Unfortunately, no one can escape fatigue. Whether we're in good health or suffering from a chronic illness, whether we're ordinary people or top athletes, after an intense day's work we're all confronted with the feeling that we no longer have sufficient resources to continue working, thinking, playing sport...

Stéphane Perrey, University of Montpellier

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However, this feeling of fatigue can be a good thing. In response to physical effort, when it remains temporary and reversible, it helps us to improve our performance. It's a normal situation, and a reminder of the famous "No pain, no gain" principle!

But its symptoms can also be signs of accumulated fatigue, which can have lasting negative consequences. In this case, it's a signal that alerts us to the risk of "overheating", and translates into altered activity in a part of our brain important for decision-making: the lateral prefrontal cortex.

Our attention span may be impaired, poor decisions may be made, our anxiety may intensify, our motivation may diminish, as may our working memory... The important thing is to assess our level of fatigue: how do we go about it? How does our body deal with it? And above all... what are we talking about?

A complex assessment

Talking about fatigue is commonplace, but measuring it remains a complex task, due to the multiple indicators (objective and subjective) that characterize it.

Various methods exist and complement each other in an attempt to quantify 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 indicators of 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: there's fatigue... and fatigue!

Today, it's well established that there are several types of fatigue. The Covid-19 pandemic, for example, revealed it to be a persistent symptom for patients, and it has also become an established feature among carers due to their work overload, or among telecommuters 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 make this easy. What's more, depending on which type of expert you're talking to, the definition of the phenomenon may vary! So much so that, like the fable of the elephant and the blind men, a whole host of different representations of "fatigue" co-exist.

In practical terms, what is "fatigue"?

Simply put, fatigue can be defined as a feeling of physical or cognitive weakening that can occur following muscular effort (in the case of physical and/or sporting activity) or cognitive effort (during intellectual or mental work), resulting in difficulty in continuing the effort.

This definition highlights two types of fatigue that might be thought of as independent: physical and mental, both of which were mentioned as early as 1891 in a book by 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 can be defined as physical (muscular) fatigue manifests itself 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 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, characterized by a feeling of exhaustion and lack of energy".

Acutely, both are considered "normal" and disappear on their own after recovery. In this context, sleep is, unsurprisingly, an essential phase in both physical and mental recovery.

But physical fatigue isn't just muscular, and mental fatigue isn't just 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 in our brain activity. In particular, the prefrontal cortex ("control tower" involved in emotions and mood disorders, working memory, decision-making, motivation and concentration) modulates its activity.

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Physical fatigue and its control

To maintain a physical effort, whether walking, cycling or swimming, we have to deal with the insidious appearance of fatigue in our muscles. If we only listened to our bodies and stopped at the first twinge, we wouldn't get far...

Neuromuscular functional fatigability is a complex phenomenon involving numerous mechanisms at different levels of the motor pathways, from the motor cortex to the muscle fibers. It stems from both peripheral factors, which impair the muscle's ability to produce force, and/or central factors, which influence the central nervous system's ability to activate the muscle.

These two types of factor interact, via neural circuitry, to adapt muscular contractions to the level of effort required. Several models for this dialogue have been proposed - such as the " central governor " model (the brain manages) or the " flush" model (accumulation of fatigue).

In addition, there are psychological factors (psychobiological model). Some are also capable of regulating the speed at which we move, delaying or precipitating the voluntary cessation of physical effort.

Our brain must integrate all these different factors, in a complex process involving several of its regions, including those related to cognitive control. The result is an estimate of our real level of fatigue and the optimum ratio between the unavoidable physiological costs and the expected benefits of effort... Or how to be tired, but not too tired, according to this good strategist.

When the going gets tough, we need to be able to push ourselves. To tolerate the unpleasant signals sent by our muscles (pain, etc.) in particular, we depend on various neurocognitive inputs under the control of the prefrontal cortex - there it is again. It is capable of inhibiting other brain structures such as the anterior cingulate cortex (involved in regulating decision-making, empathy, etc.), the amygdala (fear response, etc.) and the insula (consciousness, emotions, etc.).

The mind, so to speak, by limiting our sensitivity to the emotional response to painful effort, dominates matter and tires it out...

The biochemistry of mental fatigue

In the same way that a highly stressed muscle becomes exhausted, intense and prolonged intellectual effort generates mental fatigue. The activity of the prefrontal cortex will then diminish, 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 medium-term ones. Far from being anecdotal, this loss of control can have far-reaching consequences in the medical, aeronautical and other fields.

You might think 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 may be responsible for the effects of mental fatigue. Sustained mental effort leads to the accumulation of glutamate, a by-product of neuronal activity. Glutamate is one of the nervous system's most important excitatory neurotransmitters (a chemical signal between nerve cells), 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 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 neurometabolic factors.

Knowing how to tire without exhausting your resources

Physical and mental fatigue are therefore omnipresent, and our body has the mechanisms to assess it and warn us, via our brain, when overwork is imminent...

Almost all of us are inevitably overworked at one time or another. It's enough for everything to accumulate, professionally and/or personally, for overactivity to set in... What we need to avoid is for this to become permanent - a state which is deleterious to the body.

Hence the importance of being alert to signs of fatigue and the first signs of non-recovery, in order to ease off before burn-out... A syndrome which can also be caused by excessive physical training - or overtraining.

In addition to chronic physical fatigue, the athlete is no longer able to achieve his or her usual level of performance, even when rested. The athlete's fatigue warning systems are deregulated, and tests will reveal physiological and biological alterations: changes in the functioning of the cardiovascular system, hormonal secretions, etc. Psychologically, he'll also be more irritable, depressed and apathetic. Here again, his ability to make (good) decisions will be impaired, due to the reduced activity of his lateral prefrontal cortex.

It remains to be seen to what extent, in what proportions and for how long an overload of physical training leads to dysfunction of the cognitive control system...

Knowledge that will help develop methods to prevent burn-out in athletes, and all those affected by this debilitating syndrome.The Conversation

Stéphane Perrey, PR, Director of the Research EuroMov Digital Health in Motion Unit, University of Montpellier

This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.