Fatigue is in the mind, not the muscles, suggests a new study. But it can still have a serious impact on athletic performance. The finding could lead to treatments for conditions like chronic fatigue syndrome, or the development of illicit performance-enhancing drugs.

Traditionally, fatigue was viewed as the result of over-worked muscles ceasing to function properly. But evidence is mounting that our brains make us feel weary after exercise. The idea is that the brain steps in to prevent muscle damage.

Now Paula Robson-Ansley and her colleagues at the University of Cape Town in South Africa have demonstrated that a ubiquitous body signalling molecule called interleukin-6 plays a key role in telling the brain when to slow us down. Blood levels of IL-6 are 60 to 100 times higher than normal following prolonged exercise, and injecting healthy people with IL-6 makes them feel tired.

To work out if IL-6 affects performance, Robson-Ansley injected seven club-standard runners with either IL-6 or a placebo and recorded their times over 10 kilometres. A week later, the experiment was reversed.

On average they ran nearly a minute faster after receiving the placebo, a significant difference since their finishing times were around 41 minutes. The findings will appear in the Canadian Journal of Applied Physiology.

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