The two muscles on the side of your arm tend to turn on and off together, but your biceps often turns on when they turn off — like they’re taking turns instead of working as a team.

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.

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.

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.

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.