Research into the causes and mitigation of fatigue


11 Feb
11Feb

This blog is a piece of research I wrote with a view to improve my own performance in racing. With not racing to blog about and not wanting to write another corona virus related blog I thought that you might find this interesting. 


A study in the journal of sports science Royal,K, Farrow D, Mujika I, Shona l, Halson, Pyne and Abernethy, B (2006) 24(8) 807-815 looked into the effects of fatigue, decision making and shooting skill performance in water polo players. In this study they found that skill proficiency decreased from 80% at light intensities down to 40% at very high intensities. This deterioration in skill proficiency correlated with an increase in heart rate and blood lactate levels. This research shows that the increase in blood lactate levels is linked to a decrease in ability to perform skill. Furthermore, it showed that a high heart rate is linked to a decrease in skill ability. In a criterium race, the pace is high from the very start and therefore a high heart rate is inevitable. So, one of the determining factors in the race is my ability to deal with the build-up of lactic acid in my body. Not only to offset the degradation of my skills but also to prevent accelerated perception of fatigue. <strong>What does the body do with lactic acid?</strong> Lactic acid is deacidified by the enzyme lactate-dehydrogenas into lactate ions. These can be used as fuel.Lactate shuttling is a relatively new theory that was proposed by Professor George Brooks of the University of California, Berkeley in 1984. The theory states that lactate ions are produced constantly and are used as fuel in muscles that are not under as much strain during exercise. The glucose stored in these muscles is transferred to the muscles working. So, when cycling the lactate would be transferred to the upper body and exchanged with the glucose stored in those muscles. That glucose would the go to the legs where it is needed as it is a better fuel. So, with this research in mind I can come to the conclusion that as effort level increases the performance of skill decreases but I am still no closer to answering why this happens. As the classic answer of lactic acid appears to not be the cause, as in fact it helps the body to go for longer by becoming a fuel source. This research has got me no closer to understanding what fatigue is. It obviously exists but what is it? 


My thoughts on fatigue I know from reading and experience that fatigue exists both in the mind and in the body. ‘Mind over matter’ is what Thomas Vokkler, Shut up Legs cites as the main reason from fatigue, and I would agree with this to a certain extent. From my own observations and experience, I think that this view on the matter of fatigue is very helpful to overcoming it when training or racing. However, I believe it to be untrue that fatigue is all in the mind. I have this view because if fatigue was all in the mind and there were not physiological effects of exercise then no physiological adaptations would take place. If fatigue was all in the mind, then only psychological and neurological adaptations would continue to take place. This I know not to be true as after being injured and not riding for 5 weeks I lost 3kg of muscle mass, my legs got smaller and I was unable to put out the same amount of power as I could before injury took me off the bike. Since starting riding again and going back to the gym I have regained the muscle mass that I lost, and my legs have got bigger once again. If fatigue was all in the mind, then why did I regain muscle mass when I began to train again? If fatigue was all in the mind why does muscle size increase when the muscles are used more? From this I can conclude that fatigue is not all in the mind. Equally I would argue that fatigue is not all physiological. If fatigue was all physiological, all in the body, then it doesn’t explain why you can keep going or go faster even when your body is telling you to stop. If fatigue was all in the body, then why can you go faster at the end of an individual pursuit why not go faster every lap rather than just the last? Furthermore, when I am in a good mood races and rides feel easier than if I’m in a bad mood, I am able to be faster when I have a good emotional state. If fatigue was all in the body, then I should be able to perform the same no matter my mental state. This I know not to be true. So overall, I believe that fatigue lies somewhere in the middle of these two extremes of all in the head or all in the body. So, with this in mind the route cause of fatigue needs to be found. 


 The Department of Pharmacology, School of Pharmacy, Second Military Medical University, Shanghai, China published a paper titled Muscle fatigue General understanding and treatment in the Korean Society for Biochemistry and Molecular Biology. In this they define fatigue as the following: Fatigue is a common non-specific symptom experienced by many people and is associated with many health conditions. Often defined as an overwhelming sense of tiredness, lack of energy and feeling of exhaustion, fatigue relates to a difficulty in performing voluntary tasks. Muscle fatigue is defined as a decrease in maximal force or power production in response to contractile activity. It can originate at different levels of the motor pathway and is usually divided into central and peripheral components. The paper suggests that the central nervous system has a large part to play in muscular fatigue as it controls the individual motor units in the muscles. The rate at which the central nervous system stimulates the motor unit (bundle of muscle fibres) to fire controls the overall force output of the muscle. Basically, the central nervous system controls how much of the muscle is recruited. Overtime the repetitive activation (repeated firing) of motoneurons leads to a decrease in their excitability to excitatory synaptic input. This degradation of susceptibility to stimulation from the central nervous system in the motor units is amplified at higher rates of muscle unit recruitment: i.e. higher power outputs. Thus, explaining why, the power put out in a sprint can only be held for a small amount of time. It also explains why it is much harder to sprint at the end of longer races than shorter races. This paper also proves that there is a change in the body that causes the slowing down feeling. However, it does not explain the second wind phenomena or the effects of mood on the perception of fatigue. A series of articles on the science of sport website titled: ‘Fatigue and Exercise’ suggests that heat and specifically core temperature is the cause of fatigue. It suggests that the message of slowdown is in response to the core temperature rising. If the temperature gets too high, then the enzymes denature, and this leads to all sorts of problems because the chemical reactions in the body cease to happen. This theory does more to explain why athletes speed up toward the end of the race as they know that soon the effort will be dramatically decreased and thus core temperature reduced. That same article also suggested that scalp temperature is a contributing factor to the perception of fatigue. 


A study called Mild Dehydration and Cycling Performance During 5-Kilometer Hill Climbing published in the journal of athletic training in November-December 2013, found that over a 5k hill time trial, a 5.8% reduction in performance resulted from a 1% body mass reduction as a result of dehydration. An article on Sports Cardiology BC titled: The effects of hydration on athletic performance states that water maintains blood volume, regulates body temperature and is involved in muscle contractions. Perspiring is regulated by the autonomic nervous system and is controlled unconsciously by the hypothalamus; the structure in the brain that regulates the body’s status quo. Sweating is the body’s primary way of maintaining optimal body temperature. Consuming liquids replenishes the fluids lost during exercise. Restoring fluids maintains normal muscle function, helps prevent a decrease in physical performance and reduces the risk of heat stress. Dehydration may cause a reduction in blood volume, decreased skin blood flow, decreased sweat rate, decreased heat dissipation, increased core temperature and an increased rate of glycogen use. The most likely physiological mechanism affecting a person’s maximal aerobic power (VO2 max) and hence athletic performance is one’s maximal cardiac output. As dehydration reduces plasma volume and therefore increases blood viscosity, central venous pressure decreases and reduces the amount of blood returning to the heart. During peak athletic intensity, these changes can decrease the amount of blood entering the heart during diastole; the phase in the cardiac cycle where the heart relaxes and fills with blood. Less blood entering the heart during diastole decreases the amount of blood that may possibly leave the heart during systole, the phase where the heart contracts, consequently decreasing cardiac output.


 The other aspect to fuelling is getting in enough energy to sustain the effort. An article on the website Human Kinetics states that the human body can store around 1800-2000 calories in the liver and muscle in the form of glycogen. This is enough energy for between 90-120mins of high intensity continuous exercise. Racing is both high intensity and continuous, so the body’s glycogen stores should be sufficient around an hour and a half race. There are 2 types of carbohydrate in the body, endogenous and exogenous. Endo is the 1800-2000 cal or 500g of carbohydrate. Torq Fitness conducted research into the maximum amount of carbohydrate that the lining of the ilium can absorb per hour. They found in this research that up to 90 grams of carbohydrate can absorbed per hour. Breaking it down further they found that the ilium has 2 pathways for carbohydrate absorption. Pathway one is via the SGLT1 transport protein. This protein can move up to 60 grams of glucose derivatives from the ilium lining into the blood stream where it is broken down from a complex carbohydrate into a simple one (glucose), this energy is used to resynthesise ATP molecules which then break down into ADP. The energy from this breakdown provides the energy required for muscle motor unit contraction. The second pathway is via the GLUT5 transport protein. This protein transports fructose from the ilium into the bloodstream. Again, this is then broken down from a complex carbohydrate in the form of a fructose polymer into a simple carbohydrate (fructose monomer). this energy is used to resynthesise ATP molecules which then break down into ADP. The energy from this breakdown provides the energy required for muscle motor unit contraction. When no carbohydrate remains in the body this is when ‘bonking’ occurs.


To conclude, fatigue is an extremely complex phenomena the route of which remains unclear. The physiological side to fatigue when taken in isolation is relatively easy to pin down the route cause -the motor units become less responsive over time. This means they don’t always fire which leads to a reduction in power output. The other part to physiological fatigue is a self-preservation instinct in the body. Contraction of muscles generates heat. This heat is lost through perspiration. However, when working at high energy output levels the heat energy that can be lost through perspiration is less than the heat energy that is produced. This results in the body heating up. If the temperature in the core of the body raises above 40 degrees, then the enzymes in the body will begin to function much more slowly. Any more than 40 degrees then the enzymes begin to denature and the metabolism in the body will grind to a halt. This is what causes the slowing down feeling (fatigue) and explains why at the end of an effort more can be found as the body knows it will be over soon. The psychological side to fatigue is much harder to explain. It is unclear why fatigue causes a reduction in ability to perform skills. It is also unclear why mood has such a bearing on the perception of fatigue. The fuelling aspect to fatigue is much clearer cut than the physiological or psychological aspects. When exercising at maximum effort the body will consume more carbohydrate than can be put in so eventually there will be no more carbohydrate left. As the body runs out of carbohydrate the self-preservation instinct kicks in and the slowing down sensation (fatigue) is felt. Hydration is also clear as to why it causes the slowing down feeling. When perspiring the sweat comes from the blood plasma. This is predominantly water. If the blood plasma volume is not replaced through taking on fluids, then the blood becomes thicker. The thicker blood moves slower so less oxygen can be delivered to the muscles. This results in a reduction in power that the muscles can produce. Furthermore less blood plasma volume means less perspiration which results in the heating of the body which leads to a self-preservation instinct kicking in and resulting in the slowing down sensation also known as fatigue. Therefore, to combat some elements of fatigue it is important to ensure that fuelling and hydration is planned for carefully for each race to mitigate the negative effects that ineffective preparation could have. In addition, trying to keep the body cool prior to a race and during can also have a positive impact. The physiological aspects of fatigue are more easily quantifiable but do not tell the whole story. 

So the take away from all of this research is that fatigue is an incredibly complicated psychophysical phenomena with many different factors affecting it. But very simply hydration and fuelling is key but do not neglect your psychology.  After races evaluate what your mood was what psychological state you were in and how your legs felt and begin to draw conclusions from this data. There is no magic bullet to managing fatigue it only can be done by optimising and tailoring every aspect of your race strategy and preparation to your own unique biochemistry, metabolism and psychology. 


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