FATIGUE

Fatigue after strenuous training is a good sign that the exercise moves the limits of the athlete's physiology. Training should not be excessive and exhausting, and it should make the person a bit - excited. Fatigue that takes days shows that the physiology of people over-challenged and that the muscles and energy reserves are not effectively recharged. The peripheral and central mechanisms contribute to muscle fatigue. Peripheral fatigue is the occurrence of a metabolic endpoint when glycogen stores are discharged, the plasma glucose concentration is reduced, and levels of free fatty acids in plasma are elevated. The central mechanisms include motor neurons that are mostly found in the brain, and peripheral include motor units (motor neurons, peripheral nerves, motor plates, muscle fibers).

• PERIPHERAL FATIGUE

Peripheral muscle fatigue represents the inability of an athlete to maintain the expected level of exercise intensity. The reason for the occurrence of peripheral muscle fatigue are local changes in the internal state of the muscles. These changes can be biochemical changes, discharge of the substances stores like glycogen, high energy phosphate compounds in muscle fibers, acetylcholine in the end of the motor nerves branches, may be due to the accumulation of metabolites, such as lactates or electrolytes that are released from the muscles during muscle activity, changes resulting from a musculoskeletal mechanism disorder resulting from poor response to different types of exercise, and recently suggested changes related to the immune and genetic response.

• CENTRAL FATIGUE

Fatigue of the Central Nervous System (hereinafter referred to as CNS), or Central fatigue, is a form of fatigue that is associated with changes in the concentration of neurotransmitters within the synaptic neurons in the central nervous system (CNS; including brain and spinal cord) which affect the performance of sports activities and muscle function. This type of fatigue is manifested by a reduction in the nerve transmission of motor impulses from the CNS to the muscles. Neurochemical changes in the brain primarily include changes in the level of the neurotransmitters serotonin (5-HT), noradrenaline and dopamine.

• Increased Dopamine Concentration, IMPROVES exercise performance.
• Increased serotonin concentration, or noradrenaline, REDUCES exercise performance.

In healthy athletes, central fatigue can be a consequence of:

• High-intensity work occurring too frequently in a training cycle
• Too much high-intensity volume in a single training session
• Introducing high-intensity training too rapidly into a training program when “residual fatigue still exists.”

It’s important to mention that central fatigue also has roots in psychology. For example, the limits of physical stress may be consciously or subconsciously limited by the athlete’s pain, motivation, and subjective perception.

• The role of central fatigue

The central nervous system fatigue is a key component in the prevention of peripheral muscle injury.

The brain has numerous receptors, such as osmoreceptors, to track dehydration, nutrition, and body temperature. With that information as well as peripheral muscle fatigue information, the brain can reduce the number of motor commands sent from the central nervous system. This is crucial in order to protect the homeostasis of the body and to keep it in a proper physiological state capable of full recovery. The reduction of motor commands sent from the brain increases the amount of perceived effort which an individual experiences. By forcing the body through a higher perceived intensity, the individual becomes more likely to cease exercise by means of exhaustion. Perceived effort is greatly influenced by the intensity of corollary discharge from the motor cortex that affects the primary somatosensory cortex.

Endurance athletes learn to listen to their body. Central nervous system fatigue alerts the athlete when physiological conditions are not optimal so either rest or refueling can occur. Protecting organs from potentially dangerous core temperatures and nutritional lows is an important brain function. It is important to avoid hyperthermia and dehydration, as they are detrimental to athletic performance and can be fatal.

• Assessment of the Central Fatigue

The central fatigue is assessed by a dynamometer (the number of clamping over time, three times a day), a vertical jump (a drop in performance is usually a sign of fatigue). Another marker of the sympathetic and parasympathetic system is basal temperature, heart rate at rest, blood pressure and Achilles tendon reflex.

Some notable drowsiness, lack of concentration, or tremors after exercise can also be indicators of the Central fatigue.

References:

1. Beretta-Piccoli, Matteo, et al. "Evaluation of central and peripheral fatigue in the quadriceps using fractal dimension and conduction velocity in young females." PloS one 10.4 (2015): e0123921.
2. Zając, Adam, et al. "Central and Peripheral Fatigue During Resistance Exercise–A Critical Review." Journal of human kinetics 49.1 (2015): 159-169.
3. Boccia, G., et al. "Central and peripheral fatigue in knee and elbow extensor muscles after a long‐distance cross‐country ski race." Scandinavian journal of medicine & science in sports (2016).
4. Carroll, Timothy John, Janet L. Taylor, and Simon C. Gandevia. "Recovery of central and peripheral neuromuscular fatigue after exercise." Journal of Applied Physiology 122.5 (2017): 1068-1076.
5. Enoka, Roger M., and Jacques Duchateau. "Translating fatigue to human performance." Med. Sci. Sports Exerc 48 (2016): 2228-2238.
6. Minett, Geoffrey M., and Rob Duffield. "Is recovery driven by central or peripheral factors? A role for the brain in recovery following intermittent-sprint exercise." Frontiers in physiology 5 (2014).
7. Froyd, Christian, et al. "Central regulation and neuromuscular fatigue during exercise of different durations." Med. Sci. Sports Exerc 48 (2016): 1024-1032.
8. Hureau, Thomas J., Guillaume P. Ducrocq, and Gregory M. Blain. "Peripheral and central fatigue development during all-out repeated cycling sprints." Medicine & Science in Sports & Exercise 48.3 (2016): 391-401.
9. Buckthorpe, Matthew, Matthew TG Pain, and Jonathan P. Folland. "Central fatigue contributes to the greater reductions in explosive than maximal strength with high‐intensity fatigue." Experimental physiology 99.7 (2014): 964-973.
10. Coelho, Ana Claudia. "Measurement of central and peripheral fatigue during whole body exercise: A new method." (2015).
11. O'Leary, Thomas J., et al. "Central and peripheral fatigue following non‐exhaustive and exhaustive exercise of disparate metabolic demands." Scandinavian journal of medicine & science in sports 26.11 (2016): 1287-1300.
12. Sharples, Simon A., et al. "Cortical mechanisms of central fatigue and sense of effort." PloS one 11.2 (2016): e0149026.

• CHRONIC FATIGUE SYNDROME

Central fatigue and chronic fatigue syndrome share many common features, and they are usually grouped together in the literature. People with a normal level of internal and external motivation and the appropriate level of sensory and motor functioning may continue to have a performance reduction due to limitations such as endocrine abnormalities or autonomic dysfunction. Athletes who express abnormal levels of exertional tiredness during exercise, muscle fatigue, and intolerance to exercise have chronic fatigue syndrome.

Chronic fatigue syndrome is a name for a group of diseases that are dominated by persistent fatigue. It can be triggered by viral infection, immune system dysfunction, or abnormalities in the hormone secretion. The fatigue is not due to exercise and is not relieved by rest. The main cause of fatigue in chronic fatigue syndrome most likely lies in the central nervous system. A defect in one of its components could cause Can cause an increase in the intensity of the necessary effort in relation to the standard intensities needed to perform a particular exercise.

There is no known cure for chronic fatigue syndrome, and the most effective treatment in the official medicine is currently unknown. In order to alleviate the symptoms of chronic fatigue syndrome, drugs such as antidepressants commonly are used to treat depression that often accompanies this disorder, as well as sleeping pills to facilitate rest overnight.

References:

1. Buchwald D, Herrell R, Ashton S, et al. A twin study of chronic fatigue. Psychosom Med(2001); 63: 936–43.
2. Chaudhuri A, Behan PO. Neurological dysfunction in chronic fatigue syndrome. J Chron Fatigue Syndr (2000); 6: 51–68
3. Gremion, Gérald, and Thierry Kuntzer. "Fatigue and reduction in motor performance in sportspeople or overtraining syndrome." Revue medicale suisse 10.428 (2014): 962-964.