Being mentally exhausted can impair a person's exercise performance, a finding that may help explain why it is sometimes so hard to work out, British researchers said on Tuesday.

They said people who did a mentally tiresome task just before exercising reached exhaustion much more quickly than when they had been mentally rested.

Mental fatigue did not affect the performance of the heart or muscles, but it did affect their "perceived effort," Samuele Marcora, Walter Staiano and Victoria Manning of Bangor University in Wales wrote in the Journal of Applied Physiology.

"Our study provides experimental evidence that mental fatigue limits exercise tolerance in humans through higher perception of effort," the team wrote.

For the study, the researchers had 16 people take a demanding, 90-minute test that required close attention, memory, and left participants feeling tired and listless. Next, they rode a stationary bicycle to exhaustion, while the researchers tracked their heart rate and other vital signs.

On a different day, the same group simply watched a 90-minute documentary film before riding the bike.

The researchers found participants stopped exercising 15 percent earlier on average when they were mentally exhausted, even though there was little difference in their cardio respiratory or muscle function.

"It provides strong evidence that brain function can limit short-term endurance performance," the team wrote.

The researchers said the next step is to look at the brain to find out exactly why people with mental fatigue perceive exercise to be more difficult.