Anterior Compartment of Arm

Introduction

Introduction to the Anterior Compartment of the Arm

The arm (brachium) is divided into two compartments by the medial and lateral intermuscular septa: anterior (flexor) and posterior (extensor). The anterior compartment contains three muscles – biceps brachii, brachialis, and coracobrachialis – primarily responsible for flexion of the elbow and shoulder. It is supplied by the musculocutaneous nerve (C5-C7) and the brachial artery. This compartment also houses important neurovascular structures including the median nerve, ulnar nerve (proximally), and the basilic vein superficially.

Study

Muscles of the Anterior Compartment

1. Biceps Brachii: Two heads – long head (originates from supraglenoid tubercle of scapula, passes through intertubercular groove) and short head (originates from coracoid process). Insertion: Radial tuberosity and via bicipital aponeurosis into deep fascia of forearm. Actions: Supination of forearm (when elbow flexed), elbow flexion, and shoulder flexion (weak). Innervation: Musculocutaneous nerve (C5-C6). 2. Brachialis: Origin – distal half of anterior shaft of humerus. Insertion – coronoid process and ulnar tuberosity. Action – pure elbow flexion (primary flexor). Innervation – musculocutaneous nerve (C5-C6) with small contribution from radial nerve (C7). 3. Coracobrachialis: Origin – coracoid process (with short head of biceps). Insertion – medial mid-shaft of humerus. Action – shoulder flexion and adduction. Innervation – musculocutaneous nerve (C5-C7).

Neurovascular Structures

Musculocutaneous nerve (C5-C7): Branch of lateral cord of brachial plexus. Pierces coracobrachialis, then passes between biceps and brachialis, continuing as the lateral antebrachial cutaneous nerve (sensory to lateral forearm). Brachial artery: Continuation of axillary artery at lower border of teres major. Runs medial to biceps, giving off profunda brachii (posterior compartment) and nutrient arteries. Divides into radial and ulnar arteries at cubital fossa. Median nerve: From lateral and medial cords, runs lateral to brachial artery proximally, then crosses to medial side. No branches in arm. Ulnar nerve: From medial cord, runs medial to brachial artery in proximal arm, then pierces medial intermuscular septum to enter posterior compartment (mid-arm). Basilic vein: Superficial, penetrates deep fascia to become axillary vein.

Clinical Relevance

Rupture of the long head of biceps tendon (e.g., from heavy lifting) causes a 'Popeye deformity' – muscle belly bulges distally. Biceps brachii is tested by resisted supination with elbow flexed. Brachialis is tested by resisted elbow flexion with forearm pronated (eliminating biceps). Coracobrachialis injury may cause weakness in shoulder flexion/adduction. Musculocutaneous nerve injury (rare) causes weak elbow flexion and sensory loss over lateral forearm. A supracondylar humeral fracture can damage the brachial artery (distal ischemia) and median nerve.

Summary

Key Takeaways – Anterior Compartment of Arm

Muscles: Biceps brachii (elbow flexor, supinator), Brachialis (pure elbow flexor), Coracobrachialis (shoulder flexor/adductor). Innervation: All by musculocutaneous nerve (C5-C7). Blood supply: Brachial artery. Other structures: Median nerve, ulnar nerve (proximal), basilic vein. Clinical: Popeye deformity (biceps tendon rupture); musculocutaneous nerve injury causes weak elbow flexion; supracondylar fracture risks brachial artery injury.