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

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.

Even when you're not trying to bend your elbow, the muscle on the outside of your forearm feels stiffer if your palm is facing up than if it's facing down or straight ahead.

When your palm is facing down and you bend your elbow slightly, your forearm muscle (brachioradialis) works harder than when your palm is up — it’s like your body uses this muscle more to help when your hand is in a 'handshake' position.

The brachioradialis is preferentially activated during elbow flexion when the forearm is in a neutral (hammer) grip position compared to supinated (palms-up) grip.