A mysterious phenomenon

As with other workers subjected to intense activity, top-level athletes are particularly vulnerable to exercise overload, which can result in what doctors call "overtraining syndrome."  This condition, characterized by a feeling of great fatigue and a drop in sporting performance, can cause behavioral disorders and impair brain capacity.   

Mathias Pessiglione, research director at Inserm and team leader at the Institut du Cerveau, is the author of a study on the subject published in 2019, and has attempted to identify the causes of this phenomenon. "It's still a rather mysterious syndrome, which can arise from one day to the next," explains the researcher. While he asserts that "sport is good for the brain," he notes that fatigue caused by overtraining can have the same origins as intellectual fatigue.

Carried out on two groups of triathletes, one of which underwent longer sessions, an experiment showed that participants who did more sport saw their cerebral capacities diminish- particularly cognitive control. It is this control that enables us to make decisions based on our objectives, and helps us to overcome our cerebral automatisms. "If I'm running a marathon and I get tired, I automatically stop. You have to use cognitive control to go beyond that," explains Mathias Pessiglione.

"There is a limit to what we can do"

As a result, overtired athletes are more likely to make impulsive decisions and stop along the way, or even take illegal substitutes that can be dangerous to their health. "We can see roughly where the motivational zones are located, in the orb-frontal cortex, an area of the brain where the decision-making process takes place," adds Pessiglione. As with any kind of fatigue, the most effective remedy remains sleep. "Some athletes collapse, others stop eating. There's a limit to what you can do," advises the researcher.

Professor Jean-Michel Oppert, head of the Nutrition Department at the Pitié-Salpêtrière Hospital, adds: "We don't really know what the right dose is for balancing physical activity and a sedentary lifestyle."

Like so many other health professionals, this obesity specialist is quick to point out the benefits of physical activity. "It stimulates the immune system, physical capacity and helps maintain weight. If we could find a drug that would produce the same effects, it would be an incredible molecule," states the practitioner.

A sedentary lifestyle

Defined by the World Health Organization (WHO) as all bodily movements that induce the expenditure of energy, physical activity remains the best means of combating cardiovascular disease, the world's leading cause of death, as well as cancer and diabetes.  Adults aged 18 to 64 should devote "at least 150 to 300 minutes a week" to moderate physical activity, according to recommendations from the UN agency.

"Sport is a category of physical activity," adds Jean-Michel Oppert, who warns against our sedentary lifestyle. A state that corresponds "to having an energy expenditure very close to rest," and which has been an integral part of our developed societies since the second half of the 20th century. "This behavior has an impact on our health," assures the doctor, for whom "our sedentary lifestyles are the result of a lack of physical activity."

While he doesn't advocate a return to the hunter-gatherer era, he does insist that you don't need to run a marathon every weekend to get active. "We need to know how to control ourselves", adds the practitioner, for whom, nevertheless, "any physical activity is good."