To induce mental fatigue under controlled conditions, the research team recruited 18 participants and had them complete a one-hour cognitive task, followed by exercise on a stationary bike.
Each participant completed the protocol twice. In one condition, the cognitive task consisted of watching a documentary. In the other, it was an extended version of the Stroop test, in which the word for a colour is displayed in a font of a different colour and the subject must name the font colour. The brain must suppress an automatic response, which requires inhibitory control and cognitive flexibility.
After the cognitive task, participants pedalled at 80 per cent of their maximal aerobic power until they had to or chose to stop.
After the Stroop test, participants were able to cycle for an average of 887 seconds, compared with 999 seconds after watching the documentary—about 112 seconds less. For fifteen of the 18 participants, performance was poorer after the cognitive task.
Electromyography—a technique that records electrical muscle activity—was used to measure the activity of 10 leg muscles, including the quadriceps, hamstrings, glutes and calves, and no significant difference in muscle activation was detected between the two conditions.
Same effort, different perception
“It wasn’t because their muscles had less energy,” said Pageaux. “Rather, we must turn to a central concept in endurance research: perceived effort.”
Perceived effort, Pageaux explained, refers to consciousness of the difficulty and intensity of a physical activity. It isn’t the same as pain, although the two sensations can coexist during intense physical activity.
“Imagine two runs at the same speed,” Pageaux said. “On a day when you’re well-rested, the pace feels relatively leisurely. On another day, after a bad night’s sleep or a mentally taxing day, the exact same pace may feel much more demanding. You will adjust your effort based on how you feel. The more difficult the effort is perceived to be, the more quickly you will reach the limit of the exertion you are willing to expend.”
So a person will stop when the perceived effort becomes too great—and participants reported feeling more mentally fatigued after the Stroop test than after watching the documentary.
Only long-haul effects
Previous research by the team showed that mental fatigue does not necessarily affect all types of physical performance in the same way.
When a weightlifter lifts a heavy load, a high jumper clears the bar, a rugby player pushes in a scrum or a sprinter crosses the finish line, mental fatigue caused by prolonged cognitive effort was not found to have any effect. Mental fatigue did not appear to reduce a muscle group’s ability to produce maximum force, power or speed during anaerobic exercise.
On the other hand, tasks that require sustained effort over time appear to be more vulnerable. “The reason has to do with the nature of endurance: you have to constantly regulate your effort, decide whether to continue, slow down or speed up, and deal with the sensation of difficulty,” said Pageaux.
Mental fatigue can also impair psychomotor skills, such as precise movement, and decision-making. In a sport that requires sustained effort and fast decisions, these effects can compound.
Should you avoid thinking too hard before exercising?
Not necessarily. According to Pageaux, the takeaway for athletes isn’t to cut out all intellectual activity before training but to rest their minds before competition.
“Before a competition in which an athlete needs maximum endurance, it might be helpful to avoid significant cognitive overload just before the exertion,” Pageaux said. “On the other hand, during training, deliberately inducing some mental fatigue could be beneficial.”
That is the principle behind “brain endurance training,” which gradually exposes athletes to cognitive demands before or during physical exercise to develop their ability to tolerate mental fatigue.
“The primary determinant of performance is, of course, the athlete’s physical ability,” Pageaux concluded. “But mental preparation and training to withstand cognitive fatigue could provide an added edge.”
