Extreme Exercise Tires Both Your Body and Brain

A good workout might not just exhaust your body, but tire out your brain as well, suggests a new study conducted by researchers at Pitié-Salpêtrière Hospital in Paris and the French National Institute of Sport, Expertise, and Performance (INSEP). The study was published in Current Biology.

The study concluded that extreme physical exertion makes athletes more impulsive in their decision making, accompanied by impairment of brain activity in a region of the prefrontal cortex. The findings could have impact on how we treat the symptoms of both burnout and overtraining.

Burnout, a prolonged feeling of mental, emotional, and physical exhaustion, is a common experience for many people working in stressful jobs. For endurance athletes, a correlate of burnout is overtraining syndrome, where extreme fatigue is accompanied by a long-term loss in performance unrelated to injury. But how related are the two experiences? Can physical exercise exert as similar an action upon the brain as prolonged mental effort? To answer this question, senior author Mathias Pessiglione and colleagues recruited 37 male triathletes.

Exercise and the brain

The triathletes were divided into two groups – a control group, who, after a period of baseline exercise and rest, were asked to work at their normal training regimen for three weeks. The test group was asked to train at 40% more than their normal training load for the three-week period. After a rest, both groups were subjected to a battery of cognitive tests.

The subjects were placed in a magnetic resonance imaging (MRI) scanner, which uses radio waves to visualize the structures of the brain. They were tested on their decision making, and were asked to choose between immediate rewards, which were more tempting, or delayed rewards, which were ultimately of more value.

The results from the study showed that, unsurprisingly, athletes subjected to extra training (the scientists were careful not to induce actual overtraining syndrome in their volunteers on ethical grounds) were more physically tired, as shown by physical performance and psychometric testing. The cognitive results were much more novel – tired out athletes were more likely to make decisions with immediate outcomes as opposed to their less strained counterparts. The authors hypothesized that the increase in preference for immediacy represents loss of cognitive control, a deficit that is echoed by the inability of overtrained athletes to push through the immediate pain of physical exertion to attain the long-term goal of remaining fit.

Not a question of motivation

In a press release, Pessiglione summed up his team’s results: "Our findings draw attention to the fact that neural states matter: you don't make the same decisions when your brain is in a fatigue state.” These deficits were specific to decision making – tests of working memory showed no difference between experimental and control groups. The authors also pointed out that their findings were not a suggestion that overtraining caused a lack of motivation or an impairment in the ability to make choices, but specifically that it produced a preference for making impulsive decisions with immediate rewards as opposed to delayed ones.

So, what was the neural basis of this finding? Using the MRI scans, Pessiglione looked at brain activation in different regions related to decision making and identified a lower activation in an area of the pre-frontal cortex called the left middle frontal gyrus. This finding was the crux of the argument that physical overtraining and emotional burnout might be more similar than previously thought. Pessiglione explains, “The lateral prefrontal region that was affected by sport-training overload was exactly the same that had been shown vulnerable to excessive cognitive work in our previous studies. This brain region therefore appeared as the weak spot of the brain network responsible for cognitive control."

The team’s future research will look to understand the exact mechanisms resulting in lower cognitive control and could impact upon how athletes train, and how workers cope with burnout across other sectors. So, the next time you enter an ultramarathon with a bonus 50km swim at the end (if that ever happens) think about your poor brain, as well as your body.