We all know that “exercise is good for the heart” – but, what exactly does this mean?

It’s widely acknowledged that regular exercise cuts the risk of cardiovascular diseases such as heart disease and stroke. Less is known, however, about the direct, physical adaptations to the heart induced by exercise.  In celebration of National American Heart Month (and, perhaps somewhat spuriously, Valentine’s day), we explore some of the changes your heart undergoes in response to regular, vigorous exercise.

1. A bigger heart *

We’ve known for a long time that sustained exercise training causes enlargement of our heart. In the late 19^th Century, the Swedish-American physician Henschen noted that cross-country skiers had larger hearts than the average person, a phenomenon thought to result from adaptation to long-term, intense aerobic exercise.

With the advent of more advanced imaging techniques in the following centuries, we can now directly measure increases in the size and volume of our heart chambers.

Unlike the traditional-shaped heart that adorns Valentine’s cards, chocolate boxes and ‘MOM’ tattoos, our actual, beating hearts are formed of 4 differently-sized chambers. The top two chambers are called the atria, whereas the bottom two are called the ventricles.

While all chambers need to function properly, the left ventricle is particularly important during exercise as it pumps freshly oxygenated blood to supply our working muscles.

On this note, studies suggest that the size and volume of the left ventricle increases in response to exercise. For example, one study showed that 3 years of professional cycling led to a 2 mm increase in left ventricular diameter (the distance between opposing walls of the left ventricle) from 58.3 mm to 60.3 mm.

Other studies show that exercise in previously sedentary people leads to increases in the volume of both right and left ventricles at the end of diastole, when the ventricles are relaxed and fully filled with blood. The benefit of larger volume heart chambers is fairly intuitive – it allows more blood to be pumped with each heartbeat. In physiological terms, we say the heart has a higher stroke volume. This greater output of blood helps to supply exercising muscles.

For example, an untrained person’s heart pumps about 100 – 135 ml per beat during exercise. Due to adaptive enlargements in heart size, however, an elite athlete’s heart can pump over 200 ml per beat during exercise.

2. Thicker heart walls *

As well as causing the cavity of heart chambers to increase in volume, regular exercise also thickens the walls of our heart chambers. This results from a process called hypertrophy – an increase in size of individual heart muscle cells (myocytes). The benefit of this response is that it allows the heart to contract more forcefully and pump harder.

Interestingly, the exact nature by which your heart muscle increases in size differs according to what type of exercise you perform. Strength training (e.g. weightlifting) tends to lead to what is known as concentric hypertrophy. This leads to more pronounced increases in thickness of heart chamber walls, particularly the walls of the left ventricle.

By contrast, endurance exercise (e.g. long distance running) tends to elicit eccentric hypertrophy. This is characterised by a lengthwise (longitudinal) increase in size in individual heart muscle cells, which gives rise to a larger heart chamber and less pronounced wall thickening.

Nevertheless, both concentric and eccentric hypertrophy involve enlargement of heart muscle cells. Moreover, lots of sports and exercises will probably cause a mixture of both types of adaptation.

3. A more efficient heart

As an athlete’s heart becomes larger than average, it will also develop higher than average demands for oxygen and nutrients to sustain it. Fortunately, the more you train, the more efficient your heart becomes.

Studies show that trained athletes’ hearts are better at extracting oxygen from blood. This effect probably arises from a more efficient blood flow distribution through the heart. This includes a greater surface area of blood capillaries, which helps to supply working heart muscle cells.

4. Lower resting heart rate *

The five-time Tour de France winner, Miguel Indurain, reportedly had a resting heart rate of just 28 beats per minute! To put that figure in context, a typical adult has a resting heart rate between 60 and 90 beats per minute.

So, how did/does Miguel Indurain stay alive with such a low heart rate? The answer actually involves point (1) - a larger heart. As established, athletes, especially endurance athletes like Indurain, have larger hearts capable of pumping a greater volume of blood with each beat. By virtue of this higher ‘stroke volume’, their hearts do not need to beat as fast to sustain an adequate output of blood to tissues and organs at rest.

You don’t have to be an elite athlete to develop a lower resting heart rate. On starting and maintaining regular exercise in the long term, you may notice your resting heart rate gradually start to drop. This effect is due to increased activity in your vagus nerve, which acts to slow down the heart at rest.

5. Reduced blood pressure

One of the most touted benefits of exercise is that it helps to reduce blood pressure. In simple terms, blood pressure refers to the pressure of blood circulating in your blood vessels. If this pressure becomes too high, it puts additional strain on your heart. High blood pressure, also known as 'hypertension', also raises the risk of blood vessels bursting (e.g. as happens in haemorrhagic stroke) and blood clots forming (thrombosis).

There are several different (not to mention complicated) mechanisms that underlie exercise’s beneficial effect on blood pressure. These include changes in the patency of blood vessels, differences in the way your kidney reabsorbs water and salt and changes to your sympathetic nervous system.

Ultimately, by lowering blood pressure, exercise will lessen the strain placed on your heart. This is particularly true if you already suffer from high blood pressure.  

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* Note – we are specifically discussing these changes in the context of healthy, reversible adaptations to exercise, where heart function is normal. These beneficial physiological responses to exercise are sometimes grouped under the term ‘athlete’s heart.’

As you may know, there are also pathological conditions which also cause larger hearts (e.g. dilated cardiomyopathy), thickened walls (e.g. hypertrophic cardiomyopathy), and/or changes in heart rate (e.g. arrhythmias), sometimes in athletes. The hallmark of these pathological heart changes ( in stark contrast to the heart changes in the above article) are that they are irreversible, maladaptive and ultimately compromise heart function.

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