Origin: Posterior surface of the radius and the dorsal interosseous membrane.
Insertion: Dorsal base of the first phalanx (proximal phalanx) of the thumb.
Innervation: Radial nerve.
Action: Extends the thumb at the metacarpophalangeal joint and radially abducts the wrist.
Relations: It is related to the abductor pollicis longus and the extensor pollicis longus. It is covered by the extensor digitorum, and its tendon contributes to the border of the anatomical snuffbox along with the first and second radial wrist extensors.
Extensor Pollicis Longus
Origin: Posterior surface of the ulna, the posterior face of the interosseous membrane, and the intermuscular septum separating it from the posterior ulnar group.
Insertion: Dorsal base of the second phalanx (distal phalanx) of the thumb.
Innervation: Radial nerve.
Action: Extends the wrist, radially abducts, and extends the second phalanx of the thumb.
Relations: It is covered by the extensor digitorum, extensor digiti minimi, and extensor carpi ulnaris. It lies in the same plane as the supinator, abductor pollicis longus, extensor pollicis brevis, and extensor indicis.
Extensor Indicis
Origin: Posterior surface of the ulna, below the extensor pollicis longus, and from the interosseous membrane. It passes through the fourth compartment of the carpus.
Insertion: Third phalanx (distal phalanx) of the index finger.
Innervation: Radial nerve.
Action: Extends the index finger and supports the extension of the wrist.
Relations: It is covered by the extensor digitorum and is also related to the extensor pollicis longus.
Abductor Pollicis Longus
Origin: Posterior part of the ulna, the posterior part of the interosseous membrane, and some fibers from the posterior radius.
Insertion: Base of the first metacarpal, with a short extension to the trapezium.
Innervation: Radial nerve.
Action: Abducts the thumb, flexes the wrist, and acts as a radial abductor of the wrist.
Relations: It is in the same plane as the extensor pollicis longus and brevis. It is covered by the extensor digitorum, extensor digiti minimi, and the radial wrist extensors.
Supinator
Origin: Lateral epicondyle of the humerus, the radial collateral ligament, and the supinator crest on the ulna. It passes behind the radius, embracing it.
Insertion: Posterior surface of the lateral edge of the radius, both proximal and distal to the radial tuberosity.
Innervation: Radial nerve.
Action: Supinates the forearm.
Relations: It is related to the pronator teres and is covered by the brachioradialis and the short and long radial wrist extensors.
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