Your bicep works best at different elbow angles depending on whether your palm is up, down, or straight — it’s strongest at 45° if your palm is down, and 60° if it’s up.

When you bend your elbow with a light weight, your bicep works harder if your palm is facing up than if it’s facing down or straight ahead.

When you bend your elbow with a light weight, your bicep works hardest when your palm is facing up and your elbow is halfway bent — and it works least when your palm is facing down.

When your palm is up or straight, your bicep gets stiffest when your elbow is bent at 60 degrees — but if your palm is down, it gets less stiff the more you bend your elbow.

When your palm is facing down and you bend your elbow with a light weight, the muscle on the outside of your forearm works harder than when your palm is up.

How stiff your bicep feels when you bend your elbow depends on both how bent your elbow is and which way your palm is facing — but how hard your brain tells your bicep to work doesn’t change as much with that combination.