Fatigue: a psychophysiological phenomenon (normally) under control
Unfortunately, no one is immune to fatigue. Whether we are in good health or suffer from a chronic condition, whether we are ordinary people or top athletes, after a hard day's work we all experience that feeling of not having enough energy to continue working, thinking, playing sports, etc.
Stéphane Perrey, University of Montpellier
AdobeStock_323492614 © PheelingsMedia – stock.adobe.com
However, this feeling of fatigue can be a good thing. In response to physical exertion, when it remains temporary and reversible, it contributes to the improvement of our performance. This is a normal situation, reminiscent of the famous saying, "No pain, no gain!"
But its symptoms can also be signs of accumulated fatigue, which can have lasting negative consequences. It is then a warning sign that we are at risk of "overheating," resulting in impaired 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 may our working memory... The important thing, then, is to assess our level of fatigue: how can we do this? How does our body manage it? And above all... what are we talking about?
A complex assessment
While it is common to talk about fatigue, measuring it remains complex due to the multiple indicators (objective and subjective) that characterize it.
Various methods exist and complement each other in attempting to quantify it:
- Subjective assessments (questionnaires, visual analog scales),
- Behavioral measures (e.g., correct response rate, reaction time, speed or mechanical power, 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 via its central and peripheral components).
But that's not all: because there's fatigue... and then there's fatigue!
It is now established that there are several types of fatigue. The COVID-19 pandemic, for example, has revealed it to be a persistent symptom for patients. It has also become prevalent among healthcare workers due to their excessive workload and among people working from home who are 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, which are numerous and multifactorial, do not make this easy. Furthermore, depending on which type of expert you consult, the definition of the phenomenon may vary! So much so that, much like the fable of the elephant and the blind men, there are a myriad of different representations of "fatigue."
In concrete terms, what is "fatigue"?
Simply put, fatigue can be defined as a feeling of physical or cognitive weakness that can occur as a result of muscular effort (during physical activity and/or sports) or cognitive effort (during intellectual or mental work), resulting in difficulty continuing the effort.
This definition highlights two types of fatigue that might be thought 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 (Free University of Brussels), physical (muscular) fatigue occurs 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).
- Mental 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."
Both are acute phenomena, 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 just muscular, and mental fatigue is not just psychological...
Physical and mental fatigue interact more than we think. As a mental or physical task continues, fatigue sets in and results in changes in our brain activity. In particular, we see that the prefrontal cortex (the "control tower" involved in our emotions and mood disorders, working memory, decision-making, motivation, and concentration) modulates its activity.
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Physical fatigue and how to manage it
To maintain physical effort, whether walking, cycling, or swimming, we have to deal with the insidious onset of fatigue in our muscles. If we listened only to our bodies and stopped at the first twinge, we wouldn't get very far...
Neuromuscular functional fatigue is a complex phenomenon resulting from numerous mechanisms located at different levels of the motor pathways, from the motor cortex to the muscle fibers. It stems from both peripheral factors, which alter 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 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 " central governor" model (the brain manages) and the " flush valve" model (accumulation of fatigue).
Added to this are psychological factors (psychobiological model). Some people are also able to regulate the speed at which they move, delaying or hastening the voluntary cessation of physical effort.
Our brain must integrate all these different factors, using a complex process that involves several areas of the brain, including those related to cognitive control. The result is an estimate of our actual level of fatigue and the optimal balance between unavoidable physiological costs and the expected benefits of the effort... Or how to be tired, but not too tired, according to this skilled strategist.
When the stakes are high, we need to be able to push ourselves beyond our limits. To tolerate unpleasant signals sent by our muscles (pain, etc.), we rely on various neurocognitive information controlled by the prefrontal cortex—yes, that again. It is capable of inhibiting other brain structures such as the anterior cingulate cortex (involved in regulating decision-making, empathy, etc.), the amygdala (response to fear, 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 fatigue...
The biochemistry of mental fatigue
Just as a heavily used muscle becomes exhausted, intense and prolonged intellectual effort causes mental fatigue. Activity in the prefrontal cortex then decreases, impairing our ability to make good decisions.
More impulsive in our decisions, we choose short-term benefits over more significant medium-term ones. Far from being anecdotal, this loss of control can have serious consequences in areas such as medicine, aviation, etc.
It stands to reason that as the day progresses, fatigue sets in, making us feel less and less capable of making important decisions and leading us to make mistakes.
Recent experimental observations have shown that metabolic changes in the brain could be responsible for the effects of mental fatigue. Significant mental effort causes the accumulation of glutamate, a byproduct of neuronal activity. Although glutamate is one of the most important excitatory neurotransmitters (chemical signals between nerve cells) in the nervous system, it can become harmful in excessive quantities.
Its accumulation in certain areas of the prefrontal cortex impairs the functioning of this key region, simultaneously disrupting reasoning and decision-making, so that we make choices that are more impulsive than strategic—without this being directly due to subjective fatigue.
It should also be noted that massive amounts of glutamate are involved in the onset of migraines and a wide range of neurological diseases.
And glutamate is undoubtedly not the only molecule involved in mental fatigue, which cannot be separated from neuro-metabolic factors.
Knowing how to tire yourself out without exhausting your resources
Physical and mental fatigue are therefore omnipresent, and our bodies have mechanisms to assess this and warn us, via our brains, when we are becoming overworked...
Almost all of us are overworked at some point, inevitably. All it takes is for everything to pile up, professionally and/or personally, for overactivity to set in... What must be avoided is for this to become permanent—a state that is harmful to the body.
Hence the importance of being alert to signs of fatigue and early indications of non-recovery in order to slow down before burnout sets in... A syndrome that can also be caused by excessive physical training—or overtraining.
In addition to chronic physical fatigue, athletes are unable to perform at their usual level, even when they rest. Their fatigue warning systems are disrupted, and tests reveal physiological and biological changes, such as alterations in cardiovascular function and hormone secretion. Psychologically, they will also be more irritable, depressed, and apathetic. Here again, their ability to make (good) decisions will be impaired due to reduced activity in their lateral prefrontal cortex.
It remains to be explained to what extent, in what proportions, and for how long excessive physical training leads to cognitive control system dysfunction...
Knowledge that will help develop methods to prevent burnout in athletes and anyone else affected by this debilitating syndrome.
Stéphane Perrey, PR, Director of the Research Digital Health in Motion Research Unit, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.