Muscles of the arm: biceps, brachialis, coracobrachialis, triceps and deltoid

In anatomy the arm means the upper arm only, from shoulder to elbow; the part below the elbow is the forearm. The arm has three muscles in its anterior compartment (biceps brachii, coracobrachialis and brachialis) and one in its posterior compartment (triceps brachii). The anterior muscles are supplied by the musculocutaneous nerve and by muscular branches of the brachial artery. (TeachMeAnatomy: The Upper Arm Muscles, Anterior and Posterior Compartments.)

OpenStax describes the anterior muscles and their vessels and nerves as forming the anterior compartment of the arm, with triceps brachii and anconeus as the extensors of the forearm at the elbow. Brachioradialis, which also flexes the elbow, belongs to the forearm. (OpenStax Anatomy and Physiology 2e, 11.5: Muscles of the Pectoral Girdle and Upper Limbs.)

The deltoid is a shoulder muscle rather than an arm muscle, but it forms the rounded outline of the shoulder and inserts on the deltoid tuberosity on the lateral side of the humerus. It is one of the intrinsic shoulder muscles, together with the rotator cuff and teres major. (TeachMeAnatomy: The Intrinsic Muscles of the Shoulder.)

Biceps brachii has two heads, both arising from the scapula. The short head arises from the tip of the coracoid process; the long head arises from the supraglenoid tubercle, just above the glenoid cavity. The muscle inserts on the tuberosity of the radius and, through the bicipital aponeurosis, on the fascia of the forearm. It is supplied by the musculocutaneous nerve (C5 and C6). (University of Washington Musculoskeletal Atlas: Biceps brachii.)

The tendon of the long head runs inside the shoulder joint capsule but stays outside the synovial membrane, then descends in the bicipital groove (intertubercular sulcus) of the humerus. Biceps supinates the forearm, turning the palm up, and this supination is most powerful when the elbow is flexed. When both heads contract they also help flex the arm at the shoulder. (Kenhub: Biceps brachii muscle.)

The University of Washington atlas summarises the action as supination of the forearm and, when the forearm is supine, flexion of the elbow. (University of Washington Musculoskeletal Atlas: Biceps brachii.)

The bicipital aponeurosis (aponeurosis bicipitalis, also called the lacertus fibrosus) is a sheet of connective tissue from the biceps tendon that forms the roof of the cubital fossa and blends with the deep fascia of the forearm. The biceps tendon reflex tests spinal cord segment C6. (TeachMeAnatomy: The Upper Arm Muscles, Anterior and Posterior Compartments.)

Brachialis lies deep to biceps brachii. It arises from the anterior surface of the distal half of the humerus, starting just below the insertion of the deltoid, and from the intermuscular septa on both sides. It inserts on the tuberosity of the ulna and the coronoid process, and its tendon forms part of the floor of the cubital fossa. (Kenhub: Brachialis muscle.)

Brachialis is the only pure flexor of the elbow and flexes it in every position of the forearm. Kenhub calls it the workhorse of the elbow: it produces most of the force during elbow flexion. It can be seen and felt when the elbow is flexed against resistance. (Kenhub: Brachialis muscle.)

Brachialis has a double nerve supply. Most of it is supplied by the musculocutaneous nerve (C5 and C6), but a small lateral portion is supplied by the radial nerve (C7). Its blood comes from branches of the brachial artery and the radial recurrent artery. (Kenhub: Brachialis muscle.)

Coracobrachialis arises from the tip of the coracoid process of the scapula and inserts on the middle third of the medial surface of the humerus. It helps to flex and adduct the arm at the shoulder. It is supplied by the musculocutaneous nerve; the atlas lists roots C5, C6 and C7. (University of Washington Musculoskeletal Atlas: Coracobrachialis.)

Coracobrachialis is a useful landmark. The musculocutaneous nerve, a terminal branch of the lateral cord of the brachial plexus, leaves the axilla by piercing coracobrachialis near its insertion. It then runs down the arm between biceps brachii (in front) and brachialis (behind), supplying all three anterior muscles, and ends as the lateral cutaneous nerve of the forearm. (TeachMeAnatomy: The Musculocutaneous Nerve.)

The median nerve crosses in front of the humeral insertion of coracobrachialis. (Kenhub: Coracobrachialis.)

Triceps brachii fills the back of the arm and has three heads. The long head arises from the infraglenoid tubercle of the scapula, below the glenoid cavity. The lateral head arises from the posterior surface of the humerus above the radial groove, and the medial head from the posterior surface below the radial groove. The three heads insert by a common tendon on the olecranon of the ulna and the fascia of the forearm. (University of Washington Musculoskeletal Atlas: Triceps brachii.)

Triceps is the chief extensor of the forearm at the elbow, and its long head steadies the head of the humerus when the arm is abducted. It is supplied by the radial nerve (C6, C7 and C8) and by branches of the deep brachial artery (profunda brachii). (University of Washington Musculoskeletal Atlas: Triceps brachii.)

Because the long head crosses the shoulder, it also helps to extend and adduct the arm. The radial groove between the lateral and medial head origins carries the radial nerve and the deep brachial artery and vein, and the radial nerve gives a separate branch to each head. (Kenhub: Triceps brachii muscle.)

A tap on the triceps tendon tests spinal segment C7. In some people the long head is supplied by the axillary nerve instead of the radial nerve. (TeachMeAnatomy: The Upper Arm Muscles, Anterior and Posterior Compartments.)

The deltoid arises from the lateral third of the clavicle, the acromion and the spine of the scapula, and inserts on the deltoid tuberosity of the humerus. Its anterior (clavicular) part flexes and medially rotates the arm; its middle (acromial) part abducts it; its posterior (spinal) part extends and laterally rotates it. It is supplied by the axillary nerve (C5 and C6). (University of Washington Musculoskeletal Atlas: Deltoid.)

The acromial part is multipennate (its fibres attach at angles to several internal tendons). Kenhub describes it as abducting the arm once it is beyond the first fifteen degrees. The deltoid is also supplied by the deltoid and acromial branches of the thoraco-acromial artery. (Kenhub: Deltoid muscle.)

The axillary nerve, a continuation of the posterior cord, passes through the quadrangular space and around the surgical neck of the humerus to supply deltoid and teres minor. It can be injured by a fracture of the surgical neck or a shoulder dislocation, and it supplies the skin over the lower deltoid, the regimental badge area, through the upper lateral cutaneous nerve of the arm. (TeachMeAnatomy: The Axillary Nerve.)

After axillary nerve injury the deltoid wastes and the shoulder can look flattened rather than rounded. The nerve's position also matters when injections are given into the deltoid. (Kenhub: Deltoid muscle.)

Anconeus is a small triangular muscle behind the elbow. It arises from the lateral epicondyle of the humerus and inserts on the lateral surface of the olecranon and the upper posterior surface of the ulna. It assists triceps in extending the forearm, stabilises the elbow joint and abducts the ulna during pronation. (University of Washington Musculoskeletal Atlas: Anconeus.)

Anconeus is supplied by the radial nerve; Kenhub gives the root values as C6 to C8, while the University of Washington atlas gives C7, C8 and T1. Kenhub classes it with the superficial extensor muscles of the forearm, although it is often studied with triceps because it assists triceps at the elbow. (Kenhub: Anconeus muscle.)

Three nerves supply the arm muscles: the musculocutaneous nerve (biceps, brachialis and coracobrachialis), the radial nerve (triceps, anconeus and part of brachialis) and the axillary nerve (deltoid). Musculocutaneous nerve injury is relatively uncommon because the nerve is well protected in the axilla. (TeachMeAnatomy: The Musculocutaneous Nerve.)

The radial nerve leaves the axilla through the triangular interval and runs in the radial groove of the humerus. A fracture of the humeral shaft can damage it there, causing wrist drop. Triceps may be weakened but is not paralysed, because some of its branches leave the nerve above the groove. (TeachMeAnatomy: The Radial Nerve.)

The tendon of the long head of biceps is one of the more commonly ruptured tendons, although complete ruptures are rare overall. When the patient flexes the elbow, the muscle belly bunches up into a bulge, called the Popeye sign. Patients often notice little weakness, because other muscles compensate. (TeachMeAnatomy: The Upper Arm Muscles, Anterior and Posterior Compartments.)

Rupture of the distal biceps tendon at the elbow occurs in about 5.4 cases per 100,000 patients per year, mostly in active young athletes and people in their 40s to 60s. An intact lacertus fibrosus can stop the torn tendon retracting far. Without repair, studies report a 40 to 50% loss of maximal supination strength and about 30% loss of elbow flexion strength. The hook test is useful for diagnosing an acute complete rupture. (Vishwanathan K, Soni K. Distal biceps rupture: Evaluation and management. J Clin Orthop Trauma. 2021.)

Acromial part of deltoid

The middle fibres of the deltoid, forming the rounded contour of the shoulder.

Anconeus

A small triangular muscle on the posterior elbow.

Brachialis

The main flexor of the elbow, lying deep to biceps brachii.

Clavicular part of deltoid

The anterior fibres of the deltoid.

Coracobrachialis

A slender muscle on the medial side of the upper arm.

Lateral head of triceps brachii

The lateral head of the triceps.

Long head of biceps brachii

The long head of biceps, passing through the shoulder joint.

Long head of triceps brachii

The long head of triceps, crossing the shoulder joint.

Medial head of triceps brachii

The deep head of triceps, lying beneath the long and lateral heads.

Short head of biceps brachii

The medial head of biceps brachii at the front of the arm.

Spinal part of deltoid

The posterior fibers of deltoid arising from the scapular spine.