Forearm Muscles

The muscles in the forearm can be broadly categorized as follows:

  1. Anterior Compartment

  2. Posterior Compartment

These categories are further subdivided as seen below.

Anterior Compartment

The muscles in the anterior compartment of the forearm are flexors and are responsible for pronation, flexion at the wrist joint and flexion of the digits. This group of muscles can be further categorized into the following layers:

  1. Superficial Compartment - all originate from the medial epicondyle of the humerus

    • Flexor Carpi Ulnaris

      • This muscle originates from the medial epicondyle of the humerus (humeral head) and from the olecranon of the ulna (ulnar head) and attaches to the pisiform bone, hook of hamate, and base of the fifth metacarpal bone.

      • This muscle acts to flex and adduct at the wrist joint.

      • This muscle is innervated by the ulnar nerve.

    • Palmaris Longus

      • This muscle originates from the medial epicondyle of the humerus and attaches to the flexor retinaculum of the wrist.

      • This muscle acts to flex at the wrist joint.

      • This muscle is innervated by the median nerve.

    • Flexor Carpi Radialis

      • This muscle originates from the medial epicondyle of the humerus and attaches to the base of the second and third metacarpals.

      • This muscle acts to flex and adduct at the wrist joint.

      • This muscle is innervated by the median nerve.

    • Pronator Teres - also forms the medial border of the cubital fossa.

      • This muscle originates from the medial epicondyle of the humerus and from the coronoid process of the ulna and attaches to the midshaft region of the radius.

      • This muscle acts to pronate the forearm.

      • This muscle is innervated by the median nerve.

  2. Intermediate Compartment

    • Flexor Digitorum Superficialis - median nerve and ulnar artery pass through the two heads of this muscle.

      • This muscle originates from the medial epicondyle of the humerus and radius and splits into four tendons at the wrist and passes through the carpal tunnel before attaching to the middle phalanges of each of the four digits.

      • This muscle acts to flex the metacarpophalangeal joints and the proximal interphalangeal joints at the four digits as well as flexing the wrist joint.

      • This muscle is innervated by the medial nerve.

  3. Deep Compartment

    • Flexor Digitorum Profundus

      • This muscle originates from the ulna and splits into four tendons at the wrist and passes through the carpal tunnel before attaching to the distal phalanges of each of the four digits.

      • This muscle acts to flex the metacarpophalangeal joints at the wrists and the interphalangeal joints.

      • This muscle is innevated by the ulnar nerve (medial half, only ring and little fingers) and the interosseous branch of the median nerve (lateral half, only middle and index fingers)

    • Flexor Pollicis Longus

      • This muscle originates from the anterior surface of the radius and surrounding interosseous membrane and attaches to the distal phalanx of the thumb.

      • This muscle acts to flex the metacarpophalangeal joint and interphalangeal joint of the thumb.

      • This muscle is innervated by the anterior interosseous branch of the median nerve.

    • Pronator Quadratus

      • This muscle is square shaped and originates from the anterior surface of the ulna and attaches to the anterior surface of the radius.

      • This muscle acts to pronate the forearm.

      • This muscle is innervated by the anterior interosseous branch of the median nerve.

Posterior Compartment

The muscles in the posterior compartment are extensors and are responsible for extension at the wrist joint and digits. This group of muscles can be further categorized as:

  1. Superficial Muscles

    • Brachioradialis

      • This muscle originates from the proximal aspect of the lateral supercondylar ridge of the humerus and attaches to the distal end of the radius (before the radial styloid process).

      • This muscle acts to flex at the elbow joint.

      • This muscle is innervated by the radial nerve.

    • Extensor Carpi Radialis Longus and Brevis

      • This muscle originates from the superconyla ridge (longus) and from the lateral epicondyle of the humerus (brevis) and attaches to the second and third metacarpals.

      • This muscle acts to extend and abduct at the wrist joint.

      • This muscle is innervated by the radial nerve.

    • Extensor Digitorum

      • This muscle originates from the lateral epicondyle of the humerus and splits into four before attaching to the extensor hood of each digit.

      • This muscle acts to extend the medial four digits at the metacarpophalangeal and interphalangeal joints.

      • This muscle is innervated by the radial nerve.

    • Extensor Digiti Minimi

      • This muscle originates from the lateral epicondyle of the humerus and attaches to the extensor hood of the little finger with the extensor digitorum tendon.

      • This muscle acts to extend the little finger while also contributing to extension at the wrist joint.

      • This muscle is innervated by the radial nerve.

    • Extensor Carpil Ulnaris

      • This muscle originates from the lateral epicondyle of the humerus and attaches to the base of the fifth metacarpal.

      • This muscle acts in extension and adduction of the wrist joint.

      • This muscle is innervated by the radial nerve.

    • Anconeus

      • This muscle originates from the lateral epicondyle of the humerus and attaches to the posterolateral aspect of the olecranon.

      • This muscle acts to extend and stabilize the elbow joint as well as abducting the ulna during forearm pronation.

      • This muscle is innervated by the radial nerve.

  2. Deep Muscles

    • Supinator - this muscle forms the floor of the cubital fossa

      • This muscle originates from the lateral epicondyle of the humerus and the posterior surface of the ulna and attaches to the posterior surface of the radius.

      • This muscle acts to supinate the forearm.

      • This muscle is innervated by the radial nerve.

    • Abductor Pollicis Longus - this muscle contributes to the lateral border of the anatomical snuffbox.

      • This muscle originates from the interosseous membrane and the posterior surfaces of the radius and ulna and attaches to the lateral base of the first metacarpal.

      • This muscle acts to abduct the thumb.

      • This muscle is innervated by the radial nerve.

    • Extensor Pollicis Brevis - this muscle contributes to the lateral border of the anatomical snuffbox.

      • This muscle originates from the interosseous membrane and the posterior surface of the radius and attaches to the proximal phalanx of the thumb.

      • This muscle acts to extend at the metacarpophalangeal and carpometacarpal joints of the thumb.

      • This muscle is innervated by the radial nerve.

    • Extensor Pollicis Longus - this muscle contributes to the medial border of the anatomical snuffbox.

      • This muscle originates from the interosseous membrane and posterior surface of the ulna and attaches to the distal phalanx of the thumb.

      • This muscle acts to extend the carpometacarpal, metacarpophalangeal, and interphalangeal joints of the thumb.

      • This muscle is innervated by the radial nerve.

    • Extensor Indicis

      • This muscle originates from the interosseous membrane and posterior surface of the ulna distal to the extensor pollicis longus and attaches to the extensor hood of the index finger.

      • This muscle acts to extend the index finger.

      • This muscle is innervated by the radial nerve.