Tough mental tasks can cause people to tire more quickly at physical exercise, a new study suggests.

The study was small, however, with only 16 participants. It found that mental fatigue did not change how the heart or muscles performed, but rather the subjects' perceived effort seemed to determine when they reached exhaustion. More research is needed to figure out why, the scientists said.

The findings will be detailed next month in the Journal of Applied Physiology.

The study

The participants rode a stationary bicycle to exhaustion — when they could not maintain a cadence of at least 60 revolutions per minute for more than five seconds — under two conditions: once when they were mentally fatigued and once when they were mentally rested. The trials took place in the laboratory on different days. The participants got the same amount of sleep, drank the same amount and had the same meal before each of the sessions.

The mental fatigue sessions began with a challenging 90-minute mental task that required close attention, memory and quick reactions. After undergoing this session, participants reported being tired and lacking energy. The control session consisted of watching neutral documentaries for 90 minutes and was not mentally fatiguing.

Throughout both exercise sessions, the researchers tracked a variety of physiological measures, such as oxygen consumption, heart rate, cardiac output, blood pressure, ventilation, and blood lactate levels. The participants completed surveys to measure their motivation and perceived effort. The researchers offered monetary prizes for the best performance on the exercise and mental tasks as a way to keep motivation high.

The findings

The participants stopped exercising 15 percent earlier, on average, when they were mentally fatigued.

The participants stopped at the same perceived effort level in both the fatigued and non-fatigued trial. However, mentally fatigued participants started at a higher level of perceived effort and reached the endpoint sooner.

The cardio-respiratory and musculo-energetic measurements did not vary between the two trials when compared at specific points in time. However, because the non-fatigued trials went longer, heart rate and blood lactate levels were higher at the end of those trials.

Samuele M. Marcora, Walter Staiano and Victoria Manning of Bangor University in the UK did the study, which was funded by the university.

The researchers speculate that the perception of effort occurs in the brain. Marcora said his team is considering two possibilities: Mental fatigue lowers the brain's inhibition against quitting, or perhaps mental fatigue affects dopamine, a brain chemical known to play a role in motivation and effort.

Mental tasks activate the anterior cingulate cortex of the brain, the scientists note. Previous research has shown that rats with a lesion in the anterior cingulate cortex would not work as hard for a reward compared to rats with no lesion. This area of the brain may be where perception of effort originates, Marcora said.

The study could lead to improved understanding of people with chronic fatigue syndrome, who report they lack energy and experience "brain fog," as did the study participants when they were mentally fatigued. It could be useful to the military, where long periods of vigilance may produce mental fatigue. Finally, the researchers suggest, the study indicates that competitive athletes might want to train while mentally rested.