The bicep is one muscle with two parts. Its full anatomical name, biceps brachii, literally means “two-headed muscle of the arm.” Those two heads, the long head and the short head, start at different points on the shoulder blade but merge into a single muscle belly before crossing the elbow and attaching to the forearm bone.
The Two Heads of the Biceps
The long head sits on the outer side of your upper arm. It originates from a small bump at the top of your shoulder socket on the shoulder blade. The short head sits on the inner side and originates from a bony projection on the front of the shoulder blade called the coracoid process.
Despite starting in two different places, both heads come together about halfway down the upper arm, forming the single bulge you can see and feel when you flex. From there, the combined muscle tapers into a tendon that crosses the elbow and attaches to the radius, the forearm bone on the thumb side. A thin sheet of connective tissue also fans out from that tendon into the fascia of the forearm, which is why you can sometimes feel tension in your forearm during heavy biceps exercises.
What the Biceps Actually Does
Most people think of the biceps as the muscle that bends the elbow, and it does. But its strongest action is actually rotating the forearm so your palm faces upward, a movement called supination. Think of turning a screwdriver or opening a jar. Research on distal biceps tendon mechanics shows that the tendon’s angle of attachment on the radius is critical for generating that rotational force, which is why injuries to the lower biceps tendon make twisting motions feel noticeably weaker long before you struggle with curling movements.
The biceps also plays a secondary role at the shoulder, helping to bring the arm forward and stabilize the joint during overhead activity. This is why biceps tendon problems can show up as shoulder pain rather than elbow pain.
Some People Have a Third Head
While two heads is the standard, a notable percentage of people have a third head of the biceps. A cadaver study examining 46 limbs found a third head in about 13% of cases overall, with a striking sex difference: roughly 21% of female specimens had a third head compared to about 4.5% of males. This extra head typically originates lower on the upper arm bone and merges into the same common tendon. It rarely causes problems and most people who have one never know it. In some cases, though, it can be confused with a soft tissue mass on imaging, so radiologists and surgeons are trained to recognize the variant.
Other Muscles in the Same Area
The biceps shares the front of the upper arm with two other muscles that contribute to elbow bending, and people sometimes lump them all together when they say “bicep.”
- Brachialis: This muscle sits directly underneath the biceps, between it and the bone. It’s actually the primary elbow flexor, generating more bending force than the biceps itself, especially when your palm faces downward. You can’t see it in the mirror, but it adds thickness to the upper arm.
- Coracobrachialis: A smaller, deeper muscle that shares its origin with the short head of the biceps on the coracoid process. Its main job is pulling the arm forward and inward toward the body. It plays almost no role in bending the elbow.
So while the biceps brachii is a single two-headed muscle, the front of your upper arm contains three distinct muscles working together. If you’ve ever wondered why hammer curls (with a neutral grip) hit differently than standard curls (with palms up), it’s because changing your grip shifts the workload from the biceps toward the brachialis underneath it.