Why Do Your Muscles Get Tired?When your lungs are locked in a desperate struggle for oxygen, your mind tormented by pain, and your skin blanketed with sweat, the mechanisms of muscle fatigue are probably the last thing on your mind. Scientists and athletes have always thought that your muscles tire because they reach some kind of physical limit. Either they run out of fuel, or they drown in toxic by-products. In the past few years, researchers Tim Noakes and Alan St Clair Gibson have begun to question the standard theory. And they're convinced that fatigue simply isn't the same as a car running out of petrol. Fatigue, they argue, is an emotional response that begins in your brain. The Central Governor theoryThe essence of The Central Governor Theory is that your brain paces your muscles to keep them back from the brink of exhaustion. When the brain decides it's time to quit, it creates the distressing sensations you interpret as muscle fatigue. The theory remains controversial. But it might help to explain why interval training, a training technique where repeated bouts of high-intensity exercise are separated by recovery periods, is so effective. In one study, researchers took a group of cyclists and assigned them to a four-week interval training program [3]. Despite the fact they completed only six interval sessions, the cyclists were able to shave an average of two minutes off their 40-kilometer time trial performance (54.4 versus 56.4 minutes). According to conventional wisdom, this improvement is down to changes in the muscles that make them better at using oxygen or more able to fight fatigue. But Noakes believes that interval training works largely by teaching the central governor that going faster won't do your body any harm. In one intriguing study, Noakes and St Clair Gibson recruited seven experienced cyclists and asked them to complete two 100-kilometer time trials on exercise bikes [2]. On several occasions during the trial, the cyclists were asked to sprint for 1000 or 4000 meters. Electrical sensors taped to their legs were used to measure nerve impulses traveling to their muscles. During exercise, your body never uses all of the available muscle fibers in a single contraction. Instead, it spreads the load by recruiting fresh fibers as needed. If fatigue was due to muscle fibers hitting some kind of limit, the number of fibers used during each pedal stroke should increase as the fibers tire and the body attempts to compensate by recruiting a larger fraction of the total. But Noakes and his team found exactly the opposite.As fatigue set in, electrical activity in the cyclists' legs dropped — even during the sprints, when they were trying to cycle as fast as they could. To Noakes, this was strong evidence that the old theory was wrong. The cyclists may have felt as though they'd reached their physical limit. But there were actually considerable reserves they could theoretically tap into by using a greater fraction of the resting fibers. More evidence for The Central Governor Theory comes from the fact that fatigued muscles don't actually run out of anything critical. For example, when researchers look at a slice of muscle tissue under a microscope, they can see that carbohydrate stores decline with exercise. Carbohydrate is stored in the form of glycogen (pronounced gly-ka-jun) in your liver and muscles. Glycogen molecules are linked together like a chain of sausages. They can range in size from a few hundred to several thousand glucose molecules. However, while glycogen levels might approach zero, they never quite get there. The theory may also explain a few puzzling aspects of athletic performance.Lactic acid is a by-product of exercise and its build-up is often cited as a cause of fatigue. But when subjects exercise in conditions designed to simulate high altitude, they become fatigued — despite the fact that lactic acid levels remain low [1]. It appears that something else was making them tire well before they hit a physical limit. The Central Governor Theory doesn't mean that what's happening in the muscles is irrelevant. Instead, the governor constantly monitors signals from the muscles, along with other information, to set the level of fatigue. In other words, physiological factors (such as the level of glucose and oxygen in the blood) are not the direct cause of fatigue. Rather, they are signals the governor takes into account. The Central Governor Theory is just that — a theory.But when you think about it, there's a good reason for your body to keep something back. It means there's always something left in case of an emergency. To your Stone Age ancestors, an emergency might take the form of a lion or pack of wolves at the end of long hunt. Today, the "lion" might be a mugger hiding in an alley, or a lightning storm near the end of a long walk. But the same concept applies — life would be too dangerous if your body allowed you to become so tired that it was impossible to respond quickly to an unexpected threat. "The mind," wrote Arnold Schwarzenegger "always poops out before the body." Whether Arnold himself actually said it, or his ghostwriter Bill Dobbins, doesn't really matter. It turns out he might have been right.
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