system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it so hard (or impossible) to keep our hands steady?

I mean, I don't have particularly shaky hands, but if I shine a laser pointer across the room, the dot isn't going to be stationary.

Somewhat related (or maybe not), why is it so hard to control our bodies in general? For example: typos, mistakes when playing instruments, messing up inputs in games, etc. Would improved proprioception reduce error?
gpt
When it comes to keeping limbs steady, the issue is due to the nervous system. You see, to keep your hand steady, you need to use muscular force to keep the hand up. The muscle flexes because the brain sends impulses down the nerves which excite the muscles and the muscles contract. This is a discrete phenomenon, muscle fibers contract many times a second. The stronger the force they need to provide, the more they vibrate. That's why it's hard to keep a steady hand with your arm all out in front or to the side but pretty easy when you just let it hang on your side.

You'll notice that as your arm gets progressively more tired, your hand starts to shake more and more. Your muscle fibers are basically switching between resting and flexing. The transition between the two is not always smooth but you can get better at it with practice and improved endurance.