interaction of endurance training and AF risk.

Inclusion Criteria:

ALL participants:

• endurance athlete (cyclist, runner, triathlete or other) who participate in vigorous year-round training of 4-6 hours per week for a minimum of 20 years
• participation in atleast one major competition per year (marathon/triathlon/endurance competition)
• sinus rhythm during all assessments

Inclusion criteria specific to endurance athletes with AF:

• diagnosed with lone paroxysmal AF in the last 4 years
• able to verify diagnosis (ECG or Holter verification/proof of diagnosis),
• can be on any type of medication.

Inclusion criteria specific to healthy endurance athletes without AF:

• previous participation in Athlete's Heart Study
• has completed resting cardiac magnetic resonance imaging and resting electrocardiogram

Exclusion Criteria:

• females
• no more than >1 hour per week of resistance training
• treatment or prior diagnosis of cardiovascular disease, valvular disease, hypertension, heart failure, diabetes, thyroid disorder, sleep apnea, current/recent viral disease, chronic inflammatory disease
• previous (within 10 years) or current smoking
• recreational drug use or alcohol consumption in excess of accepted standards
• inability to provide informed consent
• inability to verify AF diagnosis (if AF athlete).

Community sample from, but not limited to, various running, cycling, and rowing clubs for middle-aged athletes. Healthy endurance athletes will be selected from an existing study for convenience (non-probability sample). Endurance athletes with AF will be selected from the community (probability sample).

Participant data may be used for future study upon participant consent for their de-identified data to be used in the event that a related study is conducted. Only principal investigators will have access to records and will have the ability to share data with approved researchers.