Fatigue and Rest
Scientists have made an interesting experiment about fatigue.
When a muscle in the body works, it produces lactic acid,and it is this lactic that makes the muscle tired.
Fatigue is not just a chemical process, it is also a biologicai process. We can`t just "remove" fatigue; we must allow the cells of the body to rest. Damages must be repaired, nerve cells of the brain must be "recharged," the joints of the body must replace used-up lubricants. Sleep will always be necessary as a way of restoring the body`s energy after fatigue.
However, a person who has been working hard at his desk for hours may not want to lie down at all when he is tired. He`d rather take a walk! Or when children come home from school they want to run out to play... not to lie down and rest.
The reason is that if only a certain part of the body is tried the best way to make that part feel fresh again is to make other parts of the body active! We can actually rest by means of activity. Of course, if you are totally exhausted, the best thing to do is to go to sleep!
[点评]
人人都懂得劳逸结合的道理。此文正是说明了人为什么会疲劳，为什么要休息的问题。文中有些比喻恰当、生动，说理性强。
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疲劳与休息
关于疲劳，科学家们做了一项有趣的实验。
人身体上的肌肉工作的时候，就会产生出乳酸，而就是这种乳酸使肌肉感到疲劳。
疲劳不仅是一种化学过程，也是一种生物过程。我们不能只是把疲劳“拿掉”；我们必须让体内细胞得到休息。损坏的部分要得到修复，大脑的神经细胞必须要“充电”，关节部位要更换润滑剂。疲劳之后，睡眠，作为恢复体能的方法总是必不可少的。
然而，有人伏案工作数小时之后，感到疲劳的时候，或许根本不想躺下休息，而更愿意散散步。还有，孩子们放学回家后就想跑出去玩……却不愿躺下来休息。
其原因是，身体的某一部分感到乏困时，使这一部分恢复活力最好的办法是让其他部分活跃起来!我们实际上通过活动也能得到休息。当然，你若浑身上下疲惫不堪，最好还是去睡觉!