The Athlete’s Heart

No, the heart I’ll be discussing in this blog entry isn’t metaphorical triumph of the human spirit in regards to sports — it’s about the actual adaptation the circulatory system of an athlete makes in response to the stimulus of endurance training. Yeah, the real athlete’s heart. I know, gross, right?

The most notable adaptations the body makes to cardiovascular exercise stimulus include:

  • Reduced heart rate during submaximal exercise and potential reduced heart rate during maximal exercise
  • Increase in stroke volume during exercise
  • Increase in the mass and chamber size of the left ventricle
  • Increase in capillary density and recruitment, which facilitates oxygen delivery to the contracting muscles

The cardiovascular system is made up of blood, blood vessels, and, of course, the heart.

During exercise, the primary functions of the cardiovascular system are to deliver oxygen to skeletal muscles and remove carbon dioxide and heat from contracting muscles. The cardiovascular system also is responsible for the maintenance of mean arterial blood pressure as exercise intensity increases.

Cardiac output refers to the volume of blood pumped by the left ventricle of the heart. It is a combination of the heart rate (beats per minute) and stroke volume (amount of blood pumped with each beat). Endurance training increases maximal cardiac output, which increases the maximal level of oxygen delivery to the body’s tissues. The heart of an endurance athlete generally has a cardiac output much higher than that of an untrained person. Long-term adherence to a proper cardiovascular exercise program will create adaptations in the heart to increase stroke volume, which is the amount of blood pumped from a ventricle with each heartbeat. It is this increase in stroke volume that determines the level of cardiac output.

Physical exertion increases the demand for oxygen in the muscles. VO2 Max is widely viewed as a measure of assessment regarding a person’s aerobic athletic potential. VO2 Max is the capacity of the body to incrementally supply itself with oxygen during physical effort. Along with respiration, the oxidative potential of the muscles, and central nervous system motor drive, circulation is a physiological determinant of VO2. The body’s ability to distribute oxygenated blood in the contracting muscles is obviously a crucial component of physical endurance.

Blood volume refers to red blood cells and blood plasma in the circulatory system. Increasing blood volume increases the efficiency of the heart by assisting it to pump more blood per beat. Additional red blood cells transport more oxygen to muscles, allowing them to perform at higher intensities for longer periods of time. Erythropoietin — better known as EPO — is a hormone that signals cells to produce more red blood cells. When synthetic EPO is added to the body, uh… hypothetically, in the form of injection into a professional cyclist, the expansion of plasma volume and production of additional red blood cells can effectively increase VO2 max, which, of course, provides the athlete with higher levels of endurance. If you’re not as fond of injecting synthetic substances into yourself as cyclists are, similar adaptation can be achieved by prolonged training at high altitudes. This is the reason why high altitude locations like Big Bear Lake, California (surface elevation 6,750), have been popular training camp locations for boxers — a sport that requires superlative cardiovascular conditioning… oh, and for those of you that were expecting metaphor after reading the title — boxing also requires a “big heart”. There. Happy now?

There you have it — the abridged version of the literal athlete’s heart. Metaphor was never my strong suit. Who do I look like to you, Mark Twain?


Ekblom B and Hermansen L. Cardiac output in athletes. J Appl Physiol 25: 619–625, 1968.

Grimby G, Nilsson NJ, and Saltin B. Cardiac output during submaximal and maximal exercise in active middle-aged athletes. J Appl Physiol 21: 1150–1156, 1966.

National Center for Biotechnology Information. EPO erythropoietin.