Palmaris longus muscle

Origin and insertion

Palmaris longus muscle originates from the medial epicondyle of humerus, via the common flexor origin. A tendinous hub that it shares with the five long forearm flexors; flexor carpi radialis, flexor carpi ulnaris, pronator teres, flexor digitorum superficialis and profundus.

The muscle has a relatively short inferior course, giving off a long tendon midway down the forearm. The tendon enters the palmar surface of the hand by passing superficially (i.e. external) to the flexor retinaculum. A few tendinous fibers blend with the superior surface of the retinaculum, while the majority widens and continues distally to finally insert into the palmar aponeurosis.

Relations

Palmaris longus is the most superficial muscle of the superficial forearm flexors, found deep to the forearm skin. It sits between the flexor carpi radialis and flexor carpi ulnaris muscles, superficial to the flexor digitorum superficialis muscle.

Just proximal to the wrist, the palmaris longus tendon sits medial to that of flexor carpi radialis, and the median nerve passes between the tendons of these two muscles. At the wrist level, the median nerve lies directly below the palmaris longus tendon.

Innervation

Palmaris longus is innervated by the median nerve (C7, C8), a branch from the medial and lateral cords of brachial plexus.

Blood supply

Blood supply to the palmaris longus muscle comes from a branch of the anterior ulnar recurrent artery, itself a branch of the ulnar artery. If the median artery is well developed, it contributes to the blood supply as well.

Function

Being located centrally in the anterior forearm, palmaris longus aids the flexor carpi ulnaris and flexor carpi radialis muscles to perform a balanced flexion of the hand on the wrist. It also acts to stabilize the elbow joint when fully extended, as does the other forearm muscles that attach to the humerus and thus cross the elbow joint.

Fibers attaching to the palmar aponeurosis tighten this fascial sheath during the muscle’s contraction, resulting in a weak flexion of the 2nd to 5th metacarpophalangeal joints as the aponeurosis is being pulled towards the wrist. Tightening of the palmar aponeurosis also contributes to maintaining the hand’s grip while holding certain objects.

If you feel ready, take this quiz below to test your knowledge about the palmaris longus and other anterior muscles of the forearm!