英语翻译For almost a century,scientists have assumed,tiredness-o

英语翻译
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…

在近世纪以来,科学家们一直以为运动员的疲惫或力竭是源于肌肉.明确的解释各异,但全是基于「极限理论」.换句话说,肌肉的疲惫是因为它们已达物理极限：要么它们缺燃料或氧气,或者已被身体分解出来有害副产品淹没了.
但是,在过去的几年中,南非开普敦大学的教授蒂莫西• 诺克斯（Timothy Noakes）,再三审查这一标准理论.他争辩说,疲惫并非由肌肉所传递的负荷过重信号所引起的,而是来自脑部的一种情绪反应.他的新理论的基础论调是大脑会在肌肉面临力竭时,不让身体开启肌肉能量.当大脑决定该退下来之时,它会制造些难以忍受的肌肉惫.这「中央指挥」理论仍存在著争议,但它确实解释了许多令人费解的运动性能的表现.
最近,诺克斯从他的「乳酸悖论」中有新发现,使他开始认真研究这领域.乳酸是一种经由运动分解出来的副产品,如它堆积起来,常被说成是构成疲惫的原因之一.但是,当研究对象在某些特定的人为条件下锻炼,即使乳酸水平仍然很低,他们也变得疲惫.
也不是他们的血液中的氧含量降至过低水平,以致阻碍他们持续下去.显然地,有别的因素导致他们在达生理极限之前变得疲惫...