Sometimes people have extra muscle parts in their upper arm, and that can squeeze nerves or blood vessels or change how nerves are shaped — this might make surgeries or diagnosing arm problems trickier.

Knowing how the biceps muscle can vary from person to person helps doctors spot nerve or blood vessel problems and avoid mistakes during arm surgery.

Someone was found to have a rare extra-headed bicep muscle in just one arm — something never seen before — during a regular anatomy class dissection.

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.

Your biceps works more efficiently when your forearm is turned palm-down — it doesn’t need to work as hard to twist your hand — but the muscle that turns your palm down doesn’t get more efficient when stretched.

The more the biceps stretched during the first tough workout, the more strength the person lost the next day.