In human anatomy, the biceps brachii is a muscle located on the upper arm. The biceps has several functions, the most important being to rotate the forearm (supination) and to flex the elbow.
Proximally, the short head of the biceps attaches to (originates from) the coracoid process of the scapula. The tendon of the long head passes along the intratubercular groove of the humerus into the joint capsule at the head of the humerus, and attaches to the scapula at the supraglenoid tubercle.
Distally, biceps attaches to (inserts into) the radial tuberosity. Because the ulnar and radial bones can rotate about each other the biceps can powerfully supinate the forearm. The biceps also connects with the fascia of the medial side of the forearm via the bicipital aponeurosis.
Two additional muscles lie underneath the biceps brachii. These are the coracobrachialis muscle, which like the biceps attaches to the coracoid process of the scapula, and the brachialis muscle which connects to the ulna and along the mid-shaft of the humerus.
The biceps is tri-articulate, meaning that it works across three joints. The most important of these functions are to supinate the forearm and flex the elbow in the supine position.
These joints and the associated actions are listed as follows in order of importance:
Proximal radioulnar joint - Contrary to popular belief, the biceps brachii is not the most powerful flexor of the forearm, a role which actually belongs to the deeper brachialis muscle. The biceps brachii functions primarily as a powerful supinator of the forearm (turns the palm upwards). This action, which is aided by the supinator muscle, requires the elbow to be at least partially flexed. If the elbow, or humeroulnar joint, is fully extended, supination is then primarily carried out by the supinator muscle.
Humeroulnar joint (Elbow) - The biceps brachii also functions an important flexor of the forearm, particularly when the forearm is supinated. Functionally, this action is performed when lifting an object, such as a bag of groceries or when performing a biceps curl. When the forearm is in pronation (the palm faces the ground), the brachialis, brachioradialis, and supinator function to flex the forearm, with minimal contribution from the biceps brachii.
Glenohumeral joint (Shoulder) - Several weaker functions occur at the glenohumeral, or shoulder, joint. The biceps brachii weakly assists in forward flexion of the shoulder joint (bringing the arm forward and upwards). It may also contribute to abduction (bringing the arm out to the side) when the arm is externally (or laterally) rotated. The short head of the biceps brachii also assists with horizontal adduction (bringing the arm across the body) when the arm is internally (or medially) rotated. Finally, the long head of the biceps brachii, due to its attachment to the scapula (or shoulder blade), assists with stabilization of the shoulder joint when a heavy weight is carried in the arm.
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