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...

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.

When lowering a barbell during a bicep curl, the biceps muscle activates more when the arms are hanging straight down than when the shoulders are bent forward, and this difference is larger with an EZ bar than a straight bar.

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.