Central Brain Fatigue - Energy And Fatigue: Chronic Fatigue

Content provided by the Faculty of the Harvard Medical School

Central (brain) fatigue

Central fatigue is, to a great degree, a perception or state of mind. Your perception of fatigue usually increases with your muscle fatigue, but the two aren't always in sync. Experiments show that a person's perception of fatigue can decline even as the muscles are expending ever more energy and, in all likelihood, showing more signs of fatigue. But sometimes the opposite happens: Your perception of fatigue can be greater than your muscle fatigue. You can feel fatigued without having expended much energy at all. Emotional and psychological factors significantly affect how fatigued you feel.

Scientists have a method of measuring the perception of fatigue. It's called the rating of perceptual effort, or RPE. To use this method, an exercising person is asked to pick a number on a scale ranging from no fatigue to maximum imaginable fatigue. Scientists have found that, in general, the more effort you think you're expending, the more fatigued you will feel. Like the sensation of pain, the perception of fatigue is highly personal. It varies from one individual to the next, depending on personality as well as mood. Fatigue is greater and comes on sooner in people with mood disorders such as depression or anxiety than in those who don't have these illnesses.

An individual's perception of fatigue can change depending on the situation. In experiments, researchers have been able to manipulate people's RPEs during physically challenging activities. For example, giving a subject feedback on his or her performance decreases the perception of fatigue, possibly because it helps keep the person motivated to improve.

Maintaining mental focus on an activity is one way that coaches aim to improve athletes' performances. Creating a distraction tends to increase a person's perception of fatigue. When people are distracted, their minds wander. They may think about how tuckered out they should be or anticipate how drained they're likely to feel if they keep going. They may also think about easier, more pleasurable things they'd rather be doing and then, subconsciously, "feel" fatigued by their present activity.

Based on experiments with humans and animals, researchers suggest that certain chemical or electrical stimuli in the brain can alter perceptions of fatigue during exercise. However, none of these experiments have yet provided a "fatigue-preventing" treatment that can be used to improve performance.

In summary, central (brain) fatigue is a much less objective and measurable phenomenon than muscle fatigue. Although brain and muscles communicate, there can be a "disconnect" between the objective measurement of how hard the muscles are working and the brain's perception.

Last updated:	January 23, 2007