Too much physical exertion appears to make the brain tired.
That's the conclusion of a study of triathletes published Thursday in the journal Current Biology.
Researchers found that after several weeks of overtraining, athletes became more likely to choose immediate gratification over long-term rewards. At the same time, brain scans showed the athletes had decreased activity in an area of the brain involved in decision-making.
The finding could explain why some elite athletes see their performance decline when they work out too much — a phenomenon known as overtraining syndrome.
The distance runner Alberto Salazar, for example, experienced a mysterious decline after winning the New York Marathon three times and the Boston Marathon once in the early 1980s. Salazar's times fell off even though he was still in his mid-20s and training more than ever.
"Probably [it was] something linked to his brain and his cognitive capacities," says Bastien Blain, an author of the study and a postdoctoral fellow at University College London. (Salazar didn't respond to an interview request for this story.)
Blain was part of a team that studied 37 male triathletes who volunteered to take part in a special training program. "They were strongly motivated to be part of this program, at least at the beginning," Blain says.
Half of the triathletes were instructed to continue their usual workouts. The rest were told to increase their weekly training by 40%.
The result was a training program so intense that these athletes began to perform worse on tests of maximal output.
After three weeks, all the participants were put in a brain scanner and asked a series of questions designed to reveal whether a person is more inclined to choose immediate gratification or a long-term reward. "For example, we ask, 'Do you prefer $10 now or $60 in six months,' " Blain says.
The answers showed a clear difference in the overtrained athletes. "Those people were, in fact, choosing more immediate gratification than the other group of athletes," Blain says.
The scanner also revealed a difference. There was less activity in "a very little brain area, a little spot of the left prefrontal cortex that's impacted during decision-making," Blain says.
When there's lots of activity in that area, athletes are able to ignore signals from screaming muscles and focus on winning, Blain says. But when an athlete trains too hard, a sort of brain fatigue sets in and the activity level remains low and the person has less ability to push their body, he says.
Other research teams also have found evidence that physical exertion can affect both decision-making and brain activity.
"We find that people as they have repeatedly exerted effort over time, they tend to be less willing to continue exerting effort for rewards," says Tanja Mueller, a graduate student at the University of Oxford who wasn't involved in the study of triathletes.
But the brain may not be simply choosing between long-term goals vs. immediate gratification, Mueller says. The calculus may be more about cost and benefit.
Research by Mueller and Matthew Apps suggests that when the body becomes physically depleted, the brain begins to experience "motivational fatigue," which affects decision-making. When that happens, the brain "may not consider it worth it anymore to wait for higher rewards."
The brain appears to be constantly reassessing the value of a goal, says Todd Braver, a professor of psychological and brain sciences at Washington University in St. Louis.
"So your brain is doing these kind of cost-benefit trade-offs all the time," he says. "Is it still worth the effort? Is it still worth the effort?"
And the answer to that question may change as the body's level of fatigue increases. "The brain might have this kind of built-in mechanism to say, 'Hey, it's time to shift from this goal to another one,' " he says.
For an athlete, Braver says, that could mean abandoning the goal of winning a race and embracing a goal that will let them recover.
RACHEL MARTIN, HOST:
Athletes who train too much don't just exhaust their bodies, they exhaust their brains. That's according to a new study from researchers in France. And it may explain why elite athletes sometimes see their performance decline as they ramp up training. Here's NPR's Jon Hamilton.
JON HAMILTON, BYLINE: The world's best distance runner in the early 1980s was Alberto Salazar.
(SOUNDBITE OF ARCHIVED RECORDING)
UNIDENTIFIED PERSON: Salazar kicking - go, go.
UNIDENTIFIED PERSON: Pushing off the final challenge by Beardsley for the win, the 1982 Boston Marathon.
HAMILTON: He won the Boston Marathon once and the New York City Marathon three times. Then Salazar stopped winning, even though he was still in his mid-20s and training harder than ever. Bastien Blain, a researcher at University College London, thinks he might know why.
BASTIEN BLAIN: Probably there was something linked to his brain and his cognitive capacities.
HAMILTON: Blain's hunch comes from a study of 37 male triathletes who took part in a special training program.
BLAIN: They were strongly motivated to be part of this program, at least at the beginning.
HAMILTON: Half the triathletes were told to continue their usual workouts. The other half were told to train a lot harder. The training was so intense, the athletes began to perform worse on a cycling test. Blaine says, after a few weeks, all the participants were put into a brain scanner and asked a series of questions.
BLAIN: For example, we asked, do you prefer $10 now or $50 in six months?
HAMILTON: The answers showed a clear difference in the over-trained athletes.
BLAIN: Those people were, in fact, choosing more immediate gratification than the other group of athletes.
HAMILTON: And Blain's team reported in the journal Current Biology that the athletes also had less activity in one part of the brain.
BLAIN: It was a very little brain area, a little spot of the left prefrontal cortex that's impacted during decision making.
HAMILTON: Lots of activity in that area seems to help an athlete decide to keep pushing. But when an athlete trains too hard, Blain says, the activity level stays low and their performance suffers.
Tanja Mueller is part of a research team at the University of Oxford that was not involved in the study. Mueller says her team also sees a link between physical exertion and decision making.
TANJA MUELLER: We find that people, as they have repeatedly exerted effort over time, they tend to be less willing to keep exerting effort for rewards, in particular high efforts.
HAMILTON: But Mueller isn't so sure the brain is simply choosing immediate gratification. She says when the body gets really tired, the brain can experience something called motivational fatigue.
MUELLER: People may not consider it worth it anymore to wait for higher rewards to be received.
HAMILTON: So they don't try as hard. Mueller says activity in the brain's left prefrontal cortex does seem to play a role in fatigue and decision making. But she says two other areas also seem to be important.
Another scientist who studies motivation is Todd Braver of Washington University in St. Louis. He says when an athlete pushes their body in a race, the brain is also working hard to answer lots of questions.
TODD BRAVER: How close am I to this goal? What is my physiological depletion versus, you know, the expected benefit I'm going to get for reaching the end of the race? Should I make a decision that it's time to quit?
HAMILTON: Braver says, to the brain, this last question is all about costs versus benefits. And as fatigue increases, he says, so do the costs until they get too high.
BRAVER: The brain might kind of have this built-in mechanism to say, hey, it's time to, you know, shift from this goal to another one.
HAMILTON: Like getting some rest. Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.