做一个简单的实验：找一扇开着的门并站在门口，面向房间。
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.
接下来向外伸展手臂直到手背是被压入门框。试一试吧，努力向外伸展手臂，接下来的几分钟你将会感到很舒服。
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.
一起来讨论一下这期间你的肌肉发挥了什么样的作用。人体肌肉需要了解他们所承受的阻力是多少以便能够保持适当的刚度。由此，肌肉组织内部有感觉感受器，通过这些感受器向神经系统传达肌肉的僵硬度或者是松弛度。例如肌梭是体内感觉肌肉伸展的细长梭形本体感受器，肌腱中的感受器感觉肌肉的收缩。有了这些从肌肉中反馈的信息，本体才能计算出为了使肌肉保持在相同位置需要多大的松紧度。在按住门口时，肌肉绷得很紧。
What will happen, though, if you suddenly take the resistance of that door frame away? Let’s find out.
如果突然移除门框的阻力会发生怎样的事情?一起来寻找答案。
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.
走进房间，彻底放松自己的手臂。如果您之前压得足够紧，您会惊讶的发现您可以毫不费力的向上抬起手臂，仿佛有一只氦气球绑在您的手腕上。
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.
一切缘由如下：在您用力压的时候，肌肉感知到需要崩得很紧才能处于同样的位置，而走出门口之后一切在瞬间改变，肌肉感受新情形需要花费一些时间，在这期间，手臂仍然保持在一种向外继续按压已经不存在的门框的状态。
