Fatigue: a psychophysiological phenomenon (usually) under control

Fatigue: a psychophysiological phenomenon (usually) under control

Nobody, 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 all face this feeling of not having enough resources to continue working, thinking, playing sports…

This feeling of fatigue, however, can be a good thing. In response to physical exertion, when it is 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 that, 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 in the activity of a part of our brain that is important in decision-making: the lateral prefrontal cortex.

So our attention span can be diminished, bad decisions can be made, our anxiety can be intensified, our motivation can decrease, as well as our working memory… The important point is therefore the evaluation of the level of fatigue: how do it? How does our body deal with it? And above all… what are we talking about?

A complex evaluation

If talking about fatigue is common, measuring it remains complex due to the multiple indicators (objective and subjective) that characterize it.

There are different methods that complement each other to try to quantify it:

  • Subjective assessments (questionnaires, visual analog scales),

  • Behavioral measures (eg, correct response rate, reaction time, mechanical speed or power, determinants of muscle strength),

  • Psychophysiological measurements (cardiac activity, electrodermal response, pupillary dilation as witnesses to autonomic nervous system responses),

  • Neurophysiological measures (brain activity via combined neuroimaging methods, neuromuscular activity via its central and peripheral components).

But that’s not all: because there is tiredness… and tiredness!

In fact, today it is established that there are several fatigues. The Covid-19 pandemic, for example, has revealed it as a persistent symptom for patients, it has also been imposed on caregivers due to their work overload or teleworkers trapped 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. Furthermore, 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 bit like in the fable of the elephant and the blind men, an endless number of different representations of “fatigue” coexist.

Specifically, what is “fatigue”?

Simply, fatigue can be defined as a feeling of physical or cognitive weakness that can occur after muscular efforts (in the case of physical and/or sports activity) or cognitive efforts (during intellectual or mental work), resulting in difficulty in continuing with the effort. .

This definition highlights two types of fatigue that could be thought to be independent, physical and mental, mentioned as early as 1891 in the work of the Italian physician Angelo Mosso.

  • According to the taxonomy proposed by Roger Enoka (University of Colorado Boulder) and Jacques Duchateau (Free University of Brussels), the physical (muscular) fatigue It manifests itself during physical exercise, causing an increase in the perception of effort for a power or force of a certain level (subjective fatigue) and/or a decrease in the maximum voluntary force after exercise (functional neuromuscular fatigue).

  • 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”.

A young man rubs his eyes in front of his computer screen at night.
Prolonged and intense intellectual exertion also causes measurable fatigue.
Earth Image/Shutterstock

Acute phenomenon, both are considered “normal” and 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…

In fact, physical and mental fatigue interact more than we think. As a mental or physical task continues, fatigue appears and translates into adaptations in the activity of our brain. We note 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 sustain physical exertion, whether walking, cycling or swimming, we must deal with the insidious onset of fatigue in our muscles. If we just listened to our body and stopped at the first jerks, we wouldn’t get very 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 stems from both peripheral factors, which alter the ability of the muscle to produce force, and central factors, which influence the ability of the central nervous system to activate the muscle.

These two types of factors interact, through neural circuits, to adapt muscle contractions to the level of effort to be performed. Several models of this dialogue have been proposed, such as the so-called “central governor” (the brain controls) 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 accelerating the voluntary cessation of physical exertion.

Our brain must integrate all these different factors, according to a complex processing that involves several of its regions, including those related to cognitive control. The result is an estimate of our real level of fatigue and the optimal relationship between the inevitable 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 worthwhile, we must be able to outdo ourselves. To tolerate unpleasant signals sent by our muscles in particular (pain, etc.), we rely on various neurocognitive information under the control of the prefrontal cortex, again. It is capable of inhibiting other brain structures such as the anterior cingulate cortex (involved in regulating decision-making, empathy…), the amygdala (response to fear…) and the insula (awareness, emotions, etc.).

The spirit, so to speak, by limiting our sensitivity to the affective response to a painful effort, dominates matter and weariness…

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 instead of 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. level.

We may think that the further the day progresses, the more fatigue sets in, so that we feel less and less able to make important decisions and make mistakes.

Recent experimental observations have shown that metabolic changes in the brain may underlie the effects of mental fatigue. In fact, substantial mental exertion causes the buildup of a byproduct of neuron activity, glutamate. If the latter is one of the most important excitatory neurotransmitters (chemical signal between nerve cells) in the nervous system, it can become harmful in too large amounts.

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 decisions than strategic ones -without this being directly due to subjective fatigue-.

It should also be noted that massive amounts of glutamate are implicated in the onset of migraines 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.

Learn to get tired without exhausting your resources

Physical and mental fatigue is therefore omnipresent, and our body has the mechanisms to evaluate it and warn us, via our brain, from the moment the overwork occurs…

Almost all of us are inevitably overworked at some point. It is enough that everything accumulates, professionally and/or personally, for hyperactivity to be established… What must be avoided is that it becomes permanent, a deleterious state for the organism.

Hence the importance of being attentive to the signs of fatigue and the first signs of lack of recovery in order to stop before burn-out… A syndrome that can also be caused by excessive physical training – or overtraining.

In addition to the physical fatigue that has become chronic, the athlete can no longer reach his usual level of performance, even if he rests. Their fatigue alert systems are out of order and tests will reveal physiological and biological changes: changes in 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 impaired, due to reduced activity in 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 appearance of burnout in athletes, and all those affected by this disabling syndrome.

#Fatigue #psychophysiological #phenomenon #control

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