A recent discovery that Noakes calls the “lactic acid paradox” made him start researching this area seriously.Lactic acid is a by­product of exercise, and the increase of it is often mentioned as a cause of tiredness.But when research subjects exercise

　　in certain conditions created artificially, they become tired even though lactic acid levels remain low.Nor has the oxygen content of their blood fallen too low for them to keep going.Obviously, something else was making them tire before they hit either of these physiological limits.

　　Noakes conducted an experiment with seven cyclists.It has long been known that during exercise, the body never uses 100% of the available muscle fibers(纤维)．The amount used varies, but in some tasks such as this cycling test the body calls on about 30%.His team found that as tiredness set in, the electrical activity in cyclist's legs declined—even when they were making a great effort to cycle as fast as they could.

　　To Noakes, this was strong evidence that the old theory was wrong.“The cyclists may have felt completely exhausted，” he says, “but their bodies actually had considerable reserves that they could theoretically tap by using a greater amount of the resting fibers.” This, he believes, is the proof that the brain is regulating the pace of the workout to hold the cyclists well back from the point of extreme tiredness.

　　文章大意：Noakes发现疲倦不是过度劳累的肌肉发出的信号，而是大脑发出的情感反应。

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