No one, unfortunately, escapes fatigue. In good health or affected by a chronic pathology, simple person or high-level athlete, after an intense day of work, we are all confronted with this feeling of no longer having sufficient resources to continue to work, think, play sports…
This feeling of fatigue, however, can be a good thing. In response to physical effort, when it remains temporary and reversible, it contributes to the progression of our performance. It is then a normal situation, which refers to the famous “No pain, no gain”!
But its symptoms can also be so many clues to the presence of accumulated fatigue which, this time, can have lasting negative consequences. It is then a signal that alerts us to a risk of “overheating” and results in an alteration of activity in a part of our brain that is important in decision-making: the lateral prefrontal cortex.
Our attention span may then be diminished, bad decisions may be made, our anxiety may intensify, our motivation may decrease, as will our working memory… The important point is therefore the evaluation of the level of fatigue: how to do it? How does our body deal with it? And above all… what are we talking about?
A complex assessment
If talking about fatigue is common, succeeding in measuring it remains complex due to the multiple indicators (objective and subjective) that characterize it.
Different methods exist and complement each other to try to quantify it:
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Subjective assessments (questionnaires, visual analogue scales),
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Behavioral measures (e.g. correct response rate, reaction time, mechanical speed or power, determinants of muscle strength),
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Psychophysiological measurements (cardiac activity, electrodermal response, pupillary dilation as witnesses of autonomic nervous system responses),
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Neurophysiological measures (brain activity via combined neuroimaging methods, neuromuscular activity via its central and peripheral components).
But that’s not all: because there is fatigue… and fatigue!
It is indeed established today that there are several fatigues. The Covid-19 pandemic has, for example, revealed it as a persistent symptom for patients, it has also imposed itself on caregivers due to their work overload or on teleworkers stuck in front of screens.
To deal with these forms of fatigue, it is necessary to identify which one(s) to consider… But their possible origins, numerous and multifactorial, do not make it any easier. Moreover, depending on whether one is addressing one type of expert or another, the definition adopted for the phenomenon may vary! So much so that, a little like the fable of the elephant and the blind men, a myriad of different representations of “fatigue” co-exist.
Concretely, what is “fatigue”?
Simply, fatigue can be defined as a feeling of physical or cognitive weakening which can occur following muscular efforts (in the case of physical and/or sporting activity) or cognitive efforts (during intellectual or mental work), resulting in difficulty in continuing the effort.
This definition highlights two types of fatigue that one might think are independent, physical and mental, mentioned as early as 1891 in the work of the Italian doctor Angelo Mosso.
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According to the taxonomy proposed by Roger Enoka (University of Colorado Boulder) and Jacques Duchateau (Free University of Brussels), the physical fatigue (muscular) manifests itself during physical exercise leading to an increase in the perception of effort for a power or force of a given level (subjective fatigue) and/or a decrease in the maximum voluntary force after exercise ( functional neuromuscular fatigability).
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The mental (cognitive) fatigue refers to “a psychobiological state experienced […] after performing an intense and/or prolonged cognitive task, which is characterized by a feeling of exhaustion and lack of energy”.
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Acute phenomenon, both are considered “normal” and they disappear on their own after recovery. In this context, sleep is, unsurprisingly, an essential phase of both physical and mental recovery.
However, physical fatigue is not only muscular and mental fatigue is not only psychological…
Indeed, physical and mental fatigue interact more than we think. As a mental or physical task is prolonged, fatigue appears and results in adaptations in the activity of our brain. We observe in particular that the prefrontal cortex (“control tower” particularly involved in our emotions and mood disorders, our working memory, our decision-making, our motivations and our concentration) will modulate its activity.
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Physical fatigue and its control
To maintain a physical effort, walking, cycling or swimming, we must face the insidious appearance of fatigue in our muscles. If we only listened to our body and stopped at the first tugs, we wouldn’t get far…
Functional neuromuscular fatigability is a complex phenomenon that results from numerous mechanisms located at different stages of the motor pathways, from the motor cortex to the muscle fibers. It comes from both peripheral factors, which alter the muscle’s ability to produce force, and/or central factors, which influence the ability of the central nervous system to activate the muscle.
These two types of factors interact, via neural circuitry, in order to adapt muscle contractions to the level of effort to be provided. Several models of this dialogue have been proposed – such as the so-called “central governor” (the brain manages) or “the flush” (accumulation of fatigue).
Added to this are psychological factors (psychobiological model). Some are, in fact, also capable of regulating the speed at which one moves, of delaying or hastening the voluntary cessation of physical effort.
Our brain must integrate all these different factors, according to a complex processing that involves several of its regions, including those relating to cognitive control. The result is an estimate of our real level of fatigue and the optimal ratio between unavoidable physiological costs and expected benefits of the effort… Or how to be tired, but not too much according to this good strategist.
When the game is worth the candle, we must be able to surpass ourselves. To tolerate the unpleasant signals sent in particular by our muscles (pain, etc.), we depend on various neurocognitive information under the control of the prefrontal cortex – again it. It is able to inhibit other brain structures such as the anterior cingulate cortex (involved in the regulation of decision-making, empathy…), the amygdala (response to fear…) and the insula (the consciousness, emotions, etc.).
The spirit, so to speak, by limiting our sensitivity to the affective response to a painful effort, dominates matter and fatigue…
The biochemistry of mental fatigue
Just as a very stressed muscle is exhausted, an intense and prolonged intellectual effort generates mental fatigue. The activity of the prefrontal cortex will then decrease, to the detriment of our ability to make good decisions.
More impulsive in our decisions, we choose short-term benefits rather than those that are more important in the medium term. Far from being anecdotal, this loss of control can have serious consequences at the medical, aeronautical, etc. levels.
We can think that the more the day advances, the more fatigue sets in, so that we feel less and less able to make important decisions and we make mistakes.
Recent experimental observations have shown that metabolic changes in the brain may underlie the effects of mental fatigue. A substantial mental effort indeed causes the accumulation of a by-product of the activity of neurons, glutamate. If the latter is one of the most important excitatory neurotransmitters (chemical signal between nerve cells) of the nervous system, it can become harmful in too large quantities.
Its accumulation in certain areas of the prefrontal cortex alters the functioning of this key region: which simultaneously disrupts reasoning and decision-making, so that we make more impulsive rather than strategic choices – without this being directly due to fatigue. subjective.
It should also be noted that massive amounts of glutamate are implicated in the onset of migraine and a wide range of neurological diseases.
And glutamate is probably not the only molecule involved in mental fatigue, which cannot be dissociated from neuro-metabolic factors.
Know how to get tired without exhausting your resources
Physical and mental fatigue are therefore omnipresent, and our body has the mechanisms to evaluate it and warn us, via our brain, from the moment the overwork happens…
Almost all of us are overworked, inevitably, at some point. All it takes is for everything to accumulate, professionally and/or personally, for overactivity to set in… What must be avoided is for this to become permanent – a deleterious state for the body.
Hence the importance of being vigilant to signs of fatigue and the first signs of non-recovery in order to slow down before burn-out… A syndrome which can also be caused by excessive physical training – or overtraining.
In addition to physical fatigue that has become chronic, the athlete can no longer reach his usual level of performance, even if he takes rest. His alert systems vis-à-vis fatigue are out of order and examinations will reveal physiological and biological alterations: modification of the functioning of the cardiovascular system, hormonal secretions, etc. Psychologically, he will also be more irritable, depressed, apathetic. Here again, his ability to make (good) decisions will be altered, due to the reduced activity of his lateral prefrontal cortex.
It remains to be explained to what extent, proportions and durations an overload of physical training induces a dysfunction of the cognitive control system…
Knowledge that will help develop methods to prevent the occurrence of burnout in athletes, and all those affected by this disabling syndrome.