The muscles of the forearm that move the wrist, hand, and digits are numerous and diverse. They can be classified into two groups based on their location and function — the anterior and posterior compartment muscles.
Anterior Compartment
The anterior compartment muscles originate from the humerus. They primarily function as flexors and are also known as flexor muscles. They typically insert on the carpals, metacarpals, and phalanges. The superficial layer includes the flexor carpi radialis, palmaris longus, and flexor carpi ulnaris. Below these lies the flexor digitorum superficialis, one of the largest superficial muscles in the forearm, which plays a crucial role in flexing the digits. There are two muscles in the deep anterior compartment. The flexor pollicis longus is responsible for flexing the distal phalanx of the thumb. The flexor digitorum profundus, however, ends in four tendons that insert into the distal phalanges of the fingers.
Posterior Compartment
The posterior compartment muscles originate from the humerus and function primarily as extensors. The superficial layer includes the extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, and extensor carpi ulnaris. The extensor digitorum occupies a significant portion of the posterior surface of the forearm and divides into four tendons that insert into the middle and distal phalanges of the fingers. The extensor carpi ulnaris is located medially in this group. In the deep posterior compartment are four muscles — the abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis. These muscles play a crucial role in extending the thumb and fingers.
The Retinaculum
The tendons of these forearm muscles that attach to the wrist or continue into the hand are held close to the bones by strong fasciae. Tendon sheaths surround the tendons, providing protection and reducing friction. At the wrist, the deep fascia forms fibrous bands called retinacula, which hold the tendons in place. The flexor retinaculum is located on the palmar surface of the carpal bones, forming the carpal tunnel where the median nerve and tendons of the flexor digitorum superficialis, flexor digitorum profundus, and flexor pollicis longus pass through. On the dorsal surface of the carpal bones, the extensor retinaculum holds the extensor tendons of the wrist and digits.
