The Sensation Of Fatigue – A Complex Emotion Which Is Vital For Human Survival

After a couple of weeks back at work after a great Christmas season break, I have noticed this week a greater than normal level of fatigue than I normally ‘feel’ at the end of a routine working week. After one of the hottest December months on record in my current home town, where temperatures for a while were consistently hovering around forty degrees Celsius, we have had a wonderful rainy, cool period, and I have noticed that I feel less fatigued in the cooler environment, and routine daily activities seem ‘easier’ to perform than when it was excessively hot. As part of a New Year’s resolution ‘action plan’ to improve my level of fitness, I have increased my level of endurance exercise, and as always have enjoyed the sensation of fatigue I feel towards the end of each long (though I know that ‘long’ is relative when compared to younger, more fitter folk) bike ride I do as part of this ‘fitness’ goal. All of these got me thinking of the sensation of fatigue, an emotional construct which I spent a great many years of my research career trying to understand, and which still is very difficult to define, let alone work out its origins and mechanisms of elicitation in our physical body structures and mental brain functions.

As described in these three very different examples from my own life, fatigue is experienced by all folk on a regular basis in a variety of different conditions and activities. Perhaps because of this, there are many different definitions of fatigue. In clinical medicine practice, fatigue is defined as a debilitating consequence of a number of different systemic diseases (or paradoxically the treatment by a variety of different drugs) or nutritional deficits. In exercise physiology, fatigue is defined as an acute impairment of exercise performance, which leads to an eventual inability to produce maximal force output as a consequence of metabolite accumulate or substrate depletion. In neurophysiology, fatigue is defined as a reduction of motor command from the brain to the active muscles resulting in a decrease in force or tension as part of a planned homeostatic process to prevent the body from damage which could result from too high a level of activity or too prolonged activity. In psychology, fatigue is defined as an emotional construct – a conscious ‘sensation’ generated by the cognitive appraisal of changing body or brain physiological activity which is influenced by the social environment in which the activity changes occur, and the mood status, temperament and background of the person ‘feeling’ these physiological changes. It will be evident from all of these different definitions how complex fatigue is an ‘entity’ / functional process, and how hard it is for even experts in the field to describe to someone asking about it what fatigue is, let alone understand it from a research perspective.

A number of different physical factors have been related to the development of the sensation of fatigue we all ‘feel’ during our daily life. During physical activity, it has been proposed that changes in the body related to the increased requirements of the physical exertion being performed cause the sensation of fatigue to ‘arise’. These include increased heart rate, increased respiratory rate, increased acid ‘build up’ in the muscles, reduced blood glucose or muscle or liver glycogen, or temperature changes in the body, particularly increased heat build-up – though for each study that shows one of these factors is ‘causal’ of the sensation of fatigue, one can find a study that shows that each of these specific factors is not related to the development of the sensation of fatigue. It has also been proposed that changes in the concentration of substrates in the brain structures associated with physical or mental activity are related to the sensation of fatigue – such as changes in neurotransmitter levels (for example serotonin, acetylcholine, glutamate), or changes in the nutrients supplied to the brain such as glucose, lactate or branched chain amino acids. But, again, for each study whose findings support these hypotheses, there are studies that refute such suggestions. It has also been suggested that a composite ‘aggregation’ of changes in all these body and brain factors may result in the development of the sensation of fatigue, via some brain process or function that ‘valences’ each in a fatigue ‘algorithm’, or via intermediate sensations such as the sensation of breathlessness associated with increased ventilation, the sensation of a ‘pounding’ heart from cardiac output increases, the sensation of being hot and sticky and sweating which result from temperature increases in the body, and / or the sensation of pain in muscles working hard, all of which are themselves ‘aggregated’ by brain structures or mental functions to create the complex sensation we know and describe as fatigue.

Which physical brain structures are involved in the creation of the sensation of fatigue is still not known, and given the complexity of the factors involved in its generation, as described above, large areas of the brain and a number of different brain systems are likely to be involved – the motor cortex as muscle activity is often involved, the sensory cortex as signals from changes in activity in numerous body ‘parts’ and functions are ‘picked up’ and assimilated by the brain, the frontal cortex as cognitive decision making on the validity of these changes and the need for potential changes in activity as a result of this ‘awareness’ of a changed state is required, the hippocampus / amygdala region as the current changes in physiological or mental activity must be ‘valenced’ against prior memories of similar changes in the past in order to make valid ‘sense’ of them as they currently occur, and the brainstem as this is the area where ventilation, heart function and a variety of other ‘basic’ life maintaining functions are primarily controlled, for example, amongst many other potential brain areas. We don’t know how the function of different brain areas is ‘integrated’ to give us the conscious ‘whole’ sensation we ‘feel’, and until we do so, it is difficult to understand how the physical brain structures ‘create’ the sensation of fatigue, let alone the ‘feeling’ of it.

How the mental ‘feeling’ of fatigue is related to these physical body and brain change ‘states’ is also challenging for us research folk to understand. Clearly some ‘change’ in structures, baseline physical values or mental states by whatever induces the fatigue process, be it physical or mental exertion or illness, is required for us to ‘sense’ these and for our brain and mental functions to ‘ascribe’ the sensation of fatigue to these changed states. It has previously been shown that the sensation of fatigue which arises during exercise is related to the distance to be covered, and increases as one gets closer to the finish line. While this sounds obvious, as one would expect the body to become more ‘changed’ as one exercises for a longer period, it has been shown that when folk run at the same pace for either five or ten kilometres, despite their pace being identical in both, at the 4km mark in the 5 km race the rating these folk give for the sensation of fatigue is higher than it is at 4km of the 10 km race, which is ‘impossible’ to explain physiologically, and suggests that folk ‘set’ their perceptual apparatus differently for the 5 and 10 km race, based on how far they have to go (what H-V Ulmer described as teleoanticipation), by changing the ‘gain’ of the relationship between the signals they get from their body depending on how far they plan to go. Two great South African scientists, Professor Ross Tucker of the University of Free State, and Dr Jeroen Swart of the University of Cape Town, have expanded on this by suggesting that there is a perceptual ‘template’ for the sensation of fatigue in the brain, and the sensation of fatigue is ‘created’ in an organized, pre-emptive ‘way’ by mental / cognitive processes in the brain, and the sensation of fatigue is ‘controlled’ by this template depending on the distance and / or duration of a sporting event. If something unexpected happens during an event, like a sudden drop in temperature, or a competitor that goes faster than expected, this will create an unexpected ‘change’ in signals from the body and requirements of the race, and the sensation of fatigue will become more pronounced and greater than what is expected at that point in the race, and one will slow down, or change plans accordingly. Ross and Jeroen’s fascinating work show how complex the mental component of the sensation of fatigue and its ‘creation’ by brain structures is.

There are multiple other factors which are involved in the generation of the sensation of fatigue, or of its modulation. I did my medical PhD (an MD) on chronic fatigue syndrome which developed in athletes who pushed themselves too hard until they eventually physically ‘broke down’ and developed the classical fatigue symptoms associated with chronic fatigue, where they felt fatigue even when not exercising, which was not relieved by prolonged periods of rest. These athletes clearly pushed themselves ‘through’ their fatigue symptoms on a regular basis until they damaged themselves. As one of the pioneer and world-leading experts in the fatigue field, Professor Sam Marcora, has pointed out, one’s ambitions and drives and ‘desire for success’ are a strong indicator both of the level of the symptom of fatigue folk will ‘feel’, and how they resist these symptoms. In these chronically fatigued folk we studied, something in their psychological makeup induced them either to constantly continue exercising despite the symptoms of fatigue, or made them ‘feel’ less sensations of fatigue for the same work-rate (assuming their fitness levels and physical capacity was similar) to most folk who do not experience this syndrome (the vast majority of folk). To make the matter even more complex, these folk with chronic fatigue described severe sensations of fatigue at rest, but when we put them on a treadmill, some of them paradoxically felt less, rather than more, sensations of fatigue when running as compared to resting, and their extreme sensations of fatigue returned (to an even greater degree) in the rest period after they completed the running bout. Furthermore, if one gives stimulants to folk when they exercise, such as caffeine, it appears to reduce the ‘awareness’ of the sensations of fatigue. Sam is doing some interesting work currently looking at the effect of caffeine on attenuating the sensation of fatigue – as did Dr Angus Hunter several years ago – and thereby using it as a ‘tool’ to get folk to exercise more ‘easily’ as they appear to ‘feel’ fatigue less after ingesting caffeine. All this shows again that the sensation of fatigue is both a very complex emotion, and a very ‘labile’ one at that, and can change, and be changed, by both external factors such as these stimulants, and internal factors such as one’s drive or ‘desire’ to resist the sensation of fatigue as they arise, or even ‘block them out’ before they are consciously generated. More research, and very advanced research techniques, will be required for us to clearly understand how and such potential ‘blockages’ of the sensation of fatigue happen, if they indeed occur.

The sensation of fatigue is therefore an immensely complex ‘derivative’ of a number of functions, behaviours, and psychological ‘filters’, and what we finally ‘feel’ as fatigue is ‘more’ than a simple one-to-one description of some underlying change in our physical body and brain that requires adjustment or attenuation. The sensation of fatigue is clearly a protective phenomenon designed to slow us down when we are exercising too hard or too long in a manner that may damage our body, or when we are working too hard or too long and need a ‘time out’, or when the environment one is performing activities of daily living in may be harming one. But there are usually more complex relationships and reasons for the occurrence of the sensation of fatigue than what on the surface may appear to be the case. For example, the increase in work related fatigue I feel is surely related not just to the fact that it is the end of a busy week – it is perhaps likely to be related to a ‘deep’ yearning to be back on holiday, or to the fact that my mind is not ‘hardened’ yet to my routine daily work requirements, or has been ‘softened’ by the holiday period so that now I feel fatigue ‘more’ than is usual. In a few weeks time this will surely be attenuated as the year progresses and my weekly routines, which have been ‘honed’ over many years of work, are re-established, and I will feel the ‘usual’ rather than excessive symptoms of fatigue as always on Thursdays and Fridays. The extreme feeling of fatigue I felt during the very hot December month may also be related to some subconscious ‘perception’ that my current living environment is perhaps not optimal for me lifestyle wise for a long term living basis, and this ‘valenced’ how I perceived the environment as one of extreme heat and therefore extreme (and greater than expected) fatigue last month. And that I am ‘enjoying’ the sensations of fatigue I feel when exercising may mean that I am perhaps not pushing my exercise bouts as hard as I could, and need to go harder, or that my mind and body is setting a pace that feels enjoyable both so I continue doing it, or to protect me from a potential heart attack if I go harder. All of these may be the case, or equally, all of these could be mere speculation – the science folk in the area of fatigue have a big mountain to climb, and many more hours in the lab, before we more fully understand the complex emotion which the sensation of fatigue is, and how and from where it arises and is controlled.

A time may come when Sam Marcora and other excellent research colleagues like him find the ‘magic bullet’ that will ‘banish’ the sensation of fatigue, and we will be able to work harder and exercise longer because of it. But then would the cold drink after exercise taste so good, or the feeling of accomplishment one gets at the end of a long exercise bout as a result of resisting the sensation of fatigue long enough to achieve one’s goals for the particular exercise bout one has just completed still occur? This is something to ponder on, when fatigued, as I am now after two hours of writing, as I sip my cup of coffee, and wait for my ‘energy’ to return so I can begin the next task of a routine Sunday, whether it be cycling with the kids, walking the dog, or any other fatigue-removing activity as I prepare for the next fatiguing cycle which is the work and sport week ahead!

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About Alan (Zig) St Clair Gibson

Professor Alan (Zig) St Clair Gibson MBChB PhD MD - Deputy Dean (Research), Faculty of Science and Health, University of Essex, Colchester, United Kingdom View all posts by Alan (Zig) St Clair Gibson

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