They found that when participants performed a mentally fatiguing task prior to a difficult exercise test, they reached exhaustion more quickly than when they did the same exercise when mentally rested.

During the study, the team including Samuele M Marcora, Walter Staiano and Victoria Manning also looked at the brain to find out why people with mental fatigue perceive exercise to be more difficult.

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.

The mental fatigue sessions began with a challenging 90-minute mental task that required close attention, memory, quick reaction and an ability to inhibit a response.

The study showed that participants stopped exercising 15 pct earlier, on average, when they were mentally fatigued.

The researchers speculate that the perception of effort occurs in the brain. Dr Marcora said there may be two possibilities.

One that mental fatigue lowers the brain’s inhibition against quitting and the other that mentally fatigue affects dopamine, a brain chemical that plays a role in motivation and effort.

The team hopes that their research could provide a way to study chronic fatigue syndrome. People with chronic fatigue report they lack energy and experience “brain fog,” just like the mentally fatigued participants in this study.

It can have important implications to military personnel. They do physically demanding tasks after long period of vigilance. Vigilance produces mental fatigue.