The bicep, properly called the biceps brachii, is the large muscle on the front of your upper arm between the shoulder and elbow. Its name literally means “two-headed muscle of the arm,” referring to the two separate muscle sections that merge into one. It’s the muscle you see bulge when you bend your elbow and flex, and it plays a central role in pulling, lifting, and rotating your forearm.
How the Biceps Is Built
The biceps has two distinct portions called the long head and the short head. The long head starts at a small bony bump on the top of the shoulder blade, just above the shoulder socket. The short head starts at a hook-shaped projection on the shoulder blade called the coracoid process. Both heads run down the front of the upper arm and join together into a single tendon that attaches to the radius, the forearm bone on the thumb side.
This two-headed design is what gives the biceps its characteristic rounded shape when contracted. The long head sits on the outer part of the arm, while the short head sits on the inner part. Together they form the visible “peak” most people associate with arm strength.
Underneath the biceps sits another muscle called the brachialis. Despite being less visible, the brachialis is actually the most powerful elbow flexor. It attaches to the ulna (the other forearm bone) rather than the radius, so it bends the elbow without rotating the forearm. The biceps gets the credit for arm strength, but these two muscles work as a team.
What the Biceps Actually Does
The biceps performs two primary movements. The first and most obvious is elbow flexion: bending your arm to bring your hand toward your shoulder. The second is forearm supination, which is the twisting motion that turns your palm face-up. Think of the motion you use to turn a screwdriver or open a jar. That rotation is driven largely by the biceps.
The muscle also plays a supporting role at the shoulder, helping to lift the arm forward and stabilize the shoulder joint during overhead movements. It shares elbow-bending duties with the brachialis and another forearm muscle called the brachioradialis, and it shares shoulder work with the front portion of the deltoid.
About 63% of the fibers in the biceps are fast-twitch, the type that generate quick, powerful contractions but fatigue faster. The remaining fibers are slow-twitch, better suited for sustained effort. This composition helps explain why the biceps responds well to both heavy, low-rep exercises and lighter, higher-rep training.
Nerve Supply and Reflexes
The biceps receives its signals from a nerve called the musculocutaneous nerve, which branches off the network of nerves in the neck and shoulder area. Specifically, the motor fibers controlling the biceps originate from the fifth and sixth cervical vertebrae (C5 and C6) in the spinal cord. This is why a neck injury at those levels can cause biceps weakness, and why doctors tap the biceps tendon during a reflex test to check whether those nerve roots are functioning properly.
Common Biceps Injuries
Biceps tendonitis is the most frequent issue, typically felt as a deep ache at the front of the shoulder or inside the elbow. It develops gradually from repetitive overhead motions or heavy lifting and usually responds to rest, ice, and a gradual return to activity.
A more serious problem is a biceps tendon tear, which can happen at either the shoulder end or the elbow end. A tear at the elbow often announces itself with an audible pop, followed by sharp pain, swelling, and bruising in the elbow and forearm. You may notice a visible bulge higher up on the arm where the muscle bunches after the tendon releases, along with a gap near the elbow where the tendon used to be. Turning a doorknob or lifting objects palm-up becomes noticeably weaker.
For tears at the elbow, surgical repair is typically necessary to restore full arm strength and rotation. The key factor is timing: surgery ideally happens within the first two to three weeks after injury. Beyond that window, the muscle and tendon begin to scar and shorten, which can make a full repair difficult or impossible. Tears at the shoulder end are more common but often less functionally limiting, and many people manage them without surgery.
How Biceps Size Varies
Upper arm circumference, which includes the biceps, triceps, and underlying bone, averages about 26.9 centimeters (roughly 10.6 inches) in adult men and 25.4 centimeters (10 inches) in adult women. These numbers reflect the mid-upper arm at rest and vary widely based on body composition, activity level, and genetics. The triceps on the back of the arm actually makes up about two-thirds of total upper arm mass, so arm size is never just a biceps measurement.
Training the Biceps Effectively
Because the biceps both bends the elbow and rotates the forearm, exercises that incorporate both motions recruit the muscle most fully. A standard curl with your palms facing up engages the biceps through its full range. Hammer curls, performed with a neutral grip (palms facing each other), shift more work to the brachialis and brachioradialis while still involving the biceps. Chin-ups, where you grip the bar with palms facing you, are one of the most effective compound movements for biceps development because they load the muscle under significant resistance through a long range of motion.
The biceps is a relatively small muscle group, so it recovers faster than larger muscles like the quadriceps or back. Most people can train it two to three times per week with adequate results, provided the total volume isn’t excessive. Because the biceps assists in nearly every pulling movement, it already gets substantial indirect work during rows, pull-ups, and other back exercises.