What does military training do to the heart?
Military training has a significant effect on the heart. Because it increases endurance, power, and strength, it changes all muscles in the body, including the heart. It modulates the structure of the heart so that it can cope with abrupt, high loads of blood demand. This remodeling is named “the soldier's heart” [1, 2].
The first thing that changes is the size of the chambers. The high-intensity training practiced in the military causes a bilateral dilation of the ventricles [2]. Ventricles are the chambers from which the blood is ejected into the pulmonary and peripheral circulations. More space inside these chambers allows the heart to deliver a bigger amount of blood without significant increases in its beating rate. Thus, the performance of every heart stroke is higher.
Additionally, with the increase of the volume capacity of the ventricles, the muscle of the heart thickens. This increase in muscular mass increases the stroke power and thus adds to the effectiveness of the strokes. In cardiology, these changes are referred to as eccentric heart remodeling.
This remodeling is most visible on the left side. It is due to higher blood pressure against which the left ventricle has to work [1]. This is what is understood under the term "the soldier's heart." Normally, this condition is a physiological reaction and is not within a harmful range.
However, these changes are not always beneficial. Because the heart increases in muscle mass and thickness, a problem with its blood supply arises. It might be that some soldiers may experience acute ST-Elevation Myocardial Infarction (STEMI). Such a condition happens when the heart fails to deliver enough oxygen to its muscle tissue. Typically, it occurs during strenuous military training, when the oxygen demand is the highest. The primary symptom is acute chest pain [3].
What is military training?
Military training is practiced in the army as part of the preparation of soldiers for duty. It is meant to enhance fitness, military skill, and mental resilience. The training includes a variety of activities. Soldiers perform a mix of endurance training, strength training, and task-specific training [4, 5, 6, 7, 8, 9].
Endurance training serves as aerobic physical conditioning. It comprises long-distance runs and prolongation of some other activities. Regular practice increases cardiovascular fitness and aerobic energy metabolism [9]. Strength training, on the other hand, focuses on enhancing muscle strength and power. It can be performed by lifting heavy weights, many sorts of drags, and other weight-linked exercises [5]. Task-specific training aims to improve performance in specific military tasks. This type of training is a combination of both the above-mentioned strength and endurance training. This mixing ensures optimal results in duty-related tasks [5].
Mental resilience training (MRT) is also a key component of military training and serves as a supplement. During MRT, soldiers are trained to cope with stress, emotions, and to increase their self-confidence [6].
Military training is not without its challenges. It places great strain on the musculoskeletal system of soldiers, leading to a high incidence of non-battle musculoskeletal injuries. Therefore, the development of safe and effective training programs is a critical area of research [10].
What is the physiology behind changes in the heart in military training?
Military training, like other forms of intense physical activity, leads to the already mentioned significant changes in the heart's structure. Thanks to a phenomenon referred to as "soldiers’ heart syndrome," heart function increases. This adaptation is a physiological response to the increased demands for oxygen in the body [11].
The remodeling of the heart includes increased capacity and muscular thickness of the chambers. A few pathways that stimulate these changes exist. One of them is lower oxygen levels in the heart itself. As the heart’s metabolism is strongly aerobic, lower levels of oxygen trigger a stress response. In terms of this response, the heart rate and blood pressure increase. During exercise, the heart is additionally stimulated by hormones like adrenaline, which also increases the heart rate. The fast and overwhelming pressure on an untrained heart causes a reaction similar to that in normal striated muscle. The micro-damage to the cardiac muscle layer triggers repair. During this muscle renewal, each time the muscle thickness increases a little bit. This process continues until the damage to the heart is minimal, and no need for more power is needed anymore.
On the other hand, the dilation, or the stretching of the ventricles, is created by the chronic filling pressure increase. It is a natural condition when the blood demand in the body is higher. With the increase in systolic blood pressure, the pressure coming back from the small (pulmonary) as well as the peripheral circulations is increased, pushing more blood into the ventricle. When repeated often, the chamber stretches under the pressure of the blood. The changes in the structure of the heart lead to functional changes. A good thing is the change in the diastolic (filling) function of the ventricle.
However, not all functional changes are positive for the heart. For instance, a study by Amir Aslani et al. found that peak systolic myocardial velocity significantly decreased after military training. This indicates that strenuous, prolonged exercise may result in depressed left ventricular contractile function [12]. A direct influence on contractility can be a phenomenon of small fibrosis in the damaged regions. The fibrous fibers are not contracting.
Changes in the heart mostly affect the left side. However, the right ventricle changes in the same manner [2]. The changes are less visible because the pressure with which the right heart works is significantly smaller than that of the left heart.
Can military training help the heart?
Strength training is a great way to improve heart health. It raises good cholesterol and lowers bad cholesterol. This reduces the risk of heart attack or stroke. The American Heart Association suggests participating in strength training exercises at least twice a week to keep the risk low [13].
On the other hand, M. R. Rinder et al. found that endurance exercise training increases the left ventricular ejection fraction (the functionality of the systolic phase) in coronary artery disease. Moreover, the passive filling of the heart was also increased [14].
The specific high-intensity aerobic exercise training for the military has been shown to generally improve the heart. It changes the peripheral resistance and increases the heart capacity [15]. Thus, the heart is less overloaded in intensive situations, decreasing the risk of heart fatigue.
However, very strenuous prolonged exercise may result in depressed left ventricular contractile function. If not balanced, military training raises the possibility of cardiac fatigue [12]. Therefore, it's crucial to slowly increase the load and respect the limits.
Can military training shorten life?
The influence of military training on life expectancy is a complex issue. Many factors play a role: genes, diet, and the living environment impact the final age with different ratios and influences on each other. Evidence exists that indicates a positive impact on longevity. For example, a study by Michael Hartal et al. found that 67.9% of retired military personnel lived longer than other non-army individuals [16].
However, as in the case of some drugs, the dose makes the toxin. Similarly, military service can induce negative effects. Prolonged high levels of stress can negatively impact the neuronal system [17]. In terms of physical performance, military training significantly improves endurance capacity and muscular strength [18, 19]. This allows for generally better well-being and avoidance of age-related injuries.