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Here's a neat experiment: Find an open doorway and standinside it, facing into the room.
做一個簡單的實驗：找一扇開著的門并站在門口，面向房間。
Now press your arms outward until the backs of your hands are pressing into the door frame.
接下來向外伸展手臂直到手背是被壓入門框。
Go ahead and try it. Keep pressing outward, as hard as you feel comfortable with, for the nextminute or so.
試一試吧，努力向外伸展手臂，接下來的幾分鐘你將會感到很舒服。
In the meantime, let's talk about what your muscles are doing.
一起來討論一下這期間你的肌肉發揮了什么樣的作用。
In order to maintain the rightamount of stiffness, your muscles need to know how much resistance they're up against.
人體肌肉需要了解他們所承受的阻力是多少以便能夠保持適當的剛度。
To check this, your muscles have sensory receptors inside the muscle tissue itself which tell your nervous system just how stiff or relaxed these muscles are.
由此，肌肉組織內部有感覺感受器，通過這些感受器向神經系統傳達肌肉的僵硬度或者是松弛度。
For example, muscle spindles are thin,fluid filled sacks that measure your muscles' stretch.
例如肌梭是體內感覺肌肉伸展的細長梭形本體感受器，
Receptors in the tendons measure your muscles' contraction.
肌腱中的感受器感覺肌肉的收縮。
With this information feeding back from the muscle, your body can figureout how tight the muscle needs to be in order to keep it in the same place.
有了這些從肌肉中反饋的信息，本體才能計算出為了使肌肉保持在相同位置需要多大的松緊度。
Which, right now asyou press the doorway, is pretty tight.
在按住門口時，肌肉繃得很緊。
What will happen, though, if you suddenly take the resistance of that door frame away? Let's findout.
如果突然移除門框的阻力會發生怎樣的事情？一起來尋找答案。
Step into the room again and completely relax your arms.
走進房間，徹底放松自己的手臂。
If you've been pressing hard enough,you'll be surprised to find your arms now drifting effortlessly upward, as though you had heliumballoons tied to your wrists.
如果您之前壓得足夠緊，您會驚訝的發現您可以毫不費力的向上抬起手臂，仿佛有一只氦氣球綁在您的手腕上。
What's going on is this: While you were pressing, your muscles learned that they needed to bepretty stiff to stay in the same place.
一切緣由如下：在您用力壓的時候，肌肉感知到需要崩得很緊才能處于同樣的位置，
Stepping out of the doorway changed all that-and very suddenly.
而走出門口之后一切在瞬間改變，
It takes your muscles a few moments to learn about this new situation.
肌肉感受新情形需要花費一些時間，
Until that happens, your arms will continue to press outward against a door frame that's no longer there.
在這期間，手臂仍然保持在一種向外繼續按壓已經不存在的門框的狀態。
