After three weeks, all athletes were put in a brain scanner. They were asked questions designed to determine a preference for immediate gratification or long-term reward.
"For example, we ask, 'Do you prefer $10 now or $60 in six months,' " Blain told NPR.
The athletes who overtrained “were, in fact, choosing more immediate gratification than the other group of athletes," Blain said.
Not only that, MRIs of the athletes’ brains showed more fatigue in the part of the brain responsible for cognitive control.
"Cognitive control in this situation is the capacity to maintain exercise despite things like muscle pain," Blain told CNN. "And what we found is there is an intellectual component involved in exercising and it has a finite capacity. You cannot use it forever."
Essentially, your brain will burn out and affect your ability to exercise. When there is a lot of activity in the brain’s cognitive control area, Blain told NPR, “athletes are able to ignore signals from screaming muscles and focus on winning.” But overtraining can fatigue that part of the brain, he said, and a person is less able to push their body.
Because this study focused only on elite athletes, the "brain fatigue" conclusions can't be applied to the average person without more research, the group said.