When you turn your palm down against resistance, the brachioradialis muscle fires more than when you turn your palm up.

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

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 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 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.

When your palm is facing down and you bend your elbow with a light weight, the muscle on the outside of your forearm works harder than when your palm is up.