What is Fatigue?

Different types of fatigue and how to combat them

We are all familiar with the feelings of fatigue in whatever form they strike: be it by the end of one of a very long endurance ride at a solid Zone 2 intensity; or when we are attempting that final 5 minute maximal aerobic power effort and our legs are screaming at us to let them stop. But there are many different types of fatigue and understanding what they are can help us combat them and reduce the onset or impact of these various fatigues.

Hydrogen ions (recovery)

The first one that we are going to talk about, and the first of the peripheral fatigue mechanisms, is one you would’ve likely heard referred to as lactic acid build-up. However, this is quite a misleading title as lactate is not something that directly fatigues us and, in fact, is something that our body uses as fuel during efforts and can be converted into pyruvate which is then used to make ATP (adenosine triphosphate), which we know to be the energy source of all bodily functions. The real culprit of fatigue here is something that comes with that lactate production: hydrogen ions. Increased levels of hydrogen ions (H+) create a more acidic environment within the muscles. This, in turn, results in something called ATP hydrolysis, which is the process of ATP being split into ADP (adenosine diphosphate) and inorganic phosphate (Pi). This happens because the enzyme that causes ATP hydrolysis (ATPases) does not work as well in an acidic environment. This not only impacts on our ability to utilise the energy available in the ATP we create from substrate utilisation, but also our capacity to produce muscle contractions.

We can reduce the negative effect of H+ by improving our buffering capacity. This can be done by using supplements, such as beta-alanine or bicarbonate of soda. Another means of reducing H+ is to improve our ability to utilise fats as fuel at higher intensity, which reduces the production of lactate and H+ that come with it due to carbohydrate metabolism. As H+ production increases exponentially above the physiological turn point Critical Power, increasing our Critical Power through training will greatly help in reducing the fatigue effect of H+ at certain power levels. This is why when H+ levels do get so high that they cause fatigue, the best thing to do is to reduce the intensity, which then enables you to clear the hydrogen ions faster than you are producing them.

Inorganic phosphate (recovery)

Another cause of peripheral fatigue (and usually the one that leads to the pain we feel in our legs) is something called inorganic phosphate. This is produced in especially large quantities when we suddenly go from perhaps a very low intensity to a very high intensity. For example, during sprints or micro intervals. Inorganic phosphate is produced during ATP hydrolysis and also when using the phosphocreatine pathway to produce energy, which occurs during very high intensity or the initial uptake of high power. So, when you go from 40 W to 400 W in your micro intervals, the phosphocreatine pathway is how we initially uptake that power. Or, even when starting an endurance ride, the initial energy system working will be the phosphocreatine one, with the aerobic system is kicking in shortly afterwards. This also explains why we feel more fatigue when we have a lot of changes in pace, such as in a criterium or after conducting micro intervals. The production of inorganic phosphate leads to a reduced cross bridge capacity within the muscles, which is how we produce our power by contracting and releasing the cross bridges (see sliding filament theory). It also reduces the potential power from each cross bridge, resulting in a double whammy of power reduction. 

We can reduce the impact of inorganic phosphate by pedalling at an easy intensity after the repeated bouts of exercise that produce it or immediately after a large sprint. Studies have found that stopping peddling completely keeps the inorganic phosphate levels high and reduces our capacity to perform sprints again. So, in races, it is a good idea to reduce the amount of surges that we do as that will reduce the inorganic phosphate production. In training, after doing a high intensity sprint, it is important to remain pedalling at a low intensity rather than stopping pedalling entirely.

Muscle Afferents (recovery)

Our body is incredibly clever, and we have parts of our muscles called Afferent Fibres. These afferents play a very important role in exercise as they monitor the state of the muscle and interact with the central nervous system (CNS). The afferents we are concerned with here are Group III and IV as they mediate cardiovascular and ventilatory reflexes. Essentially, they help regulate the CNS to control some degree of blood and oxygen delivery to the working muscles. They are essential, as studies have found that blocking these receptors from signalling the CNS reduced the blood/oxygen delivery to the working muscles by reducing blood flow and also pulmonary ventilation. However, they also play a role in Central Fatigue and are linked in a way to some of the peripheral fatigue mechanisms (H+ and Pi). Up to a certain critical threshold these afferents assist in muscular contractile performance and delivering oxygen to the muscles. However, once this threshold is crossed, the afferents provide inhibitory feedback to the CNS to reduce the central motor drive, basically our voluntary capacity to produce greater muscular force is reduced. The suggested reason for this inhibitory feedback is to limit the body from spending time above this critical threshold and producing excessive peripheral fatiguing metabolites. This is to try and protect the body and muscles from associated damage due to these metabolites. Studies have tested with inhibiting the afferents above the critical threshold, and although blood oxygen levels were slightly reduced, this negative affect was outweighed by the increased central motor drive. In the test, cyclists were able to perform at a higher power output over a 5km time trial than those without afferent inhibition.

The way to overcome this afferent feedback induced central fatigue, is to reduce intensity and decrease Group III/IV stimulus. Once that critical threshold is passed, the cascade of fatigue mechanisms, both central and peripheral, increases until intensity is reduced. We can increase the threshold at which the afferents start to provide inhibitory feedback the same way we can reduce the presence of H+ and Pi, by increasing the physiological turn point known as Critical Power.

Substrate availability (carbs)

Another form of fatigue comes from substrate availability. Simply put, this means having the fuel available to conduct the work. Higher intensity work requires carbohydrates in order to complete it, but our carbohydrate stores are not indefinite. With our muscles and liver saturated in carbohydrates (muscle glycogen when it’s in the muscles), we have perhaps enough carbohydrates to fuel 90 minutes of high intensity exercise. This is why consuming carbohydrates is vital to allow for prolonged high intensity work. When we do not consume enough carbohydrates, we are unable to complete exercise at higher intensities, as even with a lot of fuel in even a very learn person’s fat stores, fat metabolism requires more oxygen to break down than carbohydrates so can’t be done when oxygen demands are very high such as high intensity exercise. This means the only way that we can continue to exercise is to reduce the intensity dramatically. This is likely the feeling you will have encountered if you’ve ever suffered the dreaded bonk. Your legs go heavy and there is no intensity that you can produce other than one that is essentially just turning the legs very, very easily.

Fortunately, this one is probably the easiest form of fatigue to combat. We simply must ensure that A) we’ve consumed sufficient carbohydrates in both the day leading up to, and on the morning of, exercise, and B) for exercise sessions longer than 90 minutes that we consume carbohydrates during the session as well. The amount of carbohydrates that you can consume depends on your ability to process them. Traditionally, we’ve been told that 60 g an hour was the maximum we could consume. This then increased to 90 g an hour when a mix of fructose and glucose was consumed. Nowadays it is considered that up to 120 g of carbohydrates can be consumed per hour when mixing different carb sources –  as long as our gut has been trained to deal with the high carbohydrate load. It’s important to consume the carbohydrates before we feel that we need them. Once we hit the point of fatigue due to lack of fuel, it’s too late as the time required to get the carbohydrates into the working muscles means we’ll have to spend a period of time working at a very low intensity in order to recover. For those just starting out with carbohydrate fuelling, it’s best to start with maybe 40-60 g an hour. After that, you can train with increasingly higher carbohydrate loads to improve your body’s tolerance to carbs and ability to utilise them.

Heat fatigue (a good fan and cooling of the hypothalamus)

Heat fatigue affects us in several ways. The main one we’re going to talk about is heat production itself and how that affects both the muscles’ ability to contract and also our ability to utilise fuel sources. When we work in hot environments, we become less able to use fats as fuel and more reliant on carbohydrates, which itself causes greater hydrogen ion production and therefore quicker onset of fatigue. The big issue with heat is when it affects our core temperature as, when that increases, a lot of enzymes in the body are unable to function properly and we therefore lose certain functions such as the ability to break down fuel.

The other way in which heat causes eventual fatigue is by dehydration. When we become dehydrated, we experience a reduction in our salt balance as salt is required to move water from within the body to outside the body to allow evaporation to cool us down: this is essentially sweating. With salt levels reduced (often called electrolytes), we lose some of our signalling capacity which is how our muscles contract and release. You may feel particularly bad cramps after being dehydrated, for example when your calf muscles tense up without any conscious intention to do so. This happens due to electrical signals not been passed properly to the muscles, and all muscle cross bridges by nature are contracted, hence why rigor mortis occurs in bodies. We actually release the muscle cross bridges with energy so dehydration, often accentuated by heat, can cause cramping by failure to release – or relax – the muscles.

There are various ways to reduce the impact of heat fatigue. One method is heat acclimatisation which essentially involves training in a hot environment more regularly and increasing our ability to sweat in order to reduce body temperature. Additionally, it has been found that making sure the hypothalamus section of the brain is cool has a significant effect on body temperature. An easy way of doing this is by putting an ice pack on the back of your neck. Ensuring that you have a good quality fan in your turbo room, as well as maybe a dehumidifier, will also go quite a long way to reducing the impact of heat on your performance. Another very simple way to combat dehydration is not only to consume enough water but also to ensure that enough salt is being consumed. This is hard to get exactly right unless you have your sweat tested for the amount of salt per millilitre and then also have your sweat rate tested. However, the amount of salt required is often higher than you would expect when exercising hard in the heat. 

Hopefully you found this piece interesting. Let us know any stories of how fatigue has really hit you and how you combatted it.

This article only scratches the surface of what fatigue mechanisms exist within the body. For more information on different types of fatigue, there is a very comprehensive scientific review out there: Allen, D. G., Lamb, G. D., & Westerblad, H. (2008). Skeletal muscle fatigue: cellular mechanisms. Physiological reviews.

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