When your palm is down, your bicep can’t pull as well because its tendon gets twisted — so your body makes your other forearm muscle work harder to bend your elbow.

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.

Your body switches which forearm muscle does most of the work when bending your elbow — your bicep stays steady, but your other muscle steps up when your palm is down.

When your palm faces down and you bend your elbow, your forearm muscle (brachioradialis) works harder than when your palm faces up or straight ahead — but your bicep works the same no matter how your hand is turned.

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.

No matter how you turn your hand while bending your elbow, the brachioradialis muscle works about the same amount.