Understanding the Athlete’s Heart

It should be obvious that an athlete, by demanding more performance from his or her arms, legs, and core muscles than the nonathlete, simultaneously demands more performance from his or her heart. But do some of these demands create long-term damage? Researchers are only beginning to find out. Let’s first take a look at what the athlete asks of the heart that the nonathlete does not.

Number of Total Beats

The average adult male heart beats 72 times a minute at rest. The average resting heart rate for females is 80 beats per minute. (A child’s resting pulse might range from 90 to 120 beats per minute.) Assuming the same average rate while both sleeping and awake, this totals about 100,000 beats per day for men and 115,000 for women, about 38 million beats per year for men and 42 million for women, and about 3 billion beats during an 80-year lifetime for men and 3.4 billion for women.

Due to conditioning, an endurance athlete’s heart tends to beat considerably slower than 72 bpm while at rest. The amount of blood the heart pumps in a given time period (cardiac output in liters per minute) is proportional to the size of the heart’s stroke volume as well as to the heart rate. Due to the cardiac hypertrophy that is one component of a collection of features of cardiac adaptation to long-term exercise, a condition dubbed “Athlete’s Heart,” the left ventricle (the one that pumps blood throughout the body) becomes enlarged, allowing the heart to maintain sufficient blood flow to the body while beating at a lower rate (see below). This is one of the reasons why athletes generally have lower resting heart rates than nonathletes.

An athlete’s heart also tends to get revved up to very high rates almost every day, often for many hours at a time. If the athlete’s heart beats at 40 beats per minute during 8 hours of sleep per day, averages 60 bpm during waking hours, and then zooms up to 125 bpm for 3 hours of working out daily, then the daily total is actually less than the example above: around 88,000 beats per day, or 32 million beats per year and 2.5 billion beats during an 80-year lifetime. Three hours of aerobic exercise every day is already high for most athletes, but even if the athlete were to spend 6 hours per day working out at 125 bpm, assuming 40 bpm while sleeping for 8 hours and 60 bpm the rest of the time, the athlete’s heart still beats fewer total beats in 80 years than if it had been averaging 72 bpm that entire time.

Certainly during intense workouts, heart rates generally go much higher than 125 bpm, but this is counterbalanced by the fact that the higher the heart rate, the shorter the time it can be maintained. An average of 125 bpm over long endurance workouts as well as interval workouts with rest periods between is certainly in the ballpark. No matter how you slice it, it’s likely the calculation for athletes will not yield significantly higher totals over 80 years than 3 billion beats for males and 3.4 billion beats for females.

Volume of Blood Pumped

At rest, the average male heart ejects around 70 ml (0.018 gallons) of blood with each contraction. At an average resting heart rate of 72 bpm, that is 1.3 gallons per minute. Since women are generally smaller than men and thus have smaller hearts, the ejection volume is lower, and hence the average resting heart rate may be higher. Since a man has an average of 5 liters (1.32 gallons) of total blood (women average 4 liters, or 1.06 gallons), this means that, at rest, the body circulates its entire volume of blood every minute.

At 200 bpm and 0.018 gallons per beat during intense exercise, this becomes 3.6 gallons per minute. In other words, the entire volume of blood in the body will be circulated nearly three times per minute when you are working out at your maximum rate. Taking into account the enlarged heart size that results from long-term training, it is estimated that some athletes pump up to 8 gallons of blood (30 liters, or six times the total blood volume of the average man) per minute during intense exercise. A normal male heart weighs around 10.5 ounces (300 grams), whereas the average female heart is 9 ounces (250 grams). However, the hypertrophy of an athlete’s heart can bring its weight up to 500 grams or more, which explains the higher volume of blood it can move through the body.

Imagine how much blood 8 gallons (or even 3.6 gallons) is; a kitchen faucet can take two minutes just to fill a 3-gallon bucket. Even while the body is resting, the heart is pumping almost as much blood as a kitchen faucet at full blast. And, let’s not forget, blood is thicker than water. That is an awesome little pump we have in our chest.

Using that average blood output number of 1.3 gallons per minute, the average human heart pumps 78 gallons per hour, 1,872 gallons per day, or 683,280 gallons per year. Given that a railroad tank car holds about 30,000 gallons, the heart is filling the equivalent of 23 tank cars per year. In an 80-year lifespan, that’s 55 million gallons of blood, enough to fill 1,833 railroad tank cars. To gush out the same amount of water as the blood a human heart pumps in an average lifetime, a kitchen faucet that flows 1.5 gallons per minute would need to be turned on full for over 69 years.

There are about 60,000 miles of blood vessels in the adult body, enough to wrap the earth’s circumference twice with some left over (the earth’s equator is only 25,000 miles around). Those vessels range in size from the aorta, which is nearly the diameter of a garden hose, to capillaries so thin that ten of them together are only as thick as a human hair.

The output of the heart is remarkable; so too is its endurance. Until your heart weakens for other reasons, it won’t fatigue from pumping these massive volumes of blood day in and day out. Squeezing a tennis ball in your hand is similar to the force of a beating heart, yet there’s no way you could do it 100,000 times a day as your heart does. And another 100,000 times tomorrow. And the day after that. And…

Blood Pressure in Athletes

In addition to a low resting heart rate, endurance athletes also tend to have low resting blood pressure. Their systolic pressure (the top number that represents the pressure when the heart muscle contracts) goes up less with exercise than it does in nonathletes. The increase in systolic blood pressure is generally under 50 percent and temporary. Diastolic blood pressure (when the heart relaxes) stays steady and may even drop during exercise due to blood-vessel dilatation. These relaxation properties of exercise are important, in that they are probably one of the key ways by which regular exercise delivers health benefits.

Since it is the left ventricle that sends blood throughout the body, research on the work done by the right ventricle, which sends deoxygenated blood to the lungs, has lagged. However, it is often the one in which ventricular arrhythmias in athletes appear. The study of pulmonary artery hypertension (PAH) inevitably has led to investigation of the right ventricle, which is the pump that generates pulmonary pressure. To help understand how patients with PAH respond to submaximal exercise, studies have been performed on normal volunteers as well as on athletes.

The major differences between the left ventricular/arterial interaction and the right ventricular/pulmonary vascular system interaction can be found in the relative sizes of the two systems and their relative blood pressures. The total surface area of the body’s arterial system is 800 to 1,000 square meters, compared with a surface area of only 80 to 100 square meters in the pulmonary vessels. And while the blood pressure in arteries throughout the body is on the order of 120/70 mmHg in healthy people, blood pressure in the pulmonary vessels generally averages 20/8 mmHg. The same amount of blood flows through the pulmonary system, yet the pressure in it at rest is only 10 to 20 percent as high.

The right ventricle in a trained athlete, however, has the power to greatly increase pressure in the pulmonary vascular system to maximize oxygen exchange in the lungs. Elite endurance athletes can see a more than fourfold increase in peak pulmonary pressures (pushed by the right ventricle) during exercise, while peak arterial systolic pressures (pushed by the left ventricle) may remain within 10 mmHg of the at-rest recordings.

Increases in pulmonary blood pressure and corresponding increases in blood pressure in the right ventricle and left atrium (where the oxygenated blood from the lungs circulates to) may be variables that contribute to cardiac electrical problems in long-term elite athletes. Right ventricular and left atrial remodeling observed in these athletes can lead to the establishment of the compromised substrate required for arrhythmias to develop.

Heart Power Output

To compute the power output of the heart, we can multiply the blood pressure times the volume of blood flowing through the body per unit time.

To work in round numbers, consider a large person with 6 liters of blood. At resting heart rate, a person’s entire blood volume circulates approximately once per minute, so the flow rate is 6 liters in 60 seconds, or 6,000 cm³ in 60 seconds = 100 cm³/s.

Again, to work in round numbers, consider the blood pressure to be 120/80 mmHg, so the average pressure (ignoring pulsations in flow) is 100 mmHg = 10 cmHg = 10 × 1.33 × 10⁴ dynes/cm² = 133,000 dynes/cm².

The average power output is the pressure times the flow rate, or 133,000 dynes/cm² × 100 cm³/s = 1.33 × 10⁷ ergs/s × 10^-7 joules/erg = 1.33 J/s = 1.33 watts.

And when a highly trained athlete is circulating those 6 liters of blood three times per minute, rather than just once a minute, then the heart’s power output jumps to 3 × 1.33 W = 4 watts.

And in athletes who can move 8 gallons of blood per minute (30 liters), this becomes 5 × 1.33 W = 6.7 watts.

When compared to a 100-watt lightbulb or the 400 watts you can get to appear on your bicycle power meter by pedaling hard, 1.33 W or even 4 W or 7 W may not seem like much. However, unlike the lightbulb or your legs, the heart continuously produces power for your entire lifetime. The amount of work done by the heart (its energy output) at rest in a day is 1.33 J/s × 24 hrs/day × 60 min/hr × 60 s/min ~ 115,000 J/day.

Compare this amount to the kinetic energy (KE) of a 100 kg (220-pound) boulder falling off a building. KE = mgh (m = mass in kilograms, g = acceleration due to gravity, and h = height in meters), so the height of the building, h, equals h = KE/mg = 115,000 J / 100kg / 9.8m/s² = 117m = 383 feet.

So the heart produces as much energy in a single day as a 100 kg boulder does while falling from a building 383 feet tall (or as much energy as it would take to lift that boulder up to that height). That would leave a big dent in the sidewalk; 15 states in America don’t even have a building tall enough to do this experiment!

The mechanical efficiency of the heart is less than 10 percent, so the heart will demand over 10 times as much energy (in Calories) as it produces, or in this case, 10 × 115,000 J = 1,150,000 joules in a day, or 1.15 × 10⁶ J × 0.239 cal/J × 10^-3 Cal/cal = 275 Calories.

In other words, your resting heart requires about one Clif Bar per day in order to keep ticking.

So that I could have round numbers in the calculations above, I used 6 liters of blood in the body, but the average man has about 5 liters, and the average woman has perhaps 4 liters. Using 4.5 liters of blood to simplify the final calculation below, the power output of the average human heart at rest would be one watt (1 J/s), rather than 1.33 watts.

The amount of power the heart constantly generates inside you is nothing to scoff at; the average human heart at a resting output of one watt for 80 years does this much work: 1 J/s × 80 yrs × 365 days/yr × 24 hrs/day × 60 min/hr × 60 s/min = 2.5 gigajoules (in other words, 2.5 billion joules).

And if this average-sized human were to spend a tenth of each day (2.4 hours) exercising at an average cardiac power output of 3 watts, this total would become 3 gigajoules (3 billion joules) of work done by the heart in a lifetime. You will never ride your bike enough to rack up that kind of total energy output on your power meter.

The Takeaway

While an endurance athlete’s heart may not beat as many times per year as a sedentary person’s, it can pump out six times as much blood as a kitchen faucet will put out at full flow. And when it’s pumping that much blood during an intense workout, it may endure sustained pulmonary blood pressures four times greater than those of the sedentary person’s heart. When the workout is over, the heart keeps producing as much energy every day as a 220-pound boulder would represent in kinetic energy after falling from a 38-story building. It should be no surprise that the heart muscle will grow in size to deal with the extraordinary demands an endurance athlete puts on it decade after decade.

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Adapted from The Haywire Heart by Chris Case, Dr. John Mandrola, and Lennard Zinn, with permission of VeloPress.

The Haywire Heart