Grammar correction: Your sentence 'What muscles bend and unbend arms?' is understandable, but a clearer way to say it is: 'Which muscles bend and straighten the arm?'
Short answer: The biceps bend the arm at the elbow. The triceps straighten the arm at the elbow.
Where they are: The biceps are on the front of your upper arm (the side your palm faces). The triceps are on the back of your upper arm.
How to feel them (try this):
- Make a fist and pull your hand toward your shoulder (bend your elbow). Feel the muscle that gets bigger on the front of your arm — that’s the biceps.
- Straighten your arm. Feel the muscle on the back of your arm tighten — that’s the triceps.
Pronunciation: Biceps = 'BYE-seps'. Triceps = 'TRY-seps'.
Extra note: Other muscles help a little, but the biceps and triceps do most of the bending and straightening. Be gentle — don’t try to lift very heavy things when you test your muscles.