Fatigue: a psychophysiological phenomenon (normally) under control

Unfortunately, no one is immune to fatigue. Whether we’re in good health or living with a chronic condition, whether we’re ordinary people or elite athletes, after a hard day’s work, we all face that feeling of no longer having the energy to keep working, thinking, or exercising…

Stéphane Perrey, University of Montpellier

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This feeling of fatigue can, however, be a good thing. When it occurs in response to physical exertion—provided it is temporary and reversible—it helps improve our performance. It is, therefore, a normal situation, reflecting the famous saying, “No pain, no gain”!

But these symptoms can also be signs of accumulated fatigue, which, in this case, can have lasting negative consequences. It serves as a warning signal of the risk of “overheating” and results in impaired activity in a part of the brain that plays a key role in decision-making: the lateral prefrontal cortex.

Our ability to concentrate may then be impaired, leading to poor decisions, increased anxiety, and a decline in both motivation and working memory… The key point, then, is assessing our level of fatigue: how do we do that? How does our body cope with it? And above all… what exactly are we talking about?

A complex assessment

While it is common to talk about fatigue, measuring it remains a complex task due to the many indicators—both objective and subjective—that characterize it.

Various methods exist and complement one another in an effort 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 measures (heart rate, skin conductance response, and pupil dilation as indicators of autonomic nervous system responses),
• Neurophysiological measurements (brain activity using combined neuroimaging methods; neuromuscular activity, including its central and peripheral components).

But that's not all: there's tiredness… and then there's real exhaustion!

It is now well established that there are various forms of fatigue. For example, the COVID-19 pandemic has revealed it as a persistent symptom among patients, and it has also become prevalent among healthcare workers due to their heavy workloads, as well as among remote workers who are stuck in front of screens.

To address these forms of fatigue, it is necessary to identify which ones to consider… But their numerous and multifactorial possible causes do not make this an easy task. Moreover, 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, a myriad of different representations of “fatigue” coexist.

In practical terms, what is “fatigue”?

Simply put, fatigue can be defined as a sensation of physical or cognitive weakness that can occur following muscular exertion (such as during physical or athletic activity) or cognitive exertion (such as during intellectual or mental work), resulting in difficulty continuing the effort.

This definition highlights two types of fatigue—physical and mental—that might be thought of as distinct, and which were first discussed as early as 1891 in a work by the 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 occurs during physical exercise, leading to an increase in the perceived effort required for a given level of power or force (subjective fatigability) and/or a decrease in maximum voluntary force after exercise (functional neuromuscular 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.”

These are acute conditions; both are considered “normal” and resolve on their own after recovery. In this context, sleep is, unsurprisingly, an essential phase of both physical and mental recovery.

However, physical fatigue isn't just about the muscles, and mental fatigue isn't just psychological…

In fact, physical and mental fatigue interact more than we realize. As a mental or physical task continues, fatigue sets in and results in changes in our brain activity. In particular, we observe 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 keep up our physical exertion—whether walking, cycling, or swimming—we have to deal with the gradual onset of muscle fatigue. If we listened only to our bodies and stopped at the first sign of soreness, we wouldn’t get very far…

Neuromuscular functional fatigue is a complex phenomenon resulting from numerous mechanisms located at various levels of the motor pathways, from the motor cortex down to the muscle fibers. It arises from both peripheral factors, which impair the muscle’s ability to generate force, and/or central factors, which affect the central nervous system’s ability to activate the muscle.

These two types of factors interact through neural circuits to adjust muscle contractions to the level of effort required. Several models of this interaction have been proposed—such as the “central governor” model (the brain controls the process) or the “flushing” model (accumulation of fatigue).

In addition, there are psychological factors (psychobiological model). Some of these factors are, in fact, also capable of regulating the speed at which we move, and of delaying or hastening the voluntary cessation of physical exertion.

Our brain must process all these different factors through a complex mechanism that involves several of its regions, including those responsible for cognitive control. The result is an assessment of our actual level of fatigue and the optimal balance between the unavoidable physiological costs and the expected benefits of the effort… Or, as this astute strategist puts it, how to be tired, but not too tired.

When the stakes are high, we need to be able to push ourselves to the limit. To tolerate the unpleasant signals sent by our muscles (pain, etc.), we rely on various neurocognitive inputs controlled by the prefrontal cortex—it’s that region 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—if we limit our understanding to the emotional response to a painful effort—overcomes the body and fatigue…

The Biochemistry of Mental Fatigue

Just as a heavily used muscle becomes exhausted, intense and prolonged mental effort leads to mental fatigue. Activity in the prefrontal cortex then decreases, impairing our ability to make sound decisions.

Because we are more impulsive in our decision-making, we tend to choose short-term benefits over more significant ones that would materialize in the medium term. Far from being a minor issue, this loss of control can have serious consequences in the medical, aviation, and other fields.

One might think that as the day goes on, fatigue sets in, making us feel less and less capable of making important decisions and causing us to make mistakes.

Recent experimental observations have shown that metabolic changes in the brain may be responsible for the effects of mental fatigue. Significant mental effort causes the accumulation of glutamate, a byproduct of neuronal activity. While glutamate is one of the most important excitatory neurotransmitters (chemical signals between nerve cells) in the nervous system, it can become harmful in excessive amounts.

Its accumulation in certain areas of the prefrontal cortex disrupts the functioning of this key region, which in turn impairs both reasoning and decision-making, leading us to make choices that are more impulsive than strategic—without this being directly attributable to subjective fatigue.

It is also worth noting that large amounts of glutamate are implicated in the onset of migraines and a wide range of neurological disorders.

And glutamate is certainly not the only molecule involved in mental fatigue, which cannot be separated from neuro-metabolic factors.

Knowing how to push yourself without burning out

Physical and mental fatigue are therefore ever-present, and our bodies have mechanisms to assess this and alert us, through our brains, when we’re about to burn out…

Almost all of us inevitably find ourselves overwhelmed at some point. It only takes a buildup of stress—whether at work or in our personal lives—for that sense of being overwhelmed to set in… What we must avoid is letting this state become permanent—a condition that is harmful to the body.

That’s why it’s important to watch for signs of fatigue and the first signs of not being able to recover, so you can ease up before you burn out… A condition that can also be caused by excessive physical training—or overtraining.

In addition to chronic physical fatigue, the athlete is then unable to perform at their usual level, even after resting. Their fatigue warning systems are disrupted, and medical tests will reveal physiological and biological changes: alterations in cardiovascular function, hormonal secretions, and so on. 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 the lateral prefrontal cortex.

It remains to be determined to what extent, in what proportions, and over what time periods excessive physical training leads to a dysfunction of the cognitive control system…

This knowledge will help develop methods to prevent burnout among athletes and all those affected by this debilitating condition.

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.