An athletic patient presents with a resting pulse of 45 beats per minute (bradycardia). What is the most likely explanation?

Trained athletes commonly have resting heart rates well below 60 beats per minute (bradycardia) due to physiologic adaptations of the cardiovascular system. Regular aerobic exercise increases cardiac stroke volume (amount of blood pumped per beat), allowing the heart to maintain adequate cardiac output at lower rates. This is called athletic heart syndrome or physiologic sinus bradycardia. It is a normal and expected finding in conditioned athletes and does not represent pathology. A resting HR of 40-50 bpm is common in endurance athletes.