M01.03.020 Upper Arm Region

The upper arm lies between the shoulder joint and the elbow joint. It contains four primary muscles organized into two compartments:

Anterior compartment: Three muscles (biceps brachii, brachialis, coracobrachialis).
Posterior compartment: One muscle (triceps brachii).

This guide explores the anatomy of the upper arm muscles, focusing on their attachments, innervation, actions, and clinical relevance.

1. Anterior Compartment

The anterior compartment contains three muscles innervated by the musculocutaneous nerve:

A useful mnemonic: BBC (Biceps, Brachialis, Coracobrachialis).

Muscle	Attachments	Function	Innervation
Biceps Brachii	Long head: supraglenoid tubercle of scapula; Short head: coracoid process. Inserts into the radial tuberosity and forearm fascia via bicipital aponeurosis.	Supinates the forearm; flexes the elbow and shoulder.	Musculocutaneous nerve (tests C6).
Coracobrachialis	Originates from the coracoid process and attaches to the medial humerus near the deltoid tubercle.	Flexes and weakly adducts the shoulder.	Musculocutaneous nerve.
Brachialis	Originates from medial and lateral surfaces of the humeral shaft and inserts into the ulnar tuberosity.	Flexes the elbow.	Musculocutaneous nerve; radial nerve.

Common site: Long head of the biceps brachii.
Presentation: Bulge in the upper arm on elbow flexion (Popeye sign).
Impact: Minimal weakness due to compensation by brachialis and supinator muscles.

The posterior compartment contains the triceps brachii, a three-headed muscle.

Head	Attachments	Function	Innervation
Long head	Infraglenoid tubercle of the scapula.	Extension of the elbow.	Radial nerve (tests C7).
Lateral head	Humerus (superior to the radial groove).
Medial head	Humerus (inferior to the radial groove).		(Axillary nerve in rare cases).
Insertion	All heads converge on the olecranon of the ulna.

Anterior Compartment: Muscles are mainly flexors of the arm and forearm, innervated by the musculocutaneous nerve.
Posterior Compartment: Muscles are extensors of the elbow, primarily innervated by the radial nerve.
Clinical Importance:
- The long head of the biceps brachii is prone to rupture, producing a distinct “Popeye sign.”
- Reflex tests:
  - Biceps reflex (C6) for anterior compartment.
  - Triceps reflex (C7) for posterior compartment.

Include labeled diagrams of the muscles in anterior and posterior compartments to aid understanding.

Moore, K. L., Dalley, A. F., & Agur, A. M. R. Clinically Oriented Anatomy. Wolters Kluwer.
Drake, R. L., Vogl, W., & Mitchell, A. W. M. Gray’s Anatomy for Students. Elsevier.