Physical fatigue is a physiological condition manifested by a temporary reduction in the physical fitness of the body. Fatigue is a mechanism that protects the body against excessive effort and thus excessive disturbance of homeostasis. Physical fatigue is caused by intense muscular work, as a result of which certain biochemical changes occur. Functional changes also affect the nervous and humoral systems, which causes homeostasis to be disturbed. 


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Fatigue is caused by the following factors 

– ion shifts; 

– hypoglycaemia; 

– changes in CNS functional status (neuron fatigue → pulse inhibition induction). 


Weariness is a subjective symptom of fatigue. The degree of fatigue most often corresponds to the degree of real decrease in the ability to move and work. However, there are situations in which these relationships are disturbed. Weariness is caused by psychological (emotional) factors. Man has the ability to induce a state of boredom at the same time as lack of real (muscle) fatigue, eg lack of mental motivation for physical exertion (lack of mental interest in movement at a given moment) while simultaneously implementing it. Weariness is the reason for low productivity and efficiency of work (work). The situation is different, there is real muscle fatigue with a simultaneous psychic foundation, manifested by the purpose and desire to carry out effective and efficient movement (work). Then, the movement (work) performed will not show a decrease in efficiency and effectiveness. Performing work (movement) is accompanied by emotional tension related to the interest in work, awareness of the great goal. These are examples that prove the influence of I and II of the signal system and thought processes on the course of fatigue and fatigue and indirectly (through the nervous system) on the work of muscles. 

Fatigue affects the course of fatigue development. Trophic nervous system effects. Irritation of sympathetic nerves results in oxidation processes, ATP synthesis, alkaline reserve and electrical conductivity. Irritation of sympathetic nerves also stimulates the conduction of the impulse with synaptic connections. 

In tired muscles, the amplitude of contractions decreases, the diastole is extended, diastole becomes incomplete, contracture develops. The excitability is reduced (the excitability threshold is increased), the latent stimulation period is prolonged (time from the moment of stimulation of the muscle to the beginning of the contraction). Fatigue affects the first phase of the motor plate (muscle-nerve synapse). 



Theories of fatigue 

1. Theory of exhaustion (Verworn and Schiff) – fatigue is a consequence of the use of energy in the working muscle. Research has shown that muscle fatigue occurs before the exhaustion of energy substances (glycogen, glucose). 

2. The theory of acidification (Pflüger`a) – fatigue is the result of the accumulation of metabolites of anaerobic metabolism (acids, creatine, carbon dioxide). 

3. Neurogenic theory of Pawłów, Sjeczenów, Orbella and Krestownik – fatigue is the result of nervous fatigue and suppression of neurotransmission and neuromuscular transmission. The state of fatigue is caused by changes mainly in the nervous system. A large part in the development of fatigue takes the cerebral cortex and vegetative nervous system.

4. Theory of poisoning – fatigue is the result of fatigue accumulating in the working muscle. At present, theory has historical significance, because apart from typical and known metabolites, the predicted toxins of fatigue from intensely working muscles have not been isolated. 

5. The hypoxia theory – muscle fatigue during work is caused by increasing hypoxia (hypoxia in the muscle) due to the intense use of oxygen for intracellular respiration. 


None of these theories fully explains the causes of fatigue. Currently, the concept of synthetic approach to the fatigue process dominates, i.e. explaining its causes using all theories, excluding the theory of poisoning, which has not been proven. With the development of molecular biology, further theories of fatigue appear (eg dehydration theory, electrolyte theory, mediator theory, chain theory = interrelations), considering this process at the molecular level. None, however, can be accepted uncritically and can really be pulled up to previously published theories (eg broadly understood neurogenic theory or exhaustion). 

Fatigue caused by moderate and cyclical effort 

Physical effort of moderate intensity, lasting for a longer time causes gradual fatigue of the nervous centers regulating and controlling the functioning of the respiratory and vasomotor system. The consequence of this are physiological and biochemical changes in the systems under their control (ie circulation, breathing). The superfluous and harmful metabolites are removed on a regular basis, so that they do not accumulate in working tissues. 

Symptoms of fatigue are hypoglycemia, or a drop in blood glucose. However, the glycogen resources are not completely depleted. Hypoglycaemia is a signal inducing compensatory processes associated with the activation of further glycogen resources and the transformation of lipids and amino acids into sugars. 

The nervous and muscular systems are especially sensitive to hypoglycaemia. Ruptured and significant hypoglycaemia disrupts the coordination of activities in the body which is manifested by disorders of the motor apparatus and mental processes. 

Adoption of approximately 50-100 g of sugar during exercise relieves symptoms of fatigue. In this way you can also prevent fatigue. However, it is best to give sugar during exercise, not before it. 



Fatigue caused by intense cyclic effort 

In the case of intensive nervous system fatigue, hypoglycaemia, respiratory and circulatory efficiency decline occur faster than with moderate effort. The consumption of oxygen increases maximally, and the oxygen demand for tissues is not ensured. Muscle work takes place with oxygen deficiency; therefore, the oxygen debt increases. In the intensely working muscle tissue, harmful metabolites accumulate, which through the blood, nervous and endocrine systems affect the other organs, causing specific functional changes in them. 



Fatigue caused by the most intense cyclic effort 

This type of effort is created during short-distance races. They last up to 5 minutes. 

The higher the frequency and strength of contractions, the faster the fatigue occurs. Muscle cramps with high frequency and strength are the result of the intense activity of the nerve centers. Catabolism processes are faster than anabolism. Harmful and unnecessary metabolites accumulate. Neuromuscular transmission is disrupted. The muscle builds up oxygen debt. 



Fatigue with static efforts 

It is created when making a half-seat, hanging on a frame, supporting oneself on a handrail. The duration of static exercises is limited by the appearance of fatigue. During this type of effort in the nervous system, muscle stimulation processes are continuing (active exercises alternate with stimulation and inhibition), without rhythmic inhibition. This results in rapid neuronal and neural-muscular fatigue. The energy and aerobic resources of the muscle are not used up as much as during intensive cyclic exercise. It is only after the effort that the so-called phenomenon of static exercise (static symptoms syndrome), the symptoms of which are a significant acceleration of lung ventilation and cardiac function. The static effort syndrome is caused by nervous fatigue and temporary weakening of the lung ventilation and blood circulation in the muscles that take place during muscle tension. 

Static muscle tension accompanies many motor activities. In the absence of proper training, unwanted static tension of some muscles is created causing rapid fatigue and local sensation of muscle pain, eg incorrect running or swimming technique leads to pain in the neck and back muscles. 

Long-term maintenance of one position (incorrect sitting) leads to disturbances in blood and lymph circulation as well as edema, exudates, including inflammatory and muscle pain. 

Therefore, when practicing static exercises and monotonous physical work related to static tension of specific muscle groups (eg long-term standing, sitting, driving a vehicle), it is necessary to use properly selected dynamic exercises and relaxing muscles. 

Fatigue and overtraining 

Physical fatigue occurs following a one-off exercise. Repeated exercise of a motor activity (professional sport, physical work) without proper (proper, proper) rest causes the summation of fatigue states and the occurrence of fatigue syndrome. Fatigue is therefore a chronic state of homeostasis disorder. 

Fatigue resulting from multiple physical training or multiple sports competitions is called overtraining. In other words, overtraining arises as a result of adding up (putting on) fatigue symptoms caused by repeated training, during which no proper rest has been applied. 

The notion of pushing through is related to the concept of fatigue and fatigue. Forcing is a state of acute fatigue resulting from a single physical effort, exceeding the actual (physiological) physical efficiency (performance) of the body. 

Overtraining occurs in violation of the discipline of training, monotony of training classes, poor emotional preparation (with poor mental foundation) of the athlete and weakening of the body (diseases, incidents, stress). The OUN is actively involved in the development of overtraining, in which the stimulation and braking cycles are overloaded. 


The symptoms of overtraining are as follows 

– vegetative neurosis; 

– emotional instability; 

– weight loss; 

– lowered vital capacity of the lungs; 

– cardiac dysfunction; 

– movement coordination disorders. 


You can read also: BCAA reduces fatigue during training

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