Here’s a neat experiment: Find an open doorway and stand inside 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. Go ahead and try it. Keep pressing outward, as hard as you feel comfortable with, for the next minute or so.
　　接下來向外伸展手臂直到手背是被壓入門框。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. In order to maintain the right amount of stiffness, your muscles need to know how much resistance they’re up against. 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. For example, muscle spindles are thin, fluid filled sacks that measure your muscles’ stretch. Receptors in the tendons measure your muscles’ contraction. With this information feeding back from the muscle, your body can figure out how tight the muscle needs to be in order to keep it in the same place. Which, right now as you press the doorway, is pretty tight.
　　一起來討論一下這期間你的肌肉發(fā)揮了什么樣的作用。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(dāng)?shù)膭偠取Ｓ纱?，肌肉組織內(nèi)部有感覺感受器，通過這些感受器向神經(jīng)系統(tǒng)傳達肌肉的僵硬度或者是松弛度。例如肌梭是體內(nèi)感覺肌肉伸展的細長梭形本體感受器，肌腱中的感受器感覺肌肉的收縮。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。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 find out.
　　如果突然移除門框的阻力會發(fā)生怎樣的事情?一起來尋找答案。

　　Step into the room again and completely relax your arms. If you’ve been pressing hard enough, you’ll be surprised to find your arms now drifting effortlessly upward, as though you had helium balloons tied to your wrists.
　　走進房間，徹底放松自己的手臂。如果您之前壓得足夠緊，您會驚訝的發(fā)現(xiàn)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 be pretty stiff to stay in the same place. Stepping out of the doorway changed all that–and very suddenly. It takes your muscles a few moments to learn about this new situation. Until that happens, your arms will continue to press outward against a door frame that’s no longer there.
　　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，而走出門口之后一切在瞬間改變，肌肉感受新情形需要花費一些時間，在這期間，手臂仍然保持在一種向外繼續(xù)按壓已經(jīng)不存在的門框的狀態(tài)。