Compartments
The arm is divided into anterior (flexor) and posterior (extensor) compartments by the humerus and by the medial and lateral intermuscular septa, which run from the deep fascia to the supracondylar ridges. Each compartment has its own nerve and its own main artery. The ulnar nerve pierces the medial septum in the lower arm to pass into the posterior compartment, and the radial nerve pierces the lateral septum to pass forwards to the elbow.
Attachments and actions
The anterior compartment muscles flex the elbow and shoulder; the posterior compartment muscle extends the elbow.
| Muscle | Origin | Insertion | Action | Nerve |
|---|---|---|---|---|
| Biceps brachii, long head | Supraglenoid tubercle and superior glenoid labrum | Radial tuberosity; bicipital aponeurosis into forearm deep fascia | Supination, elbow flexion, weak shoulder flexion | Musculocutaneous |
| Biceps brachii, short head | Tip of coracoid process | With long head | As above | Musculocutaneous |
| Coracobrachialis | Tip of coracoid process | Middle of medial border of humerus | Flexion and adduction of shoulder | Musculocutaneous |
| Brachialis | Lower half of anterior humerus | Coronoid process and ulnar tuberosity | Elbow flexion in all forearm positions | Musculocutaneous (small lateral part from radial nerve) |
| Triceps brachii | Long head: infraglenoid tubercle; lateral head: posterior humerus above radial groove; medial head: posterior humerus below radial groove | Upper surface of olecranon | Elbow extension; long head also extends and adducts shoulder | Radial |
Biceps brachii
Biceps crosses both the shoulder and elbow. The long head tendon runs inside the shoulder joint capsule, arches over the humeral head and descends in the intertubercular sulcus, held by the transverse humeral ligament. Biceps is the strongest supinator of the forearm, most effective when the elbow is bent to 90 degrees.
Brachialis
Brachialis lies deep to biceps and is the main flexor of the elbow. Because it inserts on the ulna, which does not rotate, it flexes equally well whether the forearm is pronated or supinated.
Coracobrachialis
Coracobrachialis is a small muscle in the upper medial arm. The musculocutaneous nerve pierces it, and its insertion marks the point where the median nerve crosses the brachial artery.
Triceps brachii and anconeus
Triceps is the only muscle of the posterior compartment. The radial nerve and profunda brachii artery run in the radial groove between the lateral and medial heads. Anconeus, a small triangular muscle from the lateral epicondyle to the lateral olecranon, assists extension and is sometimes grouped with the posterior forearm.
Relations
The musculocutaneous nerve runs between biceps and brachialis, supplying both, and emerges lateral to the biceps tendon at the elbow as the lateral cutaneous nerve of the forearm. The brachial artery and median nerve run in the groove along the medial border of biceps; the median nerve starts lateral to the artery and crosses in front of it to lie medially at the elbow. The ulnar nerve runs medial to the artery in the upper arm before piercing the medial intermuscular septum. The radial nerve spirals round the back of the humerus in the radial groove, deep to triceps.
Blood supply and innervation
The anterior compartment is supplied by muscular branches of the brachial artery. The posterior compartment is supplied by the profunda brachii artery, which accompanies the radial nerve, and by the superior ulnar collateral artery.
The musculocutaneous nerve (C5, C6, C7) comes from the lateral cord of the brachial plexus. The radial nerve (C5 to T1) comes from the posterior cord. Triceps is supplied mainly by C7, and its branches leave the radial nerve in the axilla and upper arm.
Clinical relevance
Arm muscle problems are mainly tendon ruptures and nerve injuries.
- Rupture of the long head of biceps: usually in older adults with a degenerate tendon. The muscle belly bunches in the lower arm, the Popeye sign, and strength is largely preserved.
- Distal biceps tendon rupture: typically in middle-aged men lifting a heavy load. Supination strength falls markedly, and the hook test fails to find the tendon in the cubital fossa. Early repair is usually offered.
- Reflexes: the biceps jerk tests C5 and C6; the triceps jerk tests C7 (with some C6 and C8).
- Radial nerve injury at the humeral shaft: from a midshaft fracture. It causes wrist drop, but triceps is usually spared because its branches arise higher up.
- Musculocutaneous nerve injury: rare in isolation. It weakens elbow flexion and supination and numbs the lateral forearm. Elbow flexion is partly preserved by brachioradialis and by the radial nerve supply to brachialis.