The biceps brachii, which translates from latin as "two-headed muscle of the arm", is a thick muscle that lies between the shoulder and the elbow and is the most superficial muscle in the anterior compartment of the upper arm. The bicep muscle consists of two head—a long head and a short head—both of which originate at the scapula and eventually converge to form a common muscle belly that inserts into the proximal forearm.
The primary biceps brachii action is elbow flexion and forearm supination. Due to the fact that the long head crosses the shoulder joint, it also weakly assists the muscles of the glenohumeral joint with shoulder flexion and shoulder abduction. [1
Learn about the biceps brachii origin and insertion and how it relates to the function of the anterior compartment of the upper arm.
The biceps, in addition to the brachialis (another powerful elbow flexor) and the coracobrachialis (flexes and adducts the arm at the shoulder joint), is one of three muscles that make up the anterior compartment of the arm.
The biceps is known as a biarticular muscle, not because of its two heads, but because it crosses two joints, the glenohumeral joint and the elbow joint
Although both heads or the biceps originate at the scapula, the precise insertion points are notably different. The short head of the biceps originates at the coracoid process of the scapula. Its tendon is shared by the coracobrachialis, a long, thin muscle that lies beneath the biceps, and is known as the conjoint tendon. [2
The long head of the biceps originates at the supraglenoid tubercle or the scapula, which is superior to the glenoid cavity (glenoid fossa), before passing through the bicipital groove (also know as the intertubercular groove). The bicipital groove is enclosed by the transverse humeral ligament and sits between the greater and lesser tubercle of the humerus.
Both the long head and the short head then converge to form a common muscle belly at the upper arm before inserting into the radial tuberosity, however, is some individuals, there are two distinct tendons that attach the respective heads to the tuberosity.
Similarly, it is estimated 10% of people have a third bicep head that sits next to the short head. There is also research showing humans with more than four bicep heads but this is an exception rather than the rule. [3
The bicipital aponeurosis is a type of deep fascia that lies medially to the radial tuberosity and which reinforces cubital fossa to protect deep tissue structures, namely, the brachialis artery and the median nerve, both of which run under the bicipital aponeurosis.
The bicep brachii performs elbow flexion at the humeroulnar joint along with the brachialis and brachioradialis muscles. This particular biceps brachii functions involves moving the lower arm toward the biceps to decrease the angle at the elbow joint.
When the forearms are supinated (when the palms are facing up), the biceps brachii is the primary elbow flexor. Conversely, when elbow flexion is performed while the palms are in a pronated position, the brachialis and brachioradialis muscle contribute the most force toward the movement. [4
The second primary biceps brachii action is forearm supination (turning the palms up), which the bicep muscle perform at the proximal radioulnar joint when the elbow is flexed with assistance from the supinator muscle of the forearms. In contrast, when the elbow is fully extended, the supinator muscle is responsible for supinating the lower arm.
Due to the biceps brachii origin at the scapula, the bicep muscles also weakly assist with shoulder flexion (raising the arm up) and shoulder abduction (raising the arm out to the side). Additionally, the short head assists the glenohumeral joint with horizontal adduction and shoulder stabilization, especially when the deltoids are under a heavy load.
The musculocutaneous originates from the lateral cord of brachial plexus nerve and innervates the biceps brachii, the coracobrachialis, and most of the brachialis. It consists of the C5, C6, and C7 cervical nerves.
The brachial artery lies beneath the biceps muscles and is a continuation of the axillary artery. It's the major blood vessel of the upper arm. [5