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Mechanisms of Fatigue

Everyone experiences fatigue as a natural part of training. But what actually is fatigue and what strategies can minimise fatigue?



The Science

Fatigue is defined as the decrease in the ability to generate force or sustain muscle contractions when exercising. In addition, fatigue can be split into two forms, central (nervous system) and peripheral (muscular).


Central fatigue originates from the cerebral cortex (the outer layer of the brain) through a decrease in motivation. The body is sending this message as a defence mechanism to prevent overexertion. These messages are sent via the Central Nervous System (CNS) which results in a lack of coherence between motor neurons (controls muscle contraction) and both the cerebral cortex and spinal motor neurons. However, central fatigue has a minor role in contraction failure accounting for 15-22%.


On the other hand, peripheral fatigue refers to repeated muscle contractions during a period of time (a marathon for example). The force a muscle produces is dependent on the number, size and type of fibres being exerted. For example type II muscle fibres (fast-twitch) can produce high amounts of force in a sort space of time. Conversely, as a result type II fibres are also more susceptible to fatigue.

Furthermore, the depletion of muscle glycogen storages is also considered a fatigue mechanism. This particularly applies to endurance sports were the effort level requires carbohydrates. The body can store up to 2000 Kcal in the muscles and liver. However, this becomes depleted after roughly 120 minutes when working at 83% of your VO2 max, unless fuel is taken on board during. So are there ways of decreasing/accompanying fatigue?



Nutrition

In addition, training results in an inflammatory response in the musculoskeletal system due to muscle tears. In some cases, muscles can be depleted of energy after a workout – you may begin to feel hypoglycemic if this is the case. To prevent the depletion of muscle glycogen stores, consuming high GI carbohydrates during exercise is recommended. But, the amount varies depending on the length of exercise. A guide to this is below:


30-75 minutes ------ < 30 grams per hour.

1-2 hours ------------ 30 grams per hour

2-3 hours ------------ 60 grams per hour

2.5 > hours ----------90 grams per hour


Furthermore, following training is a 15-20 minutes window when the muscles are highly absorptive. To accommodate these responses, it is best to consume high GI carbohydrates and low GI protein for longer synthesis within the window.