Causes of fatigue are very important subject when talking about fitness training especially in the endurance sports.
Postponing the fatigue, as well as recovery from it, is the main factor determining the sport performance and prevention of injuries.
To counter the fatigue we must understand the mechanism of how it occurs.
Mechanism is multiple. First of all, there are two types of fatigue: central and peripheral. Central is related to the CNS while peripheral to the skeletal muscles and nerves.
Regarding the CNS fatigue it is the change in the ratio of neurotransmitters in synapses dopamine (fall) and serotonin (rise) that leads to inadequate generation of the motoric commands in the motoric cortex. Another reason of the CNS fatigue is the need for sleep which occurs due to the accumulation of the neuromodulator adenosine as a consequence of spending stores of glycogen in the brain (Heller and Benington 1995.). Adenosine is made as a metabolite of ATP decomposition. ATP is the molecule that contains high energy in the form of two phosphate bonds and it is essential for the muscle contraction. Adenosine receptors that trigger sleepiness can be tricked with usage of stimulants like caffeine with a delay of somnolency as a result. The problem is that all other fatigue mechanisms are untouched by it. So, the tiredness will continue to build up camouflaged and catch you amplified when the stimulant lose its effect (half-life). If you keep on tricking adenosine receptors, sooner or later your body will collapse or get injured. Imagine constantly filling car with gas and driving. Is it possible forever? Of course not. Each part has its shelf life. If nothing else tires shall wear down. Someone can say that it will take a very long time to happen so. Well yes, but human body is much more complex and, consequently, much more sensitive.
Regarding peripheral fatigue there are 4 reasons: energy depletion, muscles acidosis, disturbance in the concentration of the inorganic Phosphate (Pi) and Potassium (K+) and the reduced Calcium (Ca2+) release together with decreased sensitivity of contractile proteins.
Energy depletion: in human body there are 3 sources of energy – carbohydrates, proteins and fat. Proteins are primarily used as building materials for muscles not actually the energy source. Only when there is no CH and fat left body starts to eat itself meaning proteins will be used as fuel. Main source of energy are CH stored mostly in the muscles, small amount in liver and brain, in the form of glycogen. Next in line is fat. Even the skinny people have enough fat to run several marathons. Fat gives more energy than CH but it is harder and slower to transform it. The secret is in the adaptation of the body metabolism. Experienced runners use special diet: they eat less CH forcing body to switch to the fat and when training they run really long distances which activates fat consumption above normal level. Now when you think about this it becomes obvious that energy depletion as a cause of fatigue can be easily avoided. You just need to properly eat before, during and after the race. It is necessary to ingest 300g of complex CH 4 hours prior to the event because time of digestion is 3-6 hours which means you will use it exactly during the course. Simple CH are taken together with isotonic drinks during the event because they digest fast. Isotonic drinks contain similar concentration of salt (Sodium) and electrolytes (minerals) as found naturally in the human body. WARNING!: Water is not an isotonic drink! Runners who drink water during the sport event become and finish dehydrated. Sport events organisers who serve ordinary water as a refreshment, to selfishly reduce their expenses and put more money from entry fees into their purse, should be penalised in the court of law.
Muscle acidosis: accumulation of the Hydrogen (H+), resulting in the lower pH, and lactic acid (lactate) in the muscles and blood plasma. Latter not so because of mere existence or elevated level but because of the clogging of the metabolic buffering system. Actually, lactate can be used as energy source transforming into pyruvate or glucose.
Disturbance in the concentration of the inorganic Phosphate (Pi) and Potassium (K+): increase of Pi reduces force output, accumulation of K+ inside the t-tubules negatively affects action potential.
Reduced Calcium (Ca2+) release together with decreased sensitivity of contractile proteins: is the major cause of peripheral fatigue. Happens due to the oxidative stress.
Source: McKenna 2008.
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