Why do we get tired? Why does fatigue occur? What happens to the body during sports? This question is answered by sports physiologists who have been studying the body of athletes for more than a century.
In order to find out why do we get tired, first, let’s see some basic definitions.
Fatigue is the most important problem in sports physiology. Knowledge of the mechanisms of fatigue and the stages of its development allows one to correctly assess the functional state and performance of athletes and should be taken into account when developing measures aimed at maintaining health and achieving high sports results. There are many theories about the origin of fatigue.
What is Fatigue?
Fatigue is a functional state of the body caused by mental or physical work, in which there may be a temporary decrease in working capacity, changes in body functions, and the appearance of a subjective feeling – fatigue.
Based on this, it is customary to distinguish two main types of fatigue:
However, this division is rather arbitrary.
Thus, the main and objective sign of a person’s fatigue is a decrease in their working capacity. With fatigue, working capacity decreases temporarily. It quickly recovers with daily, normal rest.
The state of fatigue has its own dynamics – it increases during work and decreases during rest (active, passive, and sleep). Fatigue should be considered as a natural, normal functional state of the body during labor.
Another important criterion for assessing fatigue is the change in body functions during work.
At the same time, depending on the degree of fatigue, functional shifts can be of a different nature. In the initial stage of fatigue, clinical, physiological, and psychophysiological indicators are characterized by instability and multidirectional nature of changes, but their fluctuations, as a rule, do not go beyond physiological standards. With chronic fatigue, and especially overwork, there is a significant unidirectional deterioration of all functional parameters of the body with a simultaneous decrease in the level of a person’s professional activity.
Why Do We Get Tired? Basic Theories of Fatigue
The main theories of why do we get tired are as follows:
- theory of depletion of energy resources in muscles,
- theory of muscle clogging by metabolic products,
- theory of poisoning by metabolites,
- suffocation theory (due to lack of oxygen).
All these so-called local-humoral theories of why do we get tired do not fully reveal the mechanisms of fatigue since only local changes in muscle tissue are considered as its main cause, and partial shifts are taken as general processes. However, each of these theories correctly reflected one of the many aspects of the complex process of fatigue.
The most widespread theory of fatigue formulated by I.M. Sechenov in 1903, comprehensively developed and supplemented by A.A. Ukhtomsky, connects the occurrence of fatigue only with the activity of the nervous system, in particular, the cerebral cortex.
It was assumed that the basis of why do we get tired and the mechanism of fatigue is the weakening of the main nervous processes in the cerebral cortex, the violation of their equilibrium with the relative predominance of the excitation process over the weaker process of internal inhibition and the development of protective inhibition.
Modern electrophysiological and biochemical research methods and experimental data obtained on their basis do not allow reducing the causes of fatigue to changes in any one organ or organ system, including the nervous system.
Consequently, it is inappropriate to attribute the occurrence of primary fatigue to any system. Depending on the state of the body’s functions and the nature of the human activity, the primary occurrence of fatigue is variable and can be observed in various organs and systems of the body.
So, why do we get tired? The main factor causing fatigue is the physical or mental stress that falls on the afferent systems during work. The relationship between the magnitude of the load and the degree of fatigue is almost always linear. That is, the greater the load, the more pronounced and early fatigue is.
In addition to the absolute value of the load, the nature of the development of fatigue is also affected by a number of its features. Among them, there are the static or dynamic nature of the load, its constant or periodic nature, and the intensity of the load.
Along with the main factor (workload) that leads to fatigue, there are a number of additional or contributing factors. These factors by themselves do not lead to the development of fatigue, however, combined with the action of the main one, contribute to an earlier and more pronounced onset of fatigue.
Additional factors include:
- environmental factors (temperature, humidity, gas composition, barometric pressure, etc.);
- factors associated with the violation of work and rest regimes;
- factors due to changes in the usual daily biorhythms and the shutdown of sensory stimuli;
- social factors, motivation, team relationships, etc.
One of the main symptoms of fatigue is a decrease in working capacity, which in the process of performing various physical exercises changes for different reasons. Therefore, the physiological mechanisms of the development of fatigue are not the same. They occur due to the power of the work, its duration, the nature of the exercises, the complexity of their performance, etc.
Why Do We Get Tired in Terms of Inner Processes?
When performing cyclic work of maximum power, the main reason for a decrease in working capacity and the development of fatigue is a decrease in the mobility of the main nervous processes in the central nervous system with a predominance of inhibition due to a large flow of efferent impulses from nerve centers to muscles and afferent impulses from working muscles to centers.
The working system of interconnected activity of cortical neurons is destroyed. In addition, the level of ATP and creatine phosphate decreases in neurons, and the content of an inhibitory mediator, gamma-aminobutyric acid, increases in the brain structures.
At the same time, a change in the functional state of the muscles themselves, a decrease in their excitability, lability, and the rate of relaxation are essential in the development of fatigue.
In cyclic work of submaximal power, the leading causes of why do we get tired are inhibition of the activity of nerve centers and changes in the internal environment of the body.
The reason for this is a large lack of oxygen, as a result of which hypoxemia develops, the pH of the blood decreases, and the content of lactic acid in the blood increases by 20-25 times. Oxygen debt reaches its maximum values - 20-22 liters.
Underoxidized metabolic products, absorbed into the blood, impair the activity of nerve cells. The intense activity of the nerve centers is carried out against the background of oxygen deficiency, which leads to the rapid development of fatigue.
Cyclic work of high power leads to the development of fatigue due to the discoordination of motor and autonomic functions. For several tens of minutes, a very intense work of the cardiovascular and respiratory systems must be maintained to provide an intensively working organism with the necessary amount of oxygen. During this work, the oxygen demand slightly exceeds the oxygen consumption, and the oxygen debt reaches 12-15 liters.
The total energy consumption during such work is very high, while up to 200 g of glucose is consumed, which leads to some decrease in glucose in the blood. There is also a decrease in the blood hormones of some endocrine glands (pituitary gland, adrenal glands).
The duration of the cyclical work of moderate power leads to the development of protective inhibition in the central nervous system, the depletion of energy resources, the stress of the functions of the oxygen transport system, the glands of the internal system, and a change in metabolism.
The body’s stores of glycogen decrease, which leads to a decrease in blood glucose. A significant loss of water and salts by the body, a change in their quantitative ratio, and a violation of thermoregulation also lead to a decrease in performance and the occurrence of fatigue in athletes.
In the mechanism of the development of fatigue during prolonged physical work, changes in protein metabolism, and a decrease in the functions of the endocrine glands can play a certain role.
At the same time, the concentration of gluco- and mineralocorticoids, catecholamines, and thyroid hormones decreases in the blood.
As a result of these changes, as well as a result of the prolonged influence of monotonous afferent stimuli in the nerve centers, inhibition occurs.
The suppression of the activity of these centers leads to a decrease in the effectiveness of the regulation of movements and a violation of their coordination.
With the prolonged performance of work in different climatic conditions, the development of fatigue, in addition, can be accelerated by a violation of thermoregulation.
The mechanisms for the development of fatigue with different types of acyclic movements are also not the same.
In particular, when performing situational exercises with different forms of variable power work, the higher parts of the brain and sensory systems experience heavy loads since athletes need to constantly analyze the changing situation, program their actions, and switch the tempo and structure of movements, which leads to the development of fatigue.
In some sports (for example, football), a significant role is played by the lack of oxygen supply and the development of oxygen debt. When performing gymnastic exercises and in martial arts, fatigue develops due to a deterioration in the brain’s capacity and a decrease in the functional state of muscles (their strength and excitability decrease, the rate of contraction and relaxation decreases).
During static work, the main causes of fatigue are the continuous tension of the nerve centers and muscles, the shutdown of the activity of less stable muscle fibers, and a large flow of afferent and efferent impulses between the muscles and motor centers.
With fatigue, which is a normal functional state of the body during work, its symptoms disappear completely during normal (regulated) rest. With prolonged or intensive work, violation of work and rest regimes, the symptoms of fatigue accumulate, it can turn into chronic fatigue and overwork.
Chronic Fatigue and Overwork
Chronic fatigue is a borderline functional state of the body, which is characterized by the preservation of subjective and objective signs of fatigue from previous work by the beginning of the next work cycle, for the elimination of which additional rest is required.
Chronic fatigue occurs during long-term work in violation of work and rest regimes. Its main subjective signs are as follows:
- feeling of fatigue before starting work,
- rapid fatigability,
- unstable mood.
Why do we get tired, chronically? Objectively, there is a pronounced change in body functions, a significant decrease in sports results, and the appearance of erroneous actions.
In chronic fatigue, the required level of sports performance can be maintained only for a short time due to an increase in the biological value and the rapid consumption of the body’s functional reserves.
In order to eliminate unfavorable changes in body functions and maintain sports performance, it is necessary to eliminate violations of training regimens and rest and provide athletes with additional rest. If these measures are not followed, chronic fatigue can turn into overwork.
Overwork/overtraining is a pathological condition of the body, which is characterized by a constant feeling of fatigue, lethargy, disturbed sleep and appetite, pain in the heart, and other parts of the body.
Additional rest is not enough to eliminate these symptoms, and special treatment is required. Along with the above, objective signs of overwork are abrupt changes in body functions, some of which go beyond normal fluctuations, sweating, shortness of breath, weight loss, attention and memory disorders, atypical reactions to functional tests, which are often not completed.
The main objective criterion for overwork is a sharp decrease in sports results and the appearance of gross errors when performing special physical exercises. Athletes showing signs of overwork should be suspended from training and competition and undergo a medical correction.
A quantitative assessment of the performance of various specialists carried out in recent years by labor physiologists has made it possible to establish that a decrease in direct and indirect indicators of up to 15% in comparison with the initial indicates the development of fatigue in the body, 16-19% indicates the presence of chronic fatigue, and a decrease by 20% or more indicates the occurrence of overwork.
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