Unfortunately, no one can escape fatigue. Whether in good health or affected by a chronic pathology, whether a simple citizen or a high-level athlete, after a day of intense work, we are all confronted with this feeling of not having enough resources to continue working, thinking, playing sports...
However, this feeling of fatigue 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 indicators 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 translates into an alteration of activity in a part of our brain that is important for decision-making: the lateral prefrontal cortex.
Our attention span may be reduced, poor decisions may be made, our anxiety may increase, our motivation may decrease as well as our working memory... The important point is therefore to evaluate the level of fatigue: how to do it? How does our body manage it? And above all... what are we talking about?
A complex evaluation
If talking about fatigue is common, succeeding in measuring it remains complex due to the multiple indicators (objective and subjective) that characterize it.
- Subjective assessments (questionnaires, visual analog scales),
- Behavioral measures (e.g., correct response rate, reaction time, mechanical speed or power, muscle strength determinants),
- Psychophysiological measurements (cardiac activity, electrodermal response, pupillary dilation as controls for autonomic nervous system responses),
- Neurophysiological measurements (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 now established that there are several types of fatigue. The Covid-19 pandemic, for example, revealed it as a persistent symptom for patients, and it has also become an established feature among caregivers due to their overload of work or among teleworkers stuck in front of screens.
To deal with these forms of fatigue, it is necessary to identify the one(s) to be considered... But their possible origins, numerous and multi-factorial, do not facilitate the matter. Moreover, depending on whether one is talking to one type of expert or another, the definition of the phenomenon may vary! So much so that, like the fable of the elephant and the blind men, there are a multitude of different representations of "fatigue".
In concrete terms, what is "fatigue"?
Simply put, fatigue can be defined as a feeling of physical or cognitive weakening that can occur following muscular efforts (in the case of a 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 of as independent, physical and mental, mentioned as early as 1891 in the work of Italian physician Angelo Mosso.
- According to the taxonomy proposed by Roger Enoka (University of Colorado Boulder) and Jacques Duchateau (Université Libre de Bruxelles) physical (muscular) fatigue is manifested during physical exercise leading to an increase in the perception of effort for a given level of power or force (subjective fatigability) and/or a decrease in the maximum voluntary force after exercise (neuromuscular functional fatigability).
- 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".
Acute phenomenon, both are considered "normal" and they disappear by themselves after recovery. In this context, sleep is, not surprisingly, an essential phase of both physical and mental recovery.
However, physical fatigue is not only muscular and mental fatigue is not only psychological...
In fact, physical and mental fatigue interact more than we think. As a mental or physical task is prolonged, fatigue appears and translates into adaptations of our brain activity. In particular, we observe that the prefrontal cortex ("control tower" involved in our emotions and mood disorders, our working memory, our decision-making, our motivation and our concentration) will modulate its activity.
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Physical fatigue and its control
To maintain a physical effort, whether 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 tugging, we wouldn't get far...
Neuromuscular functional fatigability is a complex phenomenon that results from many mechanisms located at different levels of the motor pathways, from the motor cortex to the muscle fibers. It results from both peripheral factors, which alter the muscle's ability to produce force, and/or central factors, which affect the central nervous system's ability to activate the muscle.
These two types of factors interact, via neural circuitry, to adapt muscle contractions to the level of effort required. Several models of this dialogue have been proposed - such as the so-called " central governor" (the brain manages) or the " flush" (accumulation of fatigue).
In addition, there are psychological factors(psychobiological model). Some are, in fact, also capable of regulating the speed at which we move, of delaying or precipitating the voluntary cessation of physical effort.
Our brain has to integrate all these different factors, in a complex process that involves several of its regions, including those related to cognitive control. The result is an estimate of our real level of fatigue and of the optimal ratio between unavoidable physiological costs and the expected benefits of the effort... Or how to be tired, but not too tired according to this good strategist.
When the game is worth the candle, we must be able to surpass ourselves. In order to tolerate unpleasant signals sent by our muscles (pain, etc.), we depend on various neurocognitive information under the control of the prefrontal cortex - again. It is able to inhibit other brain structures such as the anterior cingulate cortex (involved in the regulation of decision making, empathy...), the amygdala (fear response...) and the insula (consciousness, emotions, etc.).
The mind, if one can say, by limiting our sensitivity to the emotional response to a painful effort, dominates the matter and tires it...
The biochemistry of mental fatigue
Just as a muscle that is highly solicited becomes 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 the more important ones which would be in the medium term. Far from being anecdotal, this loss of control can have serious consequences at the medical, aeronautical and other levels.
It can be thought that as the day progresses, 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 could be at the origin of the effects of mental fatigue. A consequent mental effort causes the accumulation of a by-product of the activity of neurons, glutamate. Glutamate is one of the most important excitatory neurotransmitters (chemical signals between nerve cells) in the nervous system, but too much of it can be harmful.
Its accumulation in certain areas of the prefrontal cortex alters the functioning of this key region: this simultaneously disrupts reasoning and decision making, so that we make more impulsive than strategic choices - without this being directly due to subjective fatigability.
It should also be noted that massive amounts of glutamate are involved in thedevelopment 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 neurometabolic factors.
Knowing how to get tired without exhausting your resources
Physical and mental fatigue are therefore omnipresent, and our body has mechanisms to evaluate it and warn us, via our brain, when overwork is coming...
Almost all of us are overworked at some point. It is enough that everything accumulates, professionally and/or personally, for overactivity to set in... What must be avoided is that it becomes permanent - a deleterious state for the body.
Hence the importance of being vigilant to the signals of fatigue and the first signs of non-recovery in order to lift your foot off the gas before burn-out... A syndrome that can also be caused by excessive physical training - or overtraining.
In addition to a physical fatigue that has become chronic, the athlete is no longer able to reach his usual level of performance, even if he takes rest. His or her fatigue alert systems are deregulated and examinations will reveal physiological and biological alterations: changes in the functioning of the cardiovascular system, hormonal secretions, etc. Psychologically, he will also be more irritable, depressed and 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 duration an overload of physical training induces a dysfunction of the cognitive control system...
Knowledge that will help develop methods to prevent the occurrence of burn-out in athletes, and all those affected by this disabling syndrome.