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.

Sleep

Aim for 10 hours sleep per night – which is considered suitable for athletes. However, if this is not practical then consider naps during the day. Naps are split into two categories, short (15-20 minutes) and long (30-60 minutes). A short nap results in poorer long-term cognitive function (decision making) but better short-term function. Whereas long naps have the opposite effect and is believed to engage the brain in REM sleep. Plus, studies have shown to have a positive effect on performance (see 'How Much Sleep Do Athletes Need' post for more detail).


Walking

Sitting at a desk or with legs crossed is bad for blood flow and can even cause turbulent blood. To combat this, utilise active recovery by completing small walks (5 minutes) every 30-60 minutes.


Compression

Compression socks have been shown to increase blood flow by causing arteries and blood vessels to constrict. This process is known as vasoconstriction and increases blood flow and velocity. The additional oxygenated blood flow enhances the recovery process.


High Cadence, Low Resistance Training

A high cadence (100 rpm) is considered more efficient compared to other Candace ranges. Therefore, if you are suffering from DOMs (Delayed Onset Muscle soreness) a high cadence low resistance ride would be useful. As workouts like these increase the blood supply to the working muscles. In turn, enhancing recovery without causing much stress on the body.


Additionally, for future reference the symptoms of overtraining include:

- lack of motivation

- increased perceived exertion for same effort

- mood swings

- struggling to sleep

- excesses fatigue


Conclusion

To conclude, there two types of fatigue - central and peripheral - both of which can be accompanied by improved lifestyle factors such as amount of sleep and nutritional intake.




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