A competitive distance runner has a resting heart rate of 44 beats/min with a normal echocardiogram. The resting bradycardia of endurance training is best explained by:

• A Increased parasympathetic tone combined with a larger resting stroke volume ✓
• B Reduced sensitivity of the sinoatrial node to catecholamines
• C Chronic downregulation of beta-1 receptors from repeated catecholamine surges
• D Baroreceptor resetting secondary to low resting blood pressure

Explanation

Endurance training increases blood volume and cardiac chamber size, raising resting stroke volume. Because resting cardiac output must remain near 5 L/min, heart rate falls proportionately, aided by enhanced vagal tone. The sinoatrial node retains normal responsiveness to catecholamines, so option B is wrong, and receptor downregulation is not the accepted mechanism. Baroreceptors regulate short-term pressure, not the chronic training bradycardia.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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