(HealthDay News) -- Mental exhaustion may lead people to believe they are physical fatigued sooner than normal, a new study suggests.

A United Kingdom study found that performing mentally fatiguing tasks before a physical exercise caused people to hit the wall faster than if they performed the same exercises while having a rested mind, even though the mental tiredness didn't affect the performance of the subjects' heart or muscles.

The authors said their findings, published in the March issue of the Journal of Applied Physiology, may offer insight into the mysteries of chronic fatigue syndrome.

Researchers with Bangor University in Wales tested 16 people by having them ride a stationary bicycle until they were exhausted. When the riding followed a 90-minute mental challenge that required using memory, quick reaction and the ability to inhibit a response, the participants tired out about 15 percent earlier on average than those who watched documentaries for 90 minutes prior to the biking.

While physiological measures, such as oxygen consumption, heart rate and blood pressure, didn't appear different during the exercise between the two trials, mentally fatigued subjects were found to start at a higher level of perceived effort and reached their capacity for this sooner.

Researcher Samuele M. Marcora said the team has two theories about their results: Mental fatigue lowers the brain's inhibition to quit, or mental tiredness affects levels of dopamine, a brain chemical tied to motivation and effort.

Those who undertook the mental challenge before exercise reported a lack of energy and a "brain fog" often associated with people with chronic fatigue. People with chronic fatigue find exercise to be more difficult, despite normal physiological responses, just as those mentally exhausted people did in the study, Marcora said.

While the study suggests that people doing high-intensity exercise, such as competitive athletes, should train when mentally rested, the authors said people who exercise after work should continue doing so because of the physical and psychological benefit, such as stress relief and improved mental performance.

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The CFIDS Association of America has more about chronic fatigue.

--Kevin McKeever

SOURCE: American Physiological Society, news release, Feb. 24, 2009

Last Updated: March 13, 2009