system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it so hard to stand/keep your hands perfectly still?

I was trying to take some long exposure camera shots just standing up (yes I know its impractical) and it got me wondering why the human body can't just keep still. And then why do some people have "a surgeon's steady hand" while others can't hold a teacup on a plate without it rattling like a snake?
gpt
Ah, well... "holding still" is hard work, as you must have learned the hard way. Being still is actually an active physical process involving opposing muscle groups relaxing and contracting in opposition. If you stand up and try to hold your arms straight out (like a T) you're going to figure this out in a hurry. You're also having to balance and maintain that stillness, which takes minute adjustments of the muscles and of course, is yet more work. 

The "shaky hands" bit is hugely varied though, and can be a matter of practice, being calm, having strong and limber forearms and hands, intake of stimulants and the like... there is no one answer.