When participants performed a mentally fatiguing task prior to a difficult exercise test, they reached exhaustion more quickly than when doing the same exercise when mentally rested.
This is the major finding of a study by Samuele M. Marcora, Walter Staiano and Victoria Manning of Bangor University, funded by the University’s School of Sport, Health and Exercise Science and published in the Journal of Applied Physiology by The American Physiological Society.
The study also found that mental fatigue did not actually cause the heart or muscles to perform any differently but instead, it is a “perceived effort” that determines when we reach exhaustion. It also indicated that motivation did not decrease as a result of mental fatigue and was not a factor.
Under the study the 16 participants rode a stationary bicycle to exhaustion under two conditions: once when they were mentally fatigued and once when they were mentally rested, with the trials taking place in the laboratory on different days.
It was found that those participants who were mentally fatigued stopped exercising 15% earlier, on average than the non-fatigued. Also, when exercising to the same level of effort, the mentally fatigued started from a higher initial perceived level of effort and reached the endpoint sooner.
Although the cardio-respiratory and musculo-energetic measurements did not vary between the two trials when compared at specific points in time, because the non-fatigued trials went longer, their heart rate and blood lactate levels were higher at the end of those trials.
The researchers speculate that the perception of effort occurs in the brain. Dr. Marcora said his team is considering two possibilities. Firstly that mental fatigue lowers the brain’s inhibition against quitting and secondly that mental fatigue affects dopamine, a brain chemical that plays a role in motivation and effort
This research could provide a way to study chronic fatigue syndrome and may also be helpful for mentally fatigued military personnel who undertake physically demanding tasks after long period of vigilance.
Finally, the study suggests that people doing high intensity training, such as competitive athletes, should do their training while mentally rested.
However, people who exercise after work should continue doing so, even if mentally fatigued. Most people work out at a moderate intensity, which still gives plenty of physiological and psychological benefit, including relief from stress and improved mental performance.
This study provides experimental evidence that the brain can limit high-intensity endurance performance and the researchers said the next step is to look at the brain to find out exactly why.