When you curl with your arms stretched out on a bench, your forearm muscle has to work harder than your biceps.

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.

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

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 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.

When you bend your elbow by lifting something, the brachioradialis muscle works much harder pushing up than it does lowering the weight slowly.