Does the brain play a role in how hard we can exercise? Until recently, the New York Times reports, many researchers would have said no—muscles tire because of biochemical reactions, like getting too little oxygen, or too much lactic acid or calcium. But the brain does seem to be implicated in muscle fatigue. “We know that people speed up at the end of exercise,” Ross Tucker, a researcher with the Sports Science Institute of South Africa, told the newspaper. If biochemical changes in the muscles “caused muscle failure, this would be impossible at the end, when these changes are at their greatest levels.” The brain, it seems, tracks fuel in the muscles and the body’s core temperature. As the amount of fuel drops and the temperature rises, the brain realizes a “danger zone” is approaching, and sends fewer signals to the muscles to contract, so they become more feeble. Researchers in England have shown that rinsing your mouth with a sports drink can mitigate fatigue. In the study, eight cyclists completed a difficult time trial on stationary bikes, while their heart rate and power output was measured. They swished liquids in their mouths, some of which contained carbohydrate, a fuel; others were just flavoured water. By the end, cyclists who rinsed with carbohydrate drinks (and spit them out) were significantly faster. Using an MRI, researchers found areas of the brain associated with motivation were aroused when the carbohydrate drink was swished, leading them to assume that group of riders sensed they were getting more caloric fuel, so the brain instructed the muscles to work harder.