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 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.

Your bicep gets stiffer as you bend your elbow — but only if your palm is up or sideways. If your palm is down, it gets looser the more you bend.

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.

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.

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.