Muscles of the Arm

The muscles of the arm are responsible for movements at the shoulder and elbow joints and are primarily involved in flexion and extension of the forearm. Anatomically, the arm refers to the region between the shoulder and elbow, and its muscles are grouped into two compartments: the anterior (flexor) compartment and the posterior (extensor) compartment. These muscles are enclosed within the brachial fascia and separated by the medial and lateral intermuscular septa.

Location

The arm is situated between the shoulder and elbow joints. The anterior compartment lies anterior to the humerus and is primarily involved in flexion and supination, while the posterior compartment is located posterior to the humerus and is involved in extension. The neurovascular structures of the arm travel through both compartments alongside these muscles.

Structure and Individual Muscles

Anterior Compartment

Biceps Brachii

• Origin: Long head – supraglenoid tubercle of scapula; Short head – coracoid process of scapula
• Insertion: Radial tuberosity and bicipital aponeurosis
• Function: Supination of the forearm, flexion of the elbow and shoulder

Brachialis

• Origin: Distal half of the anterior surface of the humerus
• Insertion: Coronoid process and tuberosity of the ulna
• Function: Primary flexor of the elbow joint, regardless of forearm position

Coracobrachialis

• Origin: Coracoid process of scapula
• Insertion: Medial surface of humerus (mid-shaft)
• Function: Flexes and adducts the shoulder

Posterior Compartment

Triceps Brachii

• Origin: Long head – infraglenoid tubercle; Lateral head – posterior surface of humerus (above radial groove); Medial head – posterior surface of humerus (below radial groove)
• Insertion: Olecranon of ulna
• Function: Extension of elbow; Long head also extends the shoulder

Anconeus

• Origin: Lateral epicondyle of humerus
• Insertion: Lateral aspect of olecranon and posterior ulna
• Function: Assists triceps in elbow extension; stabilizes elbow during pronation/supination

Function

• Flexion: Biceps brachii and brachialis are the main elbow flexors. Biceps is more active during supination, brachialis during pronation.
• Extension: Triceps brachii is the main extensor of the elbow, assisted by the anconeus.
• Supination: Biceps brachii is the most powerful supinator of the forearm.
• Shoulder actions: Long head of biceps and coracobrachialis flex the shoulder; long head of triceps extends it.

Physiological Role(s)

• Enables controlled movements of the elbow and forearm during functional tasks like lifting, throwing, and pulling.
• Supports static postures and stabilization of the upper limb.
• Assists in precision movements by coordinating with forearm and shoulder muscles.

Development

Arm muscles develop from the mesoderm of the lateral plate during limb bud formation. Myogenic precursor cells migrate and differentiate into dorsal and ventral masses. The dorsal mass forms the triceps (extensors), and the ventral mass gives rise to the biceps, brachialis, and coracobrachialis (flexors). Motor innervation from the brachial plexus is established early in development, and ossification of attachment sites continues into adolescence.

Clinical Significance

• Biceps tendon rupture: May occur at either the proximal or distal attachment, leading to weakness in elbow flexion and supination. Often visible as a “Popeye” deformity.
• Brachialis strain: Can occur due to overuse or eccentric loading; often misdiagnosed as biceps injury.
• Coracobrachialis entrapment: May compress the musculocutaneous nerve, leading to weakness or paresthesia in the lateral forearm.
• Triceps rupture: Rare but can impair elbow extension and is typically associated with direct trauma or overuse.
• Radial nerve injury: Commonly affects the posterior compartment, leading to triceps weakness and wrist drop.

Surface Anatomy and Imaging

• The biceps brachii is easily palpable in the anterior arm and becomes prominent during elbow flexion.
• The triceps forms the bulk of the posterior arm and inserts into the olecranon, which can be seen during resisted extension.
• Ultrasound and MRI are commonly used to evaluate tendon integrity and muscle tears.

Anatomical Variations

• The biceps may have an accessory head (third head), usually arising from the humerus—seen in 10–20% of people.
• The coracobrachialis may be fused with the short head of the biceps in some individuals.
• The triceps may have a fourth head, especially in certain populations.