Here's a neat experiment: Find an open doorway and stand inside it, facing into the room. 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.
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. 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.
What will happen, though, if you suddenly take the resistance of that door frame away? Let's find out.
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.
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.