【RunningonEmptyForalmostacentury，scientistshaveassumed，tiredness-orexhaustion-inathletesoriginatesinthemuscles（肌肉）．Preciseexplanationshavevaried，butallhavebeenbasedonthe"LimitationsTheory"．Inotherwo】

　　Running on Empty

　　For almost a century，scientists have assumed，tiredness-or exhaustion-in athletes originates in the muscles（肌肉）．Precise explanations have varied，but all have been based on the"Limitations Theory"．In other words，muscles tire because they hit a physical limit； they either run out of fuel or oxygen or they drown in harmful by-products （副产品）．

　　In the past few years，however，Timothy Noakes from the University of Cape Town． South Africa，has examined this standard theory．Tiredness，he argues，is caused not by signals springing from overtaxed muscles，but is an emotional response which begins in the brain．The fundamental nature of his new theory is that the brain paces the muscles to keep them well back from the edge of exhaustion．When the brain decides it's time to quit，it creates unbearable muscle tiredness．This"Central Governor"theory remains controversial，but it does-explain many puzzling aspects of athletic performance．

　　A recent discovery that Noakes calls the"lactic acid paradox"made him start researching this area seriously．Lactic acid is a by-product 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 fibres （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 e