The two-headed muscle at the front of the upper arm, worked by curls, is commonly called the biceps.
Its anatomical name is the biceps brachii, meaning “two-headed muscle of the arm.” The two heads begin at different points on the scapula, then join into a common muscle structure that attaches near the radius in the forearm. The biceps helps bend the elbow and turn the palm upward.
A curl emphasizes the biceps, but it is not the only muscle involved. The brachialis, which lies deeper beneath the biceps, is also a strong elbow flexor, while the brachioradialis in the forearm assists with the movement. The triceps is the opposing muscle group: it straightens the elbow rather than bending it.
People often say “bicep” when referring to one arm, but “biceps” is the standard English form for the muscle. The biceps also crosses the shoulder joint, so exercises and arm positions can change how much its long and short heads contribute.