Contents · 9 sections
The muscles of the upper extremity are one of the most comprehensive and practically significant sections of anatomy. Surgeons, traumatologists, and neurologists deal with these structures on a daily basis. This article provides a complete group-by-group classification, innervation, functions, and clinical correlations, complete with tables and exam mnemonics.
General classification of upper extremity muscles
The muscles of the upper extremity are divided into two major groups:
- Muscles of the upper limb girdle (pectoral girdle) — connect the scapula and clavicle to the humerus
- Muscles of the free upper extremity — muscles of the arm, forearm, and hand
There are over 50 muscles in total in the upper extremity. Each group has anterior (flexors) and posterior (extensors) compartments, determined by their position relative to the axes of joint movement.
Muscles of the shoulder girdle
The muscles of the pectoral girdle include six muscles that provide movement at the shoulder joint:
| Muscle | Origin | Insertion | Function | Innervation |
|---|---|---|---|---|
| Deltoid | Clavicle, acromion, spine of scapula | Deltoid tuberosity of humerus | Abduction of arm (anterior part — flexion, posterior — extension) | Axillary nerve (C5-C6) |
| Supraspinatus | Supraspinous fossa of scapula | Greater tubercle of humerus | Initiates abduction (first 15°) | Suprascapular nerve (C5-C6) |
| Infraspinatus | Infraspinous fossa of scapula | Greater tubercle | External rotation of the arm | Suprascapular nerve (C5-C6) |
| Teres minor | Lateral border of scapula | Greater tubercle | External rotation | Axillary nerve (C5-C6) |
| Teres major | Inferior angle of scapula | Crest of lesser tubercle | Adduction, internal rotation, extension | Subscapular nerve (C5-C7) |
| Subscapularis | Subscapular fossa | Lesser tubercle | Internal rotation | Subscapular nerve (C5-C7) |
Rotator cuff
Four muscles form the rotator cuff — a crucial structure for shoulder joint stability:
SITS mnemonic:
- Supraspinatus
- Infraspinatus
- Teres minor
- Subscapularis
The tendons of these muscles surround the head of the humerus superiorly, posteriorly, and anteriorly, forming a "cuff." Rotator cuff tears are one of the most common causes of shoulder pain in patients over 40. The supraspinatus tendon is injured most frequently (it passes beneath the acromion, the impingement zone).
Muscles of the arm
Anterior compartment (flexors)
Innervated by the musculocutaneous nerve (n. musculocutaneus, C5-C7):
- Biceps brachii (m. biceps brachii) — flexion at the elbow joint, supination of the forearm. The long head originates from the supraglenoid tubercle of the scapula, the short head from the coracoid process
- Coracobrachialis (m. coracobrachialis) — flexion and adduction of the arm
- Brachialis (m. brachialis) — main flexor of the forearm (stronger than the biceps!)
Posterior compartment (extensors)
Innervated by the radial nerve (n. radialis, C6-C8):
- Triceps brachii (m. triceps brachii) — the sole extensor of the forearm. Three heads: long, lateral, medial
- Anconeus muscle (m. anconeus) — assists extension, tenses the capsule of the elbow joint
Forearm muscles
The most complex group — 20 muscles in four layers. Key for the exam:
| Group | Main muscles | Innervation | Function |
|---|---|---|---|
| Anterior superficial | Pronator teres, flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis | Median nerve (except flexor carpi ulnaris — ulnar nerve) | Wrist and finger flexion, pronation |
| Anterior deep | Flexor digitorum profundus, flexor pollicis longus, pronator quadratus | Median + ulnar nerve | Flexion of distal phalanges, pronation |
| Posterior superficial | Extensor carpi radialis longus and brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris | Radial nerve | Wrist and finger extension |
| Posterior deep | Supinator, abductor pollicis longus, extensor pollicis brevis and longus | Posterior interosseous nerve (branch of radial) | Supination, thumb movements |
Hand muscles
Three groups of intrinsic hand muscles:
- Thenar muscles (base of the thumb): abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, adductor pollicis
- Hypothenar muscles (base of the little finger): abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi
- Central group: lumbricals (4) — flexion at metacarpophalangeal joints; palmar (3) and dorsal (4) interosseosti — adduction and abduction of fingers
Mnemonic for interossei: "PAD-DAB" — Palmar ADduct, Dorsal ABduct.
Clinical correlations
Upper limb nerve injuries produce characteristic clinical presentations:
| Injury | Clinical Practice | Cause |
|---|---|---|
| Erb-Duchenne palsy (C5-C6) | "Waiter's tip hand" — arm hangs by the side, medially rotated, extended at the elbow | Birth trauma, arm traction |
| Radial nerve injury | "Wrist drop" — inability to extend the wrist and fingers | Humeral shaft fracture |
| Median nerve injury | "Hand of benediction" — inability to flex I-III fingers into a fist | Wound in the cubital fossa |
| Carpal tunnel syndrome | Paresthesia of I-III fingers, thenar atrophy | Compression of the median nerve in the carpal tunnel |
| Ulnar nerve injury | "Claw hand" — hyperextension at metacarpophalangeal joints of IV-V fingers | Medial epicondyle fracture |
Exam mnemonics
- SITS — rotator cuff (Supraspinatus, Infraspinatus, Teres minor, Subscapularis)
- PAD-DAB — function of interosseous muscles
- "Median prepares — ulnar cleans up" — the median nerve supplies flexors (prepares), the ulnar nerve supplies adductors and fine motor skills (cleans up)
- Radial = extensors — injury → wrist drop (cannot extend)
Frequently asked questions
Which muscle initiates arm abduction?
The supraspinatus muscle (m. supraspinatus) initiates abduction in the first 15°, after which the deltoid muscle takes over the primary action. This is why in a supraspinatus tear the patient cannot initiate abduction, but if helped to raise the arm to 15-20°, they can manage the rest.
What is the difference between "claw hand" and "hand of benediction"?
"Claw hand" (ulnar nerve injury) involves hyperextension at the metacarpophalangeal joints and flexion at the interphalangeal joints of the IV-V fingers. "Hand of benediction" (median nerve injury) is the inability to flex the I-III fingers when attempting to make a fist.
Which nerve is most commonly injured in a humerus fracture?
The radial nerve — it runs in the spiral groove of the humerus and is particularly vulnerable in fractures of the middle third of the shaft.
Interactive upper limb anatomy lessons with tests and fill-in-the-blanks are available on the subject page. To reinforce your knowledge, use anatomy tests with answers, and for working with the skull, check out the article on skull bones. Also, see cervical triangles for topographical anatomy.