Mental Fatigue

Researchers are finding that loss of mental drive may limit an athlete’s endurance.

Trained marathon runners can run the 42km distance in just over two hours; other people just cannot make the distance even at a very slow jogging pace.

Our understanding of endurance is unclear. What does determine that point of utter exhaustion when one can literally go no further? Is it simply a matter of physical fitness or does the brain play a part?

Can brain chemistry tip the balance against the workings of the heart, lungs, and muscles during prolonged exercise? Recent research suggests that the brain may be a decisive factor in fatigue.

Endurance sports include golf and jogging, marathon running, long-distance swimming, and round-the-world cycling. All of these activities require some degree of fitness.

This comes from training which increases the ability of the heart, lungs, and muscles to sustain the human-machine.

Within the first few months of training, the heart and lungs become more efficient so that more oxygen (and nutrients) can be delivered to the muscles to burn up fuel and provide energy for exercise. The muscles also begin to adapt in such a way that they can use different fuels more proficiently to supply energy.

So any sort of endurance exercise was presumed to depend on the physical limits of the body, and it was presumed that these limits could be extended by training. The logistics of fatigue were quite simple.

However, sports physiologists have recently begun to question whether there might be a component of mental fatigue — that loss of drive, that feel ing of simply not being able to sustain another minute of exertion even though the body may not be at the limits of physical endurance.

Now it is beginning to look as though there is a good case for the existence of central fatigue driven by the brain. It concerns a chemical, serotonin, which can relay messages from one part of the brain to another.

For many years this neuro-transmitter has been associated with controlling the sleep-wake cycle and the perception of pain. More recently it has been linked with the sensation of fatigue as well.

Ron Maughan, working at Aberdeen University in Scotland, has shown that drugs that can enhance the effects of serotonin in the brain reduce the body’s capacity for prolonged exhaustive exercise. In other words, central fatigue may be precipitated by an increased activity of serotonin.

In a more recent study, Phil Jakeman and his colleagues at Birmingham University in England showed that endurance-trained athletes become less sensitive to the central effects of serotonin compared with people who are untrained and unused to any regular form of exercise.

They concluded that endurance training produces a kind of tolerance or desensitization to the brain chemical and this may help athletes to maintain longer periods of exercise before that point of utter fatigue is reached.

There is an interesting parallel to the idea that brain serotonin can cause central fatigue. Dr. Peter Behan and his colleagues have found that people with post-viral fatigue syndrome seem to be particularly sensitive to this neurotransmitter and it may well be that a viral infection or some other unspecified trigger causes a sort of up-regulation of a brain mechanism which uses serotonin. So people with this syndrome suffer from continual fatigue even without exercise or any physical exertion.

Ron Maughan takes the concept of central fatigue one step further. “It may explain why some people find the whole idea of exercise complete anathema, while for others regular exercise in one form or other is compulsive,” he says. “People’s capacity or need for exercise varies enormously.”

“Perhaps people who have no desire for exercise are particularly sensitive to serotonin and so have a low threshold for fatigue and a low drive. At the other end of the scale are those people who need lots of exercise in their lives. They need the challenge, the release, the buzz, or whatever. Are they less sensitive to serotonin than their sedentary counterparts? We would also like to know why some men and women choose to take up heavy endurance types of sports.”

It is no new concept that exercise and the brain are linked. Athletes show a variety of hormonal abnormalities that are controlled by the brain. One of the classic syndromes is the loss of menstruation in female athletes. The hormonal signal from the brain, which in turn stimulates the hormones of the pituitary gland to control the cyclical activity of the ovaries, simply shuts down. As a consequence these female athletes stop menstruating.

The same is seen with the brain signals which control the release of growth hormone from the pituitary gland when blood-sugar levels become low. Again, this response is shut down in the athlete.

Phil Jakeman says, “Perhaps the existence of central fatigue is a fail-safe system, a way of the body communicating with the brain, inducing feelings of fatigue and thus limiting exercise to some extent before any damaging effects are reached.”

“After all, the amount of food we eat is regulated by the brain sensing the levels of nutrients in our blood. In the same way, the brain may respond to levels of nutrients in the blood as they change in response to the needs of the exercising muscle.”

More than 20 years ago, Richard Wurtman showed that brain serotonin could be increased by raising blood concentrations of the amino acid tryptophan. Brain serotonin is made from tryptophan. He found that through careful dietary manipulation the concentrations of both tryptophan and serotonin in the brain could be increased.

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