system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can certain muscles in human bodies (like in our arms, legs, etc.) be built-up through workouts while others (like our fingers, jaw, etc.) remain the same size despite working out almost constantly?
gpt
A lot of misinformation in here. Fast twitch / slow twitch has nothing to do with this these particular examples. Every muscle can be worked out and every muscle will grow if worked out more.

You say those muscles stay the same despite being used a lot. Your muscles are the size they need to be for their usage.

The muscles which are moving your fingers are in your forearm. If you do grip exercises they will definitely grow in size and you will get bigger forearms. (The muscles that are actually in your hand are mostly for moving your fingers laterally and are very small in size but even they can be worked out)

If you use or exercise your jaw beyond normal eating then those muscles will definitely grow larger as well.