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What is Fatigue - Signs & Symptoms

Updated on July 2, 2012


“Fatigue is an inability to sustain a given power output or speed. It is a mismatch between the demand for energy by the exercising muscles and the supply of energy in the form of ATP” (Sports Nutrition, Anita Bean, 2009)

Therefore you will be unable to continue exercising at the given rate and every action becomes and feels harder to do.

This occurs when ATP and PC (ATP and PC are the currency in which Energy operates) stores are depleted yet there is still a demand for them. This may also occur when the rate at which lactic acid is building up due to anaerobic respiration is greater than that at which it is getting cleared out, “if the substance is not removed by the circulatory system, it builds up to impede muscle contraction and cause fatigue” (BTEC level 3 sport, Adams et al,2010).

The acidity level within the muscles becomes too high and muscle contraction becomes difficult and it can even lead to cell death, the burning sensation which occurs during intense exercise is not as many people think a positive thing, it is however a warning from your body to stop exercising. In steady state exercise it will not be possible to carry on at the same rate at which you started.

The aerobic energy system will fatigue once the glucose and glycogen stores within the muscles and liver have been depleted and no more ready for oxidization to create energy. Although fat can be then used as energy and has near unlimited stores, it cannot produce energy quickly enough to continue exercise at the continuous pace it started at, at best it can only fuel low intensity exercise, as the transport of fatty acids can be timely. Aerobic respiration is not possible without the presence of Oxygen so the systems potential to supply energy can be limited by the absence of Oxygen.

Neuromuscular fatigue, this involves the depletion of Acetylcholine, this is a neurotransmitter released to stimulate skeletal muscles and the parasympathetic nervous system, muscle contraction is then ended by the release of Aceltylchlineterese from the muscle sarcomere.

Signs of fatigue

Legs feel heavy – Muscle contraction becomes harder as lactic acid build up in the muscles increases

Perspiration – This is a mechanism put into place by the body to deal with increasing amounts of heat generated as a bi-product of energy production.

Increased breathing rate – The body is trying to compensate for a debt of oxygen and a still increasing demand of it for energy production.

Inability to continue performing at the intensity required – This may be the inability to sprint for a touchdown in American football, or track an opposition player as a defender in football etc

Things which influence fatigue

Acclimatization – this is the process of becoming physiologically accustomed to the climate in which you perform in. It may be that your preparing for a tournament or competition is set in a location which has a high altitude (this would indicate that a location is of a relatively great distance from sea level), the biggest difference between playing at different levels of altitude is that the higher the altitude, the thinner the air and lesser quantities of Oxygen it contains, this puts the body and the energy systems on greater strain, as it must learn to function with less Oxygen than it is accustomed to. Therefore should someone train at high altitude and their respiratory, cardiovascular and energy systems adapt to perform with lesser levels of oxygen, then when they once again return to exercising in areas of lower altitude, the body will be less likely to encounter the same level of limitation due to lack of oxygen or a debt of it than it would have previously due to its adaptation to perform in high altitude with limited oxygen. Therefore training in high altitude can not only be used to prepare to exercise is high altitude but increase your ability to prolong fatigue in your regular levels of altitude.

Fitness – The body adapts to exercise to be better equipped to deal with it in the future, energy systems and thermoregulation becomes more efficient over time. Therefore an individual that is well trained is likely to reach fatigue after a greater intensity or duration of time.

Diet – An individual who consumes a high fat diet will reach fatigue much earlier than an individual who consumes a high carbohydrate based diet (Especially in mid-high intensity activities), as expressed in (A medical physiology 6th ed, Guyton, 2010), this is because the average body already holds rich supplies of lipid stores, the consumption of extra fats is not much helpful in fueling exercise and sport as it is relatively slow and poor in its availability for metabolism, due to a the transit time of fatty acids in comparison to the greater speed of Carbohydrates as glucose in the blood (Blood sugar), making it much more readily available for energy production.

Depletion of Glycogen stores – this is the point of fatigue, where all glycogen stores have been depleted, the body isn’t able to fuel anything but very low intensity exercises in the absence of Glycogen. Storage potential doesn’t differ greatly from person to person, however a larger person is likely to have proportionately larger organs and therefore slightly larger storage capacity within the liver for Glycogen and greater overall net volume of blood within the body with the potential for more glucose to be carried within the blood overall. An individual with a higher muscle mass, is likely to have a slightly greater capacity for storage of Glycogen in the muscles, therefore as a generalization, females have less glycogen storage capacity as they tend to have to less muscle mass.
Some athletes choose to ‘Carb Load’ this involves taking onboard higher amounts of Carbohydrates than normal in the three days leading up to a sporting event or competition.
You will need to eat a high carbohydrate diet for the days immediately following exercise, in order to replenish the 1600g storage capacity of the muscles and the 400g capacity of the liver. This will need to be replenished in order to recover the lactic acid energy cycle. The Phosphocreatine system generally recovers almost instantly


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    • Ruby H Rose profile image

      Maree Michael Martin 

      6 years ago from Northwest Washington on an Island

      Interesting information. I love the picture!


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