You would expect that excessive athletic training makes the body tired, but can the brain also get tired? A new study suggests the answer is "yes." When researchers imposed an excessive training load on triathletes, they showed a form of mental fatigue. This fatigue included decreased activity in a part of the brain important for decision-making. The athletes also acted more impulsively, opting for immediate rewards rather than larger ones, which would take longer to achieve.
"The lateral prefrontal region affected by exercise overload was the exact same one that had been found vulnerable to excessive cognitive work in our previous studies," the researcher said. “This brain region therefore appeared as the weak spot of the brain network responsible for cognitive control.”
Together, the studies suggest a link between mental and physical exertion:Both require cognitive control. The reason such control is essential in demanding athletic training, they suggest, is that to maintain physical exertion and achieve a goal at a distance requires cognitive control.
The researchers explain that the original idea for the study came from the National Institute of Sport, Expertise and Performance (INSEP) in France, which trains athletes for the Olympics. Some athletes suffered from 'overtraining syndrome', where their performance plummeted when they experienced an overwhelming feeling of fatigue. The question was:Is this overtraining syndrome caused in part by neural fatigue in the brain – the same kind of fatigue that can also be caused by excessive intellectual work?
To find out, the researchers recruited 37 competitive male endurance athletes with an average age of 35. Participants were instructed to either continue their normal training or increase that training by 40% per session over a three-week period. The researchers monitored their physical performance during cycling exercises performed on rest days and assessed their subjective experience of fatigue using questionnaires every two days. They also performed behavioral testing and functional magnetic resonance imaging (fMRI) scanning experiments.
The evidence showed that physical training overload caused the athletes to feel more fatigued. They also acted more impulsively in standard tests used to evaluate how they would make economic choices. This tendency was shown as a preference for immediate over deferred rewards. The brains of athletes who were physically overloaded also showed decreased activation of the lateral prefrontal cortex, an important part of the executive control system, as they made those economic choices.
The findings show that while endurance sports are generally good for your health, too much exercise can have adverse effects on your brain, the researchers say.
