The "intense training forces the whole" organism to "adapt" to this new condition of "super work" through the development of morphological and functional modifications, which are defined adaptations. As regards the cardio-circulatory system, the most striking adaptations are observed in athletes dedicated to aerobic or endurance sports, which require the achievement and maintenance for long periods of Cardiac Output (quantity of blood that the heart pumps into the circulation in a "unit of time) ceiling. Such adaptations make the heart of these athletes appear so different from that of a sedentary that it has been coined with the term "athlete's heart".
The presence of these adaptations allows the athlete's heart to perform better than normal during exertion.
Their extent varies according to:
type, intensity and duration of competitions and training sessions;
basic physiological characteristics of the subject, largely genetically defined;
age of the subject and time of commencement of the activity;
We can distinguish the Adaptations into:
CENTRAL ADAPTATIONS
PERIPHERAL ADAPTATIONS
At the expense of the heart
Affecting the blood, arterial, venous and capillary vessels
Central Adaptations
All the adaptations of the athlete's heart are aimed at receiving and pumping out of the ventricles a quantity of blood significantly higher than that of an untrained subject; the heart thus succeeds in significantly increasing the cardiac output under stress, satisfying the greater demands of O2. by the muscles. The main changes are:
- the increase in volume of the heart (cardiomegaly);
- the reduction of heart rate (bradycardia) at rest and during exercise.
The enlargement of the volume of the heart is the most important phenomenon for the purpose of increasing the Systolic Range (quantity of blood expelled at each systole) and the Cardiac Range. In athletes who practice very high level aerobic sports, the total heart volume can even double. Observing the heart of these athletes one can ask oneself when it should be considered "pathological", due to a heart disease.
To define these limits we must take into consideration the body size of the subject (body surface area). For example, in the animal world, the size of the heart strictly depends on its size and the type of physical activity it carries out; which naturally conditions the muscular energy demands. In fact, the largest heart of all is that of the whale, while the largest in relation to body weight is that of the horse.
In relation to what has just been said, in general, the largest hearts are also those that beat more slowly and vice versa; for example the heart of a small rodent called mustiolo exceeds 1000 bpm! (to know more).
With the advent of ultrasound it was possible to discover the existence of different adaptation models of the heart in athletes who practice different sports. As regards the left ventricle, two adaptation models have been identified:
ECCENTRIC HYPERTROPHY concerns aerobic endurance athletes, in which the left ventricle increases its internal volume and the thickness of its walls, assuming a rounded shape;
CONCENTRIC HYPERTROPHY concerns athletes dedicated to static, power sports, in which the left ventricle increases the thickness of the walls without increasing the internal volume, maintaining its original ovoid shape, or assuming a more elongated shape.
Ultrasound today has great power in the hands of the cardiologist because it allows him to distinguish a physiological cardiomegaly, due to training, from a pathological one, due to heart diseases linked to alterations in the normal functioning of the heart valves (valvulopathies) or to a dysfunction of the heart muscle (myocardiopathies).
Aerobic or resistance training causes important changes in the autonomic nervous system of the heart, characterized by a reduction in sympathetic tone (adrenergic, adrenaline) with a prevalence of vagal tone (from the vagus nerve where the fibers that reach the heart flow) this phenomenon is so called "relative vagal hypertonus". The most evident consequence of this new regulation of the autonomic nervous system of the heart is the reduction of the heart rate at rest. In a sedentary subject, even after a few weeks of training, it is possible to observe a reduction in HR of 8 - 10 bpm.
At great levels of competition it is possible to reach 35 - 40 bpm, values that configure the classic bradycardia of the athlete. At this point we can ask ourselves the question: "to what extent can an athlete's heart beat slowly?" the answer is now simple thanks to holter's electrocardiogram (ECG), capable of recording on magnetic tape for periods of 24 - 48 hours; this is essential to understand if such low values of HR are within normalcy.
THE HEART OF THE ATHLETE DURING THE EFFORT
At rest, the Cardiac Output of a trained athlete is comparable to that of a sedentary subject of the same age and body surface area, about 5 L / min in an adult subject of average build.
The difference between the heart of the athlete and that of the sedentary becomes clear during the effort. In highly trained endurance athletes, the maximum GC can exceptionally reach 35 - 40 L / min, practically double those achievable by a sedentary subject. .
The trained heart increases the GS compared to the resting values to a greater extent than that of the heart of a sedentary subject; in fact, at the same intensity of exercise, the HR in the athlete is always much lower than that of the sedentary (relative bradycardia during exertion).
In addition to these differences just described, there are other differences in the behavior of the heart during exertion. As they love that the HR increases during physical exercise, the time available to the ventricles to fill up (the duration of diastole) decreases in parallel: the trained heart, being more "elastic", has greater ease in accepting blood in its ventricular cavities and consequently is able to fill well even when the HR increases a lot and the duration of diastole decreases.This mechanism contributes to the maintenance of an elevated GS.
Peripheral adaptations
It is logical that the circulatory system, consisting of arterial and venous vessels, must also adapt to this new reality. In other words, circulation must be strengthened in order to allow the flow of blood flows (equivalent to car traffic) so high without "slowing down".
At the expense of microcirculation, the most important adaptations naturally concern the muscles, particularly the most trained muscles. The capillaries, through which the exchanges between blood and muscle take place, are distributed to a greater extent around the slow red muscle fibers, with aerobic metabolism (oxidative fibers), which need a greater quantity of oxygen.
In the "endurance athlete" training achieves an absolute increase in the number of capillaries and the ratio of capillaries / muscle fibers, a phenomenon known as capillarization. Thanks to it, the muscle cells are in the best conditions to take full advantage of the increased availability of oxygen and energy substrates. The increase in the capillary surface and in the vasodilation capacity of the muscular arterioles, means that the muscles are able to receive truly remarkable quantities of blood without increasing the average arterial pressure.
In addition to the vessels of microcirculation, the arterial and venous ones of medium and large caliber also increase their size ("athlete's vessels"). The phenomenon is particularly evident in the inferior vena cava, the vessel that brings blood from the muscles back to the heart of the lower limbs, used a lot in various sports.
As a result of resistance training, there is an increase in the coronary arteries, which feed the heart. The heart of the athlete, by increasing its volume and muscle mass, needs a greater supply of blood and a greater quantity of oxygen.
The increase in the caliber of the coronary arteries (the vessels that nourish the heart) is another of the elements that differentiate the physiological hypertrophy of the heart from the pathological one linked to congenital or acquired heart diseases.
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