Alternately known as the biceps brachii, the bicep brachii is a muscle of the upper arm. Situated between the shoulder and the elbow on the front of the arm, it is responsible for two actions: elbow flexion, or the act of curling the palm towards the shoulder, and forearm rotation, which is the act of turning the forearm palm-up. The name biceps refers to this muscle’s most distinguishing anatomical feature, the fact that it has two heads, separate sections of the bicep brachii that unite to form a common tendon just above the elbow.
Arising from within the shoulder on the scapula or shoulder blade, the bicep brachii’s two heads each have a disparate point of origin. The section known as the short head originates on the coracoid process of the scapula, a curved bony projection on the top of the shoulder blade that protrudes forward just beneath the collarbone to the inside of the shoulder joint. Slightly above and behind the coracoid process is a surface on the very top of the scapula known as the supraglenoid tubercle. This is where the long head of the bicep brachii originates. It attaches via a long tendon that passes through the glenohumeral or shoulder joint capsule before forming the body of the muscle at the top of the arm.
With the short head crossing the shoulder on its anterior side and the long head exiting the shoulder joint just lateral or to the outside of the short head, the two converge at the top of the arm to form a single muscle, the bicep brachii. The most superficial muscle of the anterior upper arm, meaning that it lies closest to the skin, the biceps brachii forms a tendon that inserts just below the elbow into the middle of the anterior forearm. Specifically, it attaches to the radius bone on the radial tuberosity, a bumpy surface high on the medial side of the bone, the side that faces the adjacent ulna bone.
As it crosses the shoulder joint, the elbow joint, and a joint where the top ends of the radius and ulna meet known as the proximal radioulnar joint, the bicep brachii affects movement at all three. At the shoulder joint, it weakly assists in flexion, or the forward lifting of the arm. Most of this work is performed by the anterior deltoid muscle. Where it crosses the elbow, it assists in elbow flexion, although the brachialis muscle beneath performs most of this work as well. The bicep brachii is a prime mover of the proximal radioulnar joint, particularly when the elbow is flexed. When this muscle contracts, it rotates the radius and ulna bones past each other, turning the hand palm-up.