Unfortunately, no one can avoid fatigue. In good health or affected by a chronic pathology, a simple person or a top athlete, after a hard day at work we are confronted with the feeling that we no longer have enough resources to continue working, thinking, playing sports…
However, this tired feeling can be a good thing. In response to physical exertion, when it remains temporary and reversible, it contributes to the progression of our performance. It is then a normal situation that refers to the famous “No pain, no gain”!
But these symptoms can also be so many signs of the presence of accumulated fatigue, which this time can have lasting negative consequences. It is then a signal that alerts us to the risk of “overheating” and results in a change in the activity of a part of our brain important for decision-making: the lateral prefrontal cortex.
Our attention can then be shortened, bad decisions can be made, our anxiety can increase, our motivation and working memory can decrease… Therefore, the assessment of the level of fatigue is important: how to do it? How does our body deal with it? And above all… what are we talking about?
If it is common to talk about fatigue, success in measuring it remains complex due to the many indicators (objective and subjective) that characterize it.
different methods exist and complement each other to try to quantify it:
Subjective assessment (questionnaires, visual analogue scales).
Behavioral measures (eg correct response speed, reaction time, speed or mechanical strength, determinants of muscle strength).
Psychophysiological measurements (cardiac activity, electrodermal response, pupil dilation as indicators of autonomic nervous system responses).
Neurophysiological measurements (brain activity via combined neuroimaging methods, neuromuscular activity through its central and peripheral components).
But that’s not all: because there is fatigue… and fatigue!
Today it is actually found that there are several fatigue. For example, the Covid-19 pandemic has exposed it as a persistent symptom for patientsit has also gained ground among caregivers due to their workload or among people working from home stuck in front of screens.
To deal with these forms of fatigue, it is necessary to identify those that need to be taken into account. But their possible origin, numerous and multifactorial, does not make matters easier. Moreover, depending on whether you turn to one type of expert or another, the definition of this phenomenon may differ! So much so that, somewhat like the fable of the elephant and the blind men, there are countless different representations of “fatigue” side by side.
What exactly is “fatigue”?
Simply put, fatigue can be defined as a feeling physically or cognitively impaired which may appear as a result of muscular effort (in the case of physical and/or sports activity) or cognitive (during intellectual or mental work), resulting in difficulty in continuing the effort.
This definition emphasizes two types of fatigue that one might think are independent, physical and mental, already mentioned in 1891 in the work of the Italian doctor Angelo Mosso.
According to the taxonomy proposed by Roger Enoka (University of Colorado at Boulder) and Jacques Duchateau (Free University of Brussels) physical fatigue (muscle) it manifests itself during physical exertion leading to an increase in the perception of effort for strength or power of a given level (subjective fatigue) and/or a decrease in maximal free strength after exercise (functional neuromuscular fatigue).
The mental (cognitive) fatigue return to “an experienced psychobiological state […] after performing an intense and/or prolonged cognitive task characterized by a feeling of exhaustion and lack of energy”.
It is an acute phenomenon. Both are considered “normal” and will go away on their own after healing. In this context, sleep is unsurprisingly an essential phase of physical and mental regeneration.
However, physical fatigue is not only muscular and mental, not only psychological…
Physical and mental fatigue actually affect each other more than we think. With the prolongation of a mental or physical task, fatigue appears, which has the effect of adapting the activity of our brain. In particular, we observe that the prefrontal cortex (the “control tower” particularly involved in our emotions and mood disorders, our working memory, our decision-making, our motivation and our concentration) will modulate its activity.
Physical fatigue and its control
In order to maintain physical exertion, walking, cycling or swimming, we have to face the insidious manifestation of fatigue in the muscles. If we only listened to our body and stopped at the first pains, we wouldn’t get far…
Functional neuromuscular fatigue is a complex phenomenon that it arises from many mechanisms it is found at various levels of motor pathways, from the motor cortex to the muscle fibers. It comes from both peripheral factors that alter the muscle’s ability to produce force, and central factors that affect the central nervous system’s ability to activate the muscle.
These two types of factors interact through neural circuits to adapt muscle contractions to the level of effort exerted. Several models of this dialogue have been proposed—for example, the model known as “central governor” (the brain manages) or “flush” (accumulation of fatigue).
Added to this are psychological factors (psychobiological model). In fact, some are also capable of regulating the speed at which one moves, delaying or accelerating the voluntary cessation of physical exertion.
Our brain has to integrate all these different factors according to complex processing includes several of its regions including those related to cognitive control. The result is an estimate of our true level of fatigue and the optimal ratio between the inevitable physiological costs and the expected benefits of exertion… Or how to be tired but not too tired according to this good strategist.
When the game is worth the candle, we have to be able to outdo ourselves. In order to tolerate unpleasant signals sent especially by our muscles (pain), we depend on various neurocognitive information. under the control of the prefrontal cortex – him again. It is able to inhibit other brain structures such as the anterior cingulate cortex (involved in decision-making regulation, empathy, etc.), the amygdala (fear response, etc.) and the ‘insula (consciousness, emotion, etc.). ).
The spirit, so to speak, in limiting our sensitivity to affective response to painful exertionmatter and fatigue dominate.
The biochemistry of mental fatigue
Just as a heavily strained muscle is exhausted, intense and prolonged intellectual effort produces mental fatigue. The activity of the prefrontal cortex will then decrease, at the expense of our ability to make correct decisions.
More impulsive in our decisions, we choose short-term benefits rather than those that are more important in the medium term. This loss of control can have serious consequences in the medical, aerospace, etc. industries, which is far from anecdotal.
We may think that the more 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 this metabolic changes in the brain can be behind the effects of mental fatigue. Substantial mental effort actually causes a build-up of the byproduct of neuronal activity, glutamate. If the latter is one of the most important excitatory neurotransmitters (a chemical signal between nerve cells) of the nervous system, it can become harmful in too much.
Its accumulation in certain areas of the prefrontal cortex alters the functioning of this key area: which simultaneously impairs reasoning and decision-making, so that we make impulsive rather than strategic decisions – without being directly caused by fatigue.
Also note that huge amounts of glutamate are involvedonset of migraine and a wide range of neurological diseases.
And glutamate is probably not the only molecule involved in mental fatigue that cannot be separated from neuro-metabolic factors.
Knowing how to get tired without exhausting your resources
So physical and mental fatigue is ubiquitous and our body has mechanisms to assess it and warn us through the brain when we are overworked.
Almost all of us are inevitably overworked at some point. All it takes is for it to accumulate, professionally and/or personally, for overactivity to manifest. It must be avoided that it becomes permanent – a harmful condition for the organism.
That’s why it’s important to be alert to signs of fatigue and early signs of failure to slow down before burnout. A syndrome that can also be caused excessive physical training – or overtraining.
In addition to the physical fatigue that has become chronic, the athlete can no longer reach their usual level of performance, even when resting. His fatigue warning systems are out of order, and the examination will reveal physiological and biological changes: modification of the functioning of the cardiovascular system, hormonal secretion, etc. Psychologically, he will also be more irritable, depressed, apathetic. Here again his ability to make (good) decisions changes due to reduced activity in his lateral prefrontal cortex.
It remains to be elucidated to what degree, proportion and duration of physical training overload induces cognitive control system dysfunction…
Knowledge that will help develop methods to prevent burnout in athletes and all those affected by this disabling syndrome.