Your biceps works more efficiently when your forearm is turned palm-down — it doesn’t need to work as hard to twist your hand — but the muscle that turns your palm down doesn’t get more efficient when stretched.

Your bicep doesn’t work harder or softer when you turn your hand palm-up, palm-down, or straight ahead — it just keeps doing the same amount of work.

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 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 hardest when your palm is facing up and your elbow is halfway bent — and it works least when your palm is facing down.

When you bend your elbow and twist your forearm at the same time, your biceps work harder, but the muscles on the side of your arm work less — your body doesn’t just turn muscles on or off the same way every time.