Posterior Compartment of the Arm

The posterior compartment of the arm (extensor compartment) contains muscles primarily responsible for extension of the elbow joint. It is bounded by the brachial fascia and separated from the anterior compartment by the medial and lateral intermuscular septa. The radial nerve and profunda brachii artery run within this compartment.

Boundaries

Structure	Description
Anterior	Humerus and intermuscular septa
Posterior	Brachial fascia and skin
Medial & Lateral	Intermuscular septa (medial and lateral)

Muscles of the Posterior Compartment

Muscle	Origin	Insertion	Function	Innervation
Triceps brachii	Long head: infraglenoid tubercle of scapula Lateral head: posterior surface of humerus (above radial groove) Medial head: posterior surface of humerus (below radial groove)	Olecranon process of ulna	Extension of elbow; long head also aids in shoulder extension and adduction	Radial nerve (C6–C8)
Anconeus	Lateral epicondyle of humerus	Lateral surface of olecranon and posterior ulna	Assists triceps in elbow extension; stabilizes elbow	Radial nerve (C7–T1)

Radial nerve: Innervates all posterior arm muscles; runs in the radial groove of the humerus
Profunda brachii artery (deep brachial): Travels with the radial nerve in the radial groove
Posterior cutaneous nerve of the arm and forearm: Branches of the radial nerve supplying skin

Function

Elbow Extension: Triceps brachii is the main extensor of the elbow joint
Shoulder Movement: Long head of triceps assists in shoulder adduction and extension
Joint Stabilization: Anconeus stabilizes the elbow during forearm rotation

Blood Supply

Profunda brachii artery: Main blood supply; branches into radial collateral arteries

Innervation

Radial nerve: Sole motor nerve of the compartment (C6–C8)

Surface Anatomy

The triceps forms the large muscular mass on the back of the arm. Its tendon is prominent just above the olecranon. The anconeus is small and less visible but lies lateral to the olecranon during elbow extension.

Clinical Significance

Radial Nerve Injury: Mid-shaft humeral fractures may injure the radial nerve in the radial groove, causing wrist drop and triceps weakness
Compartment Syndrome: Rare but possible; may present with posterior swelling and loss of elbow extension
Triceps Reflex: Deep tendon reflex tested over the olecranon; evaluates C7 spinal segment

Comparative Anatomy

In quadrupeds, the triceps muscle is also well-developed due to weight-bearing needs. In humans, while less load-bearing, it is adapted for precise, forceful extension required in tasks like pushing, throwing, or lifting.