Muscles of forearm

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Muscles of the Forearm

The forearm (antebrachium) muscles are organized into three groups: anterior (flexor) compartment, posterior (extensor) compartment, and the radial (lateral) group. All are innervated by branches of the brachial plexus (C5-T1).

Anterior (Flexor) Compartment

Anterior view - superficial flexors:

Superficial Layer (5 muscles)

All 5 share a common origin from the medial epicondyle of the humerus ("common flexor head").

Muscle	Origin	Insertion	Action	Nerve
Pronator teres	Medial epicondyle (humeral head) + coronoid process of ulna (ulnar head)	Lateral surface of radius (mid-shaft)	Pronation; weak elbow flexion	Median (C6)
Flexor carpi radialis	Medial epicondyle	Base of 2nd (±3rd) metacarpal	Wrist flexion, radial deviation; weak pronation	Median (C6-C8)
Palmaris longus	Medial epicondyle	Palmar aponeurosis	Wrist flexion; tightens palmar aponeurosis	Median (C8-T1)
Flexor carpi ulnaris	Medial epicondyle (humeral head) + olecranon (ulnar head)	Pisiform, hook of hamate, base of 5th metacarpal	Wrist flexion, ulnar deviation	Ulnar (C7-T1)
Flexor digitorum superficialis	Medial epicondyle + coronoid process + proximal radius	Sides of middle phalanges (digits 2-5)	Flexion at PIP, MCP, wrist; weak elbow flexion	Median (C7-T1)

Note: Flexor digitorum superficialis is the only muscle that forms an intermediate layer (between superficial and deep groups), as it covers the deep flexors.

Deep Layer (3 muscles)

Muscle	Origin	Insertion	Action	Nerve
Flexor digitorum profundus	Proximal 2/3 of anterior ulna + interosseous membrane	Palmar surface of distal phalanges (digits 2-5)	Flexion at DIP, PIP, MCP, wrist	Median (radial half: digits 2-3, C8-T1); Ulnar (ulnar half: digits 4-5, C8-T1)
Flexor pollicis longus	Anterior radius (mid-shaft) + interosseous membrane	Palmar distal phalanx of thumb	Flexion at MCP + IP of thumb; wrist flexion, radial deviation	Median (C8-T1)
Pronator quadratus	Distal 1/4 of anterior ulna	Distal 1/4 of anterior radius	Pronation; stabilizes distal radioulnar joint	Median (C8-T1)

Posterior (Extensor) Compartment

Posterior view - superficial + deep extensors:

Superficial Layer (3 muscles)

All arise from the lateral epicondyle via the common extensor head. All are innervated by the radial nerve (C7, C8).

Muscle	Insertion	Action
Extensor digitorum	Dorsal digital expansion of digits 2-5	Extension of MCP, PIP, DIP (digits 2-5); wrist extension
Extensor digiti minimi	Dorsal digital expansion of digit 5	Extension and abduction of digit 5; wrist extension + ulnar deviation
Extensor carpi ulnaris	Base of 5th metacarpal	Wrist extension, ulnar deviation

Deep Layer (5 muscles)

All are innervated by the radial nerve (C7, C8) (deep branch/posterior interosseous nerve).

Muscle	Origin	Insertion	Action
Supinator	Lateral epicondyle + olecranon/supinator crest of ulna + radial collateral ligament + anular ligament	Radius (between radial tuberosity and pronator teres insertion)	Supination
Abductor pollicis longus	Dorsal radius, ulna + interosseous membrane	Base of 1st metacarpal	Thumb abduction; radial deviation of wrist
Extensor pollicis brevis	Posterior radius + interosseous membrane	Base of proximal phalanx of thumb	Extension of thumb (CMC, MCP); radial deviation
Extensor pollicis longus	Posterior ulna + interosseous membrane	Base of distal phalanx of thumb	Full thumb extension (CMC, MCP, IP); wrist radial deviation
Extensor indicis	Posterior ulna + interosseous membrane	Dorsal digital expansion of digit 2	Extension of index finger (MCP, PIP, DIP); wrist extension

The tendons of abductor pollicis longus and extensor pollicis brevis form the anatomical snuffbox (anterior boundary), while extensor pollicis longus forms its posterior boundary.

Radial (Lateral) Group

These muscles sit between the anterior and posterior compartments and are innervated by the radial nerve. They are sometimes grouped with the posterior compartment.

Muscle	Origin	Insertion	Action	Nerve
Brachioradialis	Lateral surface of distal humerus (lateral supracondylar ridge) + lateral intermuscular septum	Styloid process of radius	Elbow flexion; semipronation (brings forearm to neutral)	Radial (C5-C6)
Extensor carpi radialis longus	Lateral supracondylar ridge + lateral intermuscular septum	Dorsal base of 2nd metacarpal	Wrist extension + radial deviation; assists in fist closure	Radial (C6-C7)
Extensor carpi radialis brevis	Lateral epicondyle	Dorsal base of 3rd metacarpal	Wrist extension + radial deviation; assists in fist closure	Radial (C7-C8)

Quick Reference: Innervation Summary

Nerve	Muscles Supplied
Median nerve	All anterior compartment muscles EXCEPT flexor carpi ulnaris and medial half of flexor digitorum profundus
Ulnar nerve	Flexor carpi ulnaris + medial half of flexor digitorum profundus (digits 4-5)
Radial nerve	All posterior compartment muscles + radial group

Key Clinical Points

Medial epicondyle is the common origin for all superficial flexors - inflammation at this point is golfer's elbow (medial epicondylitis)
Lateral epicondyle is the common origin for superficial extensors - inflammation is tennis elbow (lateral epicondylitis)
Pronator teres syndrome: median nerve compressed between the two heads of pronator teres, causing weakness of all median-innervated forearm/hand muscles
Palmaris longus is absent in ~14% of people and is the most common tendon used for grafts
Anterior interosseous nerve (branch of median) supplies the deep flexors - its injury causes inability to make an "OK" sign (no flexion at DIP of index + IP of thumb)

Source: General Anatomy and Musculoskeletal System, THIEME Atlas of Anatomy, pp. 333-340

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