Researchers claims that excessive athletic training not only makes one's body tired but also drains the brain.
When researchers imposed an excessive training load on triathletes, they showed a form of mental fatigue. This fatigue included reduced activity in a portion of the brain important for making decisions.
The athletes also acted more impulsively, opting for immediate rewards instead of bigger ones that would take longer to achieve, reported the study published in the journal 'Current Biology'.
"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," said corresponding author Mathias Pessiglione of Hopital de la Pitie-Salpetriere in Paris. "This brain region, therefore, appeared as the weak spot of the brain network responsible for cognitive control."
Together, the studies suggest a connection between mental and physical effort: both require cognitive control. The reason such control is essential in demanding athletic training, they suggest, is that to maintain physical effort and reach a distant goal requires cognitive control.
"You need to control the automatic process that makes you stop when muscles or joints hurt," Pessiglione said.
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The researchers, including Pessiglione and first author Bastien Blain, explained that the initial idea for the study came from the National Institute of Sport, Expertise, and Performance (INSEP) in France, which trains athletes for the Olympic games.
Some athletes had suffered from "overtraining syndrome," in which their performance plummeted as they experienced an overwhelming sense of fatigue.
The researchers monitored athletes physical performance during cycling exercises performed on rest days and assessed their subjective experience of fatigue using questionnaires every two days. They also conducted behavioural testing and functional magnetic resonance imaging (fMRI) scanning experiments.
The evidence showed that physical training overload led the athletes to feel more fatigued. They also acted more impulsively in standard tests used to evaluate how they'd make economic choices.
This tendency was shown as a bias in favouring immediate over delayed rewards. The brains of athletes who'd been overloaded physically also showed diminished activation of the lateral prefrontal cortex, a key region of the executive control system, as they made those economic choices.
The findings show that, while endurance sport is generally good for your health, overdoing it can have adverse effects on your brain, the researchers said.
"Our findings draw attention to the fact that neural states matter: you don't make the same decisions when your brain is in a fatigued state," Pessiglione said.
These findings may be important not just for producing the best athletes but also for economic choice theory, which typically ignores such fluctuations in the neural machinery responsible for decision-making, the researchers said.
It suggested it may also be important to monitor fatigue level in order to prevent bad decisions from being made in the political, judicial, or economic domains.