Sinus Bradycardia in Endurance Athletes From Physiological Adaptation to Potential Pathology

Sinus bradycardia is a common electrocardiographic finding in trained athletes and a hallmark feature of the athlete’s heart. Reduced resting heart rate, particularly in endurance athletes, reflects physiological adaptation to long-term intensive training and is usually asymptomatic. In highly trained individuals, heart rate values may fall well below standard clinical thresholds while maintaining normal cardiovascular function. Exercise-induced bradycardia has traditionally been attributed to increased parasympathetic activity and reduced sympathetic tone. However, recent evidence indicates that intrinsic remodeling of the sinoatrial node also contributes to this phenomenon. Molecular and electrophysiological adaptations, including alterations in ion channel expression and pacemaker cell function, play an important role in the reduction of intrinsic heart rate. Although generally considered benign, sinus bradycardia in athletes has raised concerns regarding its potential long-term clinical implications. Emerging data suggest that prolonged endurance training may be associated with an increased risk of clinically significant bradyarrhythmias later in life, including sinus node dysfunction and atrioventricular conduction abnormalities. Differentiating physiological adaptation from early pathological remodeling remains a key challenge in sports cardiology. Accurate evaluation requires integration of clinical assessment, electrocardiography, ambulatory monitoring, and cardiac imaging. This review summarizes current knowledge on the mechanisms, clinical significance, and diagnostic evaluation of sinus bradycardia in athletes, with particular emphasis on the balance between beneficial adaptation and potential adverse remodeling associated with long-term endurance training.