Biceps Brachii Muscle

Of the four major arm muscles, three flexors (biceps brachii, brachialis, and coracobrachialis) are in the anterior (flexor) compartment, supplied by the musculocutaneous nerve, and one extensor (triceps brachii) is in the posterior compartment, supplied by the radial nerve.

Biceps Brachii Muscle Anatomy

As the term biceps brachii indicates, the proximal attachment of this fusiform muscle usually has two heads (bi = two). The two heads of the Biceps Brachii arise proximally by tendinous attachments to processes of scapula, their bellies unite just distal to the middle of the arm.

Approximately 10% of people have a third head to the biceps brachii. When present, the third head extends from the superomedial part of the brachialis (with which it is blended), usually lying posterior to the brachial artery. In either case, a single biceps tendon forms distally and attaches primarily to the radius.

Although the biceps brachii is located in the anterior compartment of the arm, it has no attachment to the humerus. The biceps is a “three-joint muscle,” crossing and capable of effecting movement at the glenohumeral, elbow, and radio-ulnar joints, although it primarily acts at the latter two.

See Also: Biceps Tendonitis

Arising from the supraglenoid tubercle of the scapula and crossing the head of the humerus within the cavity of the glenohumeral joint, the rounded tendon of the long head of the biceps brachii continues to be surrounded by synovial membrane as it descends in the intertubercular sulcus of the humerus. A broad band, the transverse humeral ligament, passes from the lesser to the greater tubercle of the humerus and converts the intertubercular groove into a canal. The ligament holds the tendon of the long head of the biceps in the groove.

See Also: Long Head of Biceps Tendon

Distally, the major attachment of the biceps brachii is to the radial tuberosity via the biceps tendon. However, a triangular membranous band, the bicipital aponeurosis, runs from the biceps tendon across the cubital fossa and merges with the antebrachial (deep) fascia covering the flexor muscles in the medial side of the forearm. It attaches indirectly by means of the fascia to the subcutaneous border of the ulna.

See Also: Distal Biceps Tendon Tear

The proximal part of the aponeurosis can be easily felt where it passes obliquely over the brachial artery and median nerve. The aponeurosis affords protection for these and other structures in the cubital fossa. It also helps lessen the pressure of the biceps brachii tendon on the radial tuberosity during pronation and supination of the forearm.

Its action and effectiveness are markedly affected by the position of the elbow and forearm. When the elbow is extended, the biceps brachii is a simple flexor of the forearm; however, as elbow flexion approaches 90° and more power is needed against resistance, the biceps is capable of two powerful movements, depending on the position of the forearm.

When the elbow is flexed close to 90° and the forearm is supinated, the biceps is most efficient in producing flexion. Alternately, when the forearm is pronated, the biceps brachii is the primary (most powerful) supinator of the forearm. For example, it is used when right-handed people drive a screw into hard wood, and when inserting a corkscrew and pulling the cork from a wine bottle. The biceps barely operates as a flexor when the forearm is pronated, even against resistance. In the semiprone position, it is active only against resistance.

To test the biceps brachii, the elbow joint is flexed against resistance when the forearm is supinated. If acting normally, the muscle forms a prominent bulge on the anterior aspect of the arm that is easily palpated.

References & More

1. Clinically Oriented Anatomy – 8th Edition
2. Tiwana MS, Charlick M, Varacallo M. Anatomy, Shoulder and Upper Limb, Biceps Muscle. [Updated 2023 Aug 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK519538/