Anterior Compartment of the Arm

1. Muscles of the Anterior Compartment of the Arm

2. Biceps Brachii - Origin and Insertion

Origin

• The long head tendon passes through the glenohumeral joint (intra-articular but extrasynovial), exits through the intertubercular sulcus (bicipital groove), and is held in position by the transverse humeral ligament
• The short head passes vertically through the axilla and joins the long head in the arm

Insertion

• Both heads converge to form a single distal tendon inserting onto the radial tuberosity of the radius (in the forearm)
• A flat sheet of connective tissue - the bicipital aponeurosis - fans out medially from the distal tendon to blend with the deep fascia of the forearm

Actions

1. Powerful flexor of the forearm at the elbow joint
2. Most powerful supinator of the forearm (especially when elbow is flexed)
3. Accessory flexor of the arm at the glenohumeral joint (both heads cross the shoulder)
4. The long head prevents superior migration of the humeral head on the glenoid

3. Musculocutaneous Nerve - Demonstration

• Arises from the lateral cord of the brachial plexus (C5, C6, C7)
• Leaves the axilla by piercing through the coracobrachialis muscle (this is a key anatomical landmark)
• Passes diagonally down the arm in the plane between biceps brachii (anteriorly) and brachialis (posteriorly)
• Emerges lateral to the biceps tendon at the elbow
• Penetrates the deep fascia and continues as the lateral cutaneous nerve of the forearm

4. Nerve Supply of the Muscles of the Front of the Arm

5. Brachialis - Origin, Insertion, Nerve Supply, and Action

Origin

• Superficial head: Anterior surface of the humerus close to the deltoid tuberosity (distal half of humerus, medial and lateral surfaces, and adjacent intermuscular septae)
• Deep head: Distal third of the humeral shaft

Insertion

• Superficial head: Ulnar tuberosity distal to the coronoid process
• Deep head: All but the tip of the coronoid process of the ulna, via three units - a medial aponeurosis, a lateral aponeurosis, and direct muscle fiber insertion into the ulna

Nerve Supply

• Primary: Musculocutaneous nerve (C5, C6) - runs between brachialis and biceps brachii, lying on the surface of brachialis
• Secondary (81% of cases): Radial nerve branches to inferolateral fibers of the deep head
• Rare (15%): Median nerve to the lower quarter

Action

• Powerful flexor of the forearm at the elbow joint - regardless of forearm position (pronated or supinated), making it the "workhorse" of elbow flexion
• The superficial head has a mechanical advantage due to its more proximal origin and more distal insertion, providing the bulk of flexion strength
• The deep head also contributes to elbow stability by reinforcing the coronoid process as a buttress against posterior dislocation

6. Brachial Artery, Median Nerve, and Ulnar Nerve

Brachial Artery

• Continuation of the axillary artery (from the inferior margin of teres major)
• Runs on the medial side of the arm in the cleft between biceps brachii and triceps brachii
• Major branch: Profunda brachii artery (accompanies the radial nerve into the posterior compartment through the triangular interval)
• At the cubital fossa: divides into radial and ulnar arteries
• Clinical use: Surface for blood pressure measurement (auscultatory point)

Median Nerve

• Enters the arm from the axilla at the inferior margin of teres major
• Travels vertically down the medial side of the anterior compartment
• Proximal arm: Lies lateral to the brachial artery
• Distal arm: Crosses anterior to the brachial artery to lie on its medial side
• Passes anterior to the elbow joint
• No major branches in the arm (motor branches begin in the forearm)

Ulnar Nerve

• Enters the arm with the median nerve, initially medial to the axillary/brachial artery
• In the middle of the arm: pierces the medial intermuscular septum and enters the posterior compartment
• Runs anterior to the medial head of triceps
• Passes posterior to the medial epicondyle (where it can be palpated - the "funny bone")
• Then passes into the anterior forearm
• No major branches in the arm

• Gray's Anatomy for Students, Table 7.8 and Figs. 7.64, 7.68, 7.119
• Imaging Anatomy Text and Atlas, Vol. 3: Bones, Joints, Vessels and Nerves, Figs. 3.11, 3.12, 3.13

Posterior Compartment of the Arm

1. Muscle of the Posterior Compartment of the Arm

2. Origin and Insertion of Triceps Brachii

Origins

Key relationship: The radial groove is the anatomical landmark that separates the lateral head origin (above) from the medial head origin (below). The long head is the only head that originates from the scapula, making it the only head that acts at the shoulder joint.

Insertion

• The tendon has a bipartite appearance at insertion:
  • The common tendon of the long + lateral heads = superficial and tendinous
  • The medial head = deeper and mainly muscular at insertion

3. Nerve Supply of the Three Heads of Triceps

Clinical note: Because the branch to the medial head of triceps arises proximal to the radial groove, the medial head is often spared in midshaft humeral fractures (Saturday night palsy), even when the rest of the radial nerve is damaged. This is why elbow extension is preserved in radial nerve injuries at the radial groove.

4. Actions of Triceps Brachii

• The long head also acts as a dynamic stabilizer of the glenohumeral joint, particularly in resisting inferior subluxation
• Triceps is the primary antagonist of the biceps brachii and brachialis
• The medial head is considered the "workhorse" of extension during low-load activities; the long and lateral heads are recruited during high-load extension

5. Radial Groove and Its Contents

The Radial Groove (Spiral Groove / Musculospiral Groove)

• A shallow groove on the posterior surface of the humerus, running obliquely from medial-proximal to lateral-distal
• Located between the origins of the lateral head (above) and medial head (below) of triceps
• The groove represents the path where structures pass diagonally around the back of the humerus

Contents of the Radial Groove

1. Radial nerve - lying directly on bone in the groove
2. Profunda brachii artery (deep brachial artery) - accompanies the radial nerve throughout

Clinical importance: Both the radial nerve and profunda brachii artery are tightly bound together between the medial and lateral heads of triceps in the radial groove. A midshaft humeral fracture can stretch or transect the radial nerve here, causing wrist drop and loss of sensation over the dorsum of the hand.

6. Branches of the Radial Nerve at the Radial Groove

Muscular Branches (in the groove)

1. Branches to lateral head of triceps brachii
2. Branches to medial head of triceps brachii (distal part - some branches arise more proximally before entering the groove)
3. Branch to brachioradialis (arises as radial nerve exits the groove anterolaterally)
4. Branch to extensor carpi radialis longus (arises as nerve exits the groove)
5. Small contribution to brachialis (lateral part - C7 fibers)

Cutaneous Branches (arising in the posterior compartment)

1. Inferior lateral cutaneous nerve of the arm - supplies skin on the lateral arm distally
2. Posterior cutaneous nerve of the forearm - supplies skin on the posterior forearm

Note: Before entering the groove, the radial nerve gives off branches to the long head and medial head (proximal) of triceps, plus the posterior cutaneous nerve of the arm - all from the axilla/triangular interval level.

7. Profunda Brachii Artery

Key Facts about the Profunda Brachii Artery

The profunda brachii artery is the main blood supply to the posterior compartment of the arm and is the vascular companion of the radial nerve throughout the radial groove. Both structures are vulnerable together in midshaft humeral fractures.

• Gray's Anatomy for Students, Figs. 7.66, 7.69, Table 7.9
• Imaging Anatomy Text and Atlas, Vol. 3 (Figs. 3.16, 3.17)
• THIEME Atlas of Anatomy - General Anatomy and Musculoskeletal System