The resting heart rate (RHR) is the number of times a person’s heart beats per minute while the body is in a state of complete rest. For the average, healthy adult, this rate typically falls between 60 and 100 beats per minute (bpm). Highly trained endurance athletes often exhibit athlete bradycardia, where their RHR is lower, sometimes dropping into the 40 to 60 bpm range, or even below 40 bpm. This physiological adaptation reflects a cardiovascular system that has become exceptionally efficient through consistent, rigorous training.
How Training Changes Heart Structure and Output
Endurance training causes the heart muscle to undergo mechanical changes that enhance its pumping power, a process known as physiological ventricular hypertrophy. This adaptation is seen most clearly in the left ventricle, the heart’s main pumping chamber, which becomes enlarged and stronger. Unlike pathological heart enlargement, this change is symmetrical and allows the chamber to hold more blood.
The physical remodeling of the left ventricle directly leads to an increase in stroke volume, the volume of blood the heart ejects with each single beat. Because the heart is stronger and can fill with more blood, it pushes a greater amount of oxygenated blood into the circulatory system per contraction. A lower RHR is the consequence of this mechanical efficiency, as the trained heart achieves the necessary blood output with fewer efforts per minute.
The Role of the Nervous System in Slowing the Beat
The nervous system plays a substantial role in resetting the athlete’s baseline heart rate. The heart rate is regulated by the autonomic nervous system (ANS), which consists of two opposing branches: the sympathetic nervous system, which accelerates the heart (the “fight or flight” response), and the parasympathetic nervous system, which slows it down (the “rest and digest” response).
Sustained endurance training increases the influence of the parasympathetic system, a phenomenon often described as heightened vagal tone. The vagus nerve, the main component of the parasympathetic system supplying the heart, acts like a constant brake on the heart’s natural pacemaker, the sinoatrial (SA) node. This sustained parasympathetic dominance effectively lowers the intrinsic rate at which the heart’s electrical system fires.
While increased vagal tone is a major contributor, intrinsic changes within the SA node itself are also involved. These intrinsic changes involve the remodeling of ion channels, which sets the inherent pace of the heart. Regardless of the exact mechanism, the result is that the heart’s electrical rhythm is reset to a slower baseline, contributing to the observed low RHR.
When Low Heart Rate is a Medical Concern
A low RHR in a highly conditioned person, known as physiological bradycardia, is a healthy adaptation. This type of bradycardia is asymptomatic, meaning the athlete feels fine and has high exercise tolerance.
However, a slow heart rate can also be a symptom of pathological bradycardia, caused by heart disease, medication, or electrical system dysfunction. Pathological bradycardia occurs when the heart rate is too slow to supply enough oxygen-rich blood to the body, leading to symptoms. Individuals with a concerning slow heart rate often experience symptoms such as dizziness, chronic fatigue, fainting, or shortness of breath. Anyone who experiences these symptoms alongside a low RHR should seek a medical evaluation to determine the underlying cause.