Physiological changes caused by sports

Regular high-intensity training causes structural and functional changes in the heart. The concept of athlete’s heart refers to changes arising in the structure and functions of the heart as a result of high-intensity physical training. According to normal criteria, many changes are abnormal and therefore an indication of an intestinal defect or disease. However, it is a matter of the body’s normal physiological adaptation to the situation.

In the case of an athlete’s heart, we can distinguish between structural and electrical remodelling. Before going into more detail, let us address the benefits of sports on a general level.

Health benefits of sports

Regular, medium-intensity endurance sports have many beneficial effects on the body. In the recommendations for exercise, the concept of healthy exercise involves walking briskly for at least 150 minutes every week (for example, 30 minutes at a time five days a week) or heavy exercise, such as running, for 75 minutes every week.

The sports-related Finnish “Käypä hoito” (good treatment) recommendation also states that additional health benefits can be gained from exercise that goes beyond the minimum recommendation. Besides endurance exercise, it is recommended to practise medium-intensity exercise that maintains or increases muscular strength and endurance at least two days every week. It is obvious that the majority of athletes can easily reach these goals.

Effects of endurance training

According to studies, exercise lowers blood pressure and positively affects blood fat levels, blood sugar balance and system inflammation as well as helping to control weight. All of these changes improve your health and prognosis. The health benefits of exercise are clearly more significant than any potential harmful effects.

According to studies, exercise and good physical condition reduce mortality related to cardiovascular diseases: the risk of death related to a heart condition is 20–35% lower in people who exercise actively and are in good shape compared to persons who exercise little or are in bad shape.

Endurance sports increases the circulation, strength and aerobic energy production of the bone muscles and improves the performance of the cardiovascular system. Exercise causes the heart rate and blood pressure to rise more slowly than otherwise. Exercise affects the blood coagulation system by reducing the tendency of blood platelets to clot and reducing the amount of fibrinogen that increases the occurrence of blood clots.

A dysfunction in the interior layer of the arteries, endothelium, greatly contributes to the development of the coronary artery disease. Exercise improves the functionality of endothelium, resulting in lower blood pressure and more effective circulation in the system. The flow resistance decreases in the small branches of the coronary arteries, which in turn improves circulation in the coronary arteries.

Autonomic nervous system

The sympathetic and parasympathetic nervous system form the system’s autonomic, or involuntary nervous system, which plays a major role in regulating the functions of the cardiovascular system. Furthermore, the heart has its own rich neural system that is not as well known.

As the influence of the sympathetic nervous system grows, this causes the blood pressure, heart rate and respiratory frequency to rise and performance capacity to improve. This is needed during an athlete’s training periods and competitive performances. The parasympathetic nervous system becomes dominant when system functions need to be soothed. Due to the impact of the parasympathetic nervous system, the heart rate and respiratory frequency decrease. We can also observe an increase in heartbeat variability. Therefore, under the influence of the parasympathetic nervous system, heart rate variability will increase.

Regulating human heart rate

The heart’s own “pacemaker”, the sinus node, is located in the right atrium of the heart. Each heartbeat is initiated through an electrical stimulus in the sinus node. The sinus node strongly depends on the functions of the autonomic nervous system. The heart rate frequency remains low in rest, usually at 50–70/min, due to the slowdown caused by the parasympathetic nervous system. During physical exertion, there is less parasympathetic slowdown. The influence of the sympathetic nervous system manifests itself partly through the impact of noradrenalin and adrenalin in the heart cells. Muscle work in the feet, in particular, causes more artery blood to return, which accelerates the cardiovascular centre and increases the heart rate.

Heart rate at rest varies according to the pace of breathing and in relation to the amount of blood returning to the heart. This means that the time difference between heartbeats is not standard even at rest, with small variations occurring. This “sinus arrhythmia” cannot be easily detected by checking the pulse, but it can be easily diagnosed based on the ECG of children and young people. “Arrhythmia” in the name refers to normal arrhythmia. Heart rate variability transmitted by the sinus node slows down with age.

Changes in an athlete’s autonomic nervous system

Endurance training changes the balance of the autonomic nervous system so that the parasympathetic nervous system becomes more active. This prevents hyperactivity in the sympathetic nervous system, which reduces stress hormone secretion.

The increasing influence of the parasympathetic nervous system is most clearly visible in endurance sports athletes as well as athletes whose training includes plenty of endurance training. The impact is manifested as lower resting heart rate. The resting heart rate may be below 40/min. However, the intensity of the phenomenon depends on the individual: not everybody has very slow heartbeats.

With the lower resting heart rate, the athlete often receives a “diagnosis” in their ECG graph. ECG devices have an automatic analysis programme that produces measurement results and diagnostic suggestions for the ECG graph. Heart rate is one of these measurement values provided. The lowest limit for normal heart rate has been set to 60/min (50/min in some devices) in the analysis algorithms of the device. Because of this, an athlete often sees a diagnosis of “sinus bradycardia” in their ECG graph. This indicates a normal rhythm of the heart, a sinus rhythm with a low heart rate.

Endurance training strengthens the aforementioned heartbeat variability. This is considered a beneficial impact. Continuous, reduced heartbeat variability is a sign of the influence of the sympathetic nervous system, which could worsen the prognosis of, for example, heart patients. Many kinds of research methods have been developed for heart rate variability, including mobile applications (mobile health application; mHealth application). The significance of the method in terms of monitoring an athlete’s performance is under debate.

In athletes, the strong influence of the parasympathetic nervous system at rest is also visible in the slowdown of the electrical conduction between the atria and the ventricles. Electrical conduction between the atria and ventricles passes through the atrioventricular node, and this structure is regulated by the autonomic nervous system. Conduction between the atria and ventricles can be assessed in the ECG with a PR interval (also referred to as PQ interval). At night in particular, we can see that the PR interval is slightly longer, indicating that the conduction between the atria and ventricles has slowed down a little. This phenomenon occurs more frequently among athletes, and the ECG graph may display a diagnosis of “First degree AV bloc”. This is a normal occurrence for athletes.

It is important to distinguish between sports-induced changes in regulating the heart’s autonomic nervous system and various heart defects. This can be done by putting the heart under strain. An athlete’s heart rate rises during exertion while the PR interval becomes shorter and the heart rate variability grows smaller. This can be easily seen in a stress test or long-term ECG monitoring (Holter examination). In case of defects in the heart’s conduction pathways, abnormal changes may be highlighted in a stress situation.

The heart’s functionality as a pump

The heart is a pumping muscle that contracts approximately 100,000 times in 24 hours and pumps, on average, 10,000 kg blood in the same period of time. Cardiac output, or the amount of blood the heart pumps in a minute, is the product of the heart rate and the stroke volume (the amount of blood the heart pumps in one contraction). At rest, when the heart rate is approximately 50/min, a young person in good shape has the stroke volume of approximately 100 ml. Therefore, their cardiac output is approximately 5 litres. During heavy exertion, when the heart rate frequency may even quadruple, there could be a five-time cardiac output increase – in our example, approximately 25 litres per minute. An endurance sport athlete’s heart starts pumping more intensively, and a top athlete is able to increase their stroke volume and thus their cardiac output even up to the value of 40 l/min.

During exertion, there is a 3–5 fold increase in a normal person’s coronary artery flow. Furthermore, the transfer of oxygen from blood to tissue intensifies during exertion, which increases the oxygen supply of the system.

Structural changes related to athlete’s heart

The heart has an extraordinary ability to respond to changes induced by a growing workload in multiple ways. If a person has a severe valve leak, their heart grows in size to ensure that enough oxygen and nutrients are transferred into vital organs. This phenomenon is referred to as volume overload. Valve obstruction or continuously high blood pressure leads to a pressure overload in which the heart ventricle’s wall becomes thicker.

In athlete’s heart, the changes induced by an increased load vary a little based on the nature of the training. Changes typical of volume overload are developed during endurance training, wherein the growth of heart cavity volume is proportionally larger than the thickening of the ventricle wall. The changes resulting from weight training bear closer resemblance to the consequences of pressure load, wherein the growth in wall thickness is proportionally larger than the increasing volume of the cavities.

We do not know exactly what is the age when changes typical of athlete’s heart start to develop and what kind of physical training is involved – how long it takes and how straining it is. Even teenagers training in endurance sports have been found to have a thickened wall in the left ventricle.

We know little about what kind of schedule is needed for recovering from training-related structural and functional changes. There have been suggestions that the enlarged volume of the left ventricle and the thickness of the wall will both decrease and the ECG changes related to athlete’s heart will return to normal in 1–3 months after the end of training.

Athlete’s heart or a heart disease?

Based on the above, we can draw the conclusion that many sports-related changes in the structures and function of the heart resemble various heart diseases. It is crucial to distinguish these from each other. An endurance sport athlete often has changes in their ECG, and special expertise is often required to interpret the changes. However, a good ECG interpretation will not always help to tell the difference between changes related to athlete’s heart and changes resulting from a heart condition. Other examinations are needed in this case.

A heart ultrasound examination is crucial in order to diagnose a structural defect in the heart or to rule it out. In addition to this, a magnetic resonance imaging examination of the heart is rarely needed. A stress test and Holter examination help distinguish the electrical system of the heart when diagnosing a suspected disease. The number of examinations and their selection are decided case-specifically.

Text: Kjell Nikus, Professor of Cardiology (emeritus), Doctor of Medicine, Cardiologist, Heart Hospital