When you train regularly, it's not only your muscles or your endurance that change. Your heart also adapts to the load. This cardiac adaptation through training is a central mechanism that influences your performance in the long term.
In sport, people often speak of the so-called athlete's heart. This refers to a series of structural and functional changes in the heart that arise from regular physical activity. But how exactly does this cardiac adaptation through training occur, and what role do different forms of training play?
Why the heart adapts to training
The heart is a muscle whose task is to pump blood through the body. During physical activity, the demand for oxygen and nutrients increases so that enough energy can be produced. To meet this demand, the heart must transport more blood per minute.
This output is called cardiac output and results from two factors:
Cardiac output = stroke volume × heart rate
The stroke volume describes how much blood the heart ejects with a single heartbeat. The heart rate determines how often the heart beats per minute.
An important cardiac adaptation through training is that the stroke volume increases. As a result, your heart can transport more blood per beat and needs to beat less often for the same performance.
The athlete's heart: structural changes
The changes in stroke volume are closely linked to structural adaptations of the heart. During physical exertion, both the amount of blood the heart must transport per minute and the pressure against which it pumps the blood into the circulation increase. When the heart regularly has to move larger volumes of blood, the heart muscle also adapts over time – especially the left ventricle, that is, the heart chamber that pumps the blood into the systemic circulation.
Two fundamental adaptation tendencies can be observed here:
Volume-focused adaptation
Here it is above all the volume of the heart chamber that increases. As a result, more blood can be taken in and then ejected again. This form of cardiac adaptation through training is frequently observed in endurance athletes.
Pressure-focused adaptation
Here it is rather the wall thickness of the heart muscle that increases. This can be a response to increased pressure loads, for example when the heart has to work against a higher blood pressure during intense exertion.
In practice, however, these adaptations usually do not appear in isolation and in such a simplified way. Often it is a combination of these and further adaptation mechanisms.
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The role of training intensity
How strongly and in what form the heart adapts depends, among other things, on the training intensity.
During moderate exertion – for example a relaxed endurance run or cycling at a steady pace – the heart rate remains relatively stable. At the same time, the venous return, that is, the flow of blood back to the heart, is increased. As a result, the heart fills up well before it ejects the blood again.
This situation can be a favourable condition for a cardiac adaptation through training in which the stroke volume increases in the long term. The repeated filling of the heart causes the heart walls to stretch slightly during diastole. This stretching of the heart walls can strengthen the subsequent contraction of the heart muscle. When this load occurs regularly, it can contribute in the long term to the heart filling more strongly and thus being able to eject more blood per beat. The stroke volume increases.
During very intense exertion, the heart rate rises significantly. In many people, it also becomes apparent that the stroke volume is already close to its maximum during moderate exertion. In people not specifically trained for endurance, it is frequently observed that from about 50–60 % of maximum performance capacity it increases only slightly further. If the training intensity rises beyond that, the additional cardiac output is therefore achieved above all through a further increase in heart rate. One reason for this is that as the heart rate rises, the filling phase of the heart – the diastole – becomes ever shorter.
In well-trained endurance athletes, by contrast, the stroke volume can in part continue to increase even at higher intensities.
Due to the described increase in heart rate, the so-called diastole shortens, that is, the phase in which the heart fills with blood. At the same time – as already mentioned – blood pressure also rises during physical activity. This means that the heart muscle has to contract more strongly to pump the blood into the circulation.
This stimulus too can influence structural adaptations of the heart muscle. Among other things, adaptations of the heart musculature can occur that may be associated with a thicker wall structure of the ventricle.
Interval training and cardiac function
Interval training is today one of the frequently used training methods in endurance training. Phases of higher exertion alternate with recovery phases. This repeatedly creates different loading conditions for the cardiovascular system.
During intense intervals, the heart works close to its maximum performance capacity. The contractions are strong, while the filling phase of the heart is relatively short due to the elevated heart rate.
In the subsequent breaks, the load decreases again. However, the heart rate often remains elevated, while the venous return – that is, the flow of blood back to the heart – can remain increased. As a result, the heart can fill up more strongly with blood again during this phase.
In such situations, different loading stimuli act on the heart: short phases of high pumping work alternate with phases of stronger filling. These repeated changes in load can help to improve the pumping function of the heart and, in the long term, also increase the stroke volume.
In many training concepts, interval training is therefore used as a supplement to longer, moderate endurance sessions.
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What role does strength training play?
Also, strength training too can influence the heart, though usually through somewhat different loading mechanisms than classic endurance training.
During heavy strength loads, the heart rate and blood pressure rise significantly in the short term. The heart therefore has to work against increased pressure. This form of load differs from the rather continuous volume load that arises during many endurance efforts.
Cardiac adaptation through training can therefore, in strength training, be more strongly associated with pressure loads. Studies show, however, that even with regular strength training, adaptations of circulatory regulation can be observed that are frequently associated with endurance training and repeated volume loads.
Why different forms of training can be worthwhile
Different forms of training exert different stimuli on the cardiovascular system.
Moderate endurance loads enable steady cardiac work over a longer period and are frequently associated with adaptations of the stroke volume.
More intense intervals place higher demands on the maximum performance capacity of the heart.
Strength training too can influence the cardiovascular system. Although it is traditionally less associated with the classic cardiac adaptations of endurance training, studies show that adaptations of cardiovascular function can occur here as well.
From a training science perspective, a combination of different forms of training can therefore be worthwhile. The concrete design always depends on goals, training level and individual conditions.
Conclusion
Through regular physical activity, it is not only your performance that changes, but also the structure and function of your cardiovascular system.
Cardiac adaptation through training can show itself, among other things, in a higher stroke volume, an improved pumping function of the heart and structural changes in the heart muscle.
Training duration, intensity and form of training play an important role here. Different forms of training set different stimuli, which is why many training programmes combine several methods with one another.
