Biceps Brachii Muscle Anatomy

Location & Overview

The biceps brachii muscle, commonly known as ‘the biceps’, is a prominent superficial muscle of the upper arm, situated in its anterior compartment (front of the arm). This anterior compartment also houses the brachialis and coracobrachialis muscles, both deep to the biceps brachii.^[1,2]

The biceps brachii is a two-headed muscle, typically consisting of a short head (caput breve) and a long head (caput longum). Both heads span the shoulder and elbow regions, but the long head has a more direct relationship with the glenohumeral joint because it originates from the supraglenoid tubercle and superior glenoid labrum, whereas the short head originates from the coracoid process of the scapula.^[3,4] The deltoid muscle covers the proximal part of the long head, enveloping the bicipital groove where it resides. Despite being part of the same muscle, these two heads have distinct locations and contribute to the muscle’s characteristic curved appearance.^[1]

The triceps brachii, located in the posterior compartment of the arm, is the antagonist muscle of the biceps brachii. Antagonist muscles work in opposition to each other. In this scenario, while the biceps brachii contributes to flexing the arm, the triceps brachii is involved in extending the arm.^[3,2]

Origin & Insertion

The biceps brachii comprises two heads: a long head and a short head. The long head originates from a bony prominence on the scapula known as the supraglenoid tubercle, which is a small, rough projection superior to the glenoid cavity. The long head also originates from the superior glenoid labrum, which is a fibrocartilaginous complex attaching like a rim to the articular cartilage of the glenoid fossa. The short head of the biceps brachii originates from the tip of the coracoid process, another prominent hook-like structure on the lateral edge of the superior and anterior portion of the scapula.^[3,4,5,6] These origins allow the two heads to span a considerable distance.

Following their respective origins on the scapula, both heads travel down the arm. The long head crosses obliquely through a ridge in the upper arm known as the bicipital groove. Upon convergence, the two heads form a unified muscle belly around the mid-upper arm region. This single muscle belly then narrows and passes across the anterior aspect of the elbow to insert at two sites: the radial tuberosity, a small prominence on the proximal radius, and the fascia of the forearm. This connection to the fascia is facilitated by a broad, flat tendon known as the bicipital aponeurosis.^[3,4,5,6]

An interesting point to note is the existence of anatomical variations in the biceps brachii. Studies have reported that the muscle can sometimes display additional heads originating from the humerus. These are termed ‘supernumerary heads’ and can number anywhere from three to seven. In addition, the distal tendon of the biceps brachii (the part that inserts into the radius) can sometimes bifurcate, forming two distinct tendons. It has also been observed that the biceps brachii can insert via a single tendon (most common), double tendon, or even triple tendon, reflecting the considerable variability in human anatomy.^[3,4]

Action & Function

The biceps brachii serves several important roles in facilitating movement and stability of the arm and shoulder. One of the primary actions of the biceps brachii is to facilitate the flexion of the elbow.^[1,2,6] Notably, this action depends on the angle of the elbow. When the elbow is extended, the muscle acts primarily as an elbow flexor until the elbow reaches 90-degree flexion. Past 90 degrees of flexion, its function changes based on the orientation of the forearm. If the forearm is pronated, the biceps brachii then becomes the primary muscle involved in forearm supination. If the forearm is supinated, the biceps brachii primarily flexes the elbow.^[4]

An intriguing aspect of the biceps brachii’s biomechanics is the role played by the long head of the muscle in the dynamic stability of the shoulder joint.^[3,5] Despite ongoing academic debate, evidence from research suggests that this tendon plays a stabilizing role at the shoulder joint. Landin et al. (2017) found that beyond 30° of arm elevation, the long head, “even though contracting, does not create a noteworthy elevation moment, which suggests that it cannot serve as a dynamic shoulder stabilizer in higher ranges of elevation”.^[7] Some sources often overstate the role of the biceps brachii in arm elevation, claiming the biceps brachii is a strong flexor of the shoulder joint. However, the biceps brachii plays a role as a dynamic stabilizer of the shoulder during elevation of the arm (primarily in the scapular plane) and any shoulder flexion provided is weak compared with other primary shoulder flexors.^[7] Moreover, as previously stated, it seems this stabilization role is only notable in the first 30 degrees of arm elevation.

The role of the bicipital aponeurosis, a part of the biceps brachii, should also be noted. It not only increases the tension of the main tendon of the biceps brachii but also serves as a protective layer for the median nerve and brachial artery running posteriorly. In addition, it contributes to reinforcing the antebrachial fascia, underscoring its multifaceted function.^[4,1,2]

In summary, the actions of the biceps brachii muscle encompass a range of movements, including flexion of the elbow, supination of the forearm, dynamic stability of the shoulder joint, and a weak contribution to shoulder flexion.

Innervation

The innervation of the biceps brachii muscle is provided primarily by the musculocutaneous nerve, with roots at the C5 and C6 levels of the spinal cord. The musculocutaneous nerve is part of the brachial plexus, which is a complex network of nerves that supplies innervation to the upper limb.^[3,6]

The musculocutaneous nerve branches off from the lateral cord of the brachial plexus. As a whole, this nerve carries fibers from the cervical spinal nerves C5 through C7 (but the biceps brachii is mainly innervated by the part of the nerve associated with C5 and C6). The musculocutaneous nerve typically emerges at the inferior border of the pectoralis major muscle and immediately pierces the coracobrachialis muscle fibers to continue its course through the arm.^[6]

In the context of the biceps brachii, the musculocutaneous nerve descends between this muscle and the brachialis, which reside in the anterior compartment of the arm, providing innervation as it travels.^[6] Variations in the branching pattern to the biceps brachii have been observed, including a single branch serving the entire muscle, or two branches serving either the individual heads or the common belly of the muscle.^[8]

Once it has provided motor fibers to both the biceps brachii and the brachialis, the musculocutaneous nerve continues its course distally, emerging just lateral to the biceps brachii tendon to become the lateral cutaneous nerve of the forearm, also known as the lateral antebrachial cutaneous nerve.^[6]

Despite being considered one of the more easily identifiable nerves of the brachial plexus, it is essential to note that the musculocutaneous nerve’s course and branching patterns may exhibit variability. For instance, the nerve’s entry point into the coracobrachialis muscle and the distances at which it provides branches to the biceps brachii and brachialis have been found to vary between individuals.^[6]

Blood Supply

The biceps brachii muscle obtains its blood supply mainly via the muscular branches of the brachial artery.^[9,1,2]

The brachial artery, which is a direct extension of the axillary artery, begins at the lower margin of the teres major muscle and is the primary artery that supplies blood to the upper limb. This artery runs along the ventral surface of the arm, giving rise to several smaller branches before reaching the cubital fossa.^[10,1]

Among these branching arteries are the deep brachial artery, the superior ulnar collateral artery, and the inferior ulnar collateral artery. As the brachial artery arrives at the cubital fossa, it separates into its terminal branches, the radial and ulnar arteries of the forearm.^[10,2]

Notably, the brachial artery and its various branches play a significant role in supplying blood to the biceps brachii muscle, among others, including the triceps brachii muscle and the coracobrachialis muscle. Additionally, the median nerve, a part of the brachial plexus, initially lies lateral to the brachial artery at its proximal segment. However, at its distal segment, the median nerve crosses to the medial side of the brachial artery, lying in the ventral cubital fossa.^[10,2]

The inferior ulnar collateral artery is of particular relevance to the biceps brachii muscle. This branch of the brachial artery supplies blood to the biceps brachii muscle, as well as the brachialis muscle and the coracobrachialis muscle.^[10] The inferior ulnar collateral artery emerges anterior to the lower segment of the humerus and runs medially along the brachialis muscle before it pierces the medial intermuscular septum. It then forms an anastomosis with the anterior ulnar recurrent artery and the superior ulnar collateral artery.^[10]

In summary, the primary arterial blood supply to the biceps brachii muscle is a collaborative network involving muscular branches of the brachial artery, particularly the inferior ulnar collateral artery.^[9,10]