Flexor Carpi Ulnaris Muscle Anatomy

Location & Overview

The flexor carpi ulnaris muscle is the most powerful wrist flexor. It has two heads, a humeral head and an ulnar head, which are linked by a tendinous arch. It is located in the anterior compartment of the forearm. It is one of the superficial flexor muscles of the forearm and the most medial of these superficial flexor muscles. The other superficial forearm flexors are: the palmaris longus, pronator teres, flexor carpi radialis, and the flexor digitorum superficialis ^[1].

The flexor carpi ulnaris muscle plays an important role in various activities. For instance, it is helpful in weightlifting exercises that involve wrist flexion and adduction. In racquet sports such as tennis or badminton, the flexor carpi ulnaris stabilises the wrist during hitting and provides the required force for wrist flexion and adduction during the swing. Similarly, in golf, it assists with generating the necessary force for wrist flexion and adduction during the swing, particularly in the follow-through. The flexor carpi ulnaris muscle also assists in everyday activities such as typing, writing by hand, and DIY home improvements by aiding in stabilising and holding tools like hammers and saws.

Here we can see the flexor carpi ulnaris from a posterior, lateral, and anterior view.

Here we can see the flexor carpi ulnaris from a superficial view amongst the other muscles of the arm. The flexor carpi ulnaris is highlighted in red.

Origin & Insertion

The flexor carpi ulnaris muscle has two heads, a humeral head and an ulnar head. These two heads arise at the origin points of this muscle. The humeral head originates from the common flexor tendon, on the medial epicondyle of the humerus. The ulnar head originates from the olecranon and the upper two thirds of the posterior border of the ulna. The two heads converge via a tendonous arch. The muscle then continues down the forearm crossing the wrist joint to insert onto the fifth metacarpal bone, the hook of hamate, and the pisiform bone of the wrist. The flexor carpi ulnaris inserts into the hook of hamate through the pisohamate ligament and inserts into the fifth metacarpal bone through the pisometacarpal ligament ^{[2] [3]}.

Here we can see the origin points of the flexor carpi ulnaris. The origin is highlighted in red on the olecranon of the ulna and medial epicondyle of the humerus (via the common flexor tendon).

Here we can see the insertion points of the flexor carpi ulnaris. The insertions are highlighted in blue on the pisiform, hamate and fifth metacarpal bones.

Here we can see the bones which the flexor carpi ulnaris inserts onto. These bones are colour coded for easy identification. We can also see the pisohamate ligament and the pisometacarpal ligament. The flexor carpi ulnaris inserts into the hamate through the pisohamate ligament and inserts into the fifth metacarpal bone through the pisometacarpal ligament.

This is why most depictions of this muscle do not show it connecting onto the hamate or the fifth metacarapal. The flexor carpi ulnaris does not directly connect to them. Rather, it is those two ligaments (which are is close proximity to the flexor carpi ulnaris) that are connecting these bones together and by proxy allowing the flexor carpi ulnaris to interact with them. However, these insertions on the hamate and fifth metacarpal are still considered insertions of the flexor carpi ulnaris.

Actions

The actions of the flexor carpi ulnaris are flexion and adduction of the hand at the wrist joint (radiocarpal joint). In addition to flexion and adduction, the flexor carpi ulnaris can also flex the elbow joint. However, it is very weak when it comes to elbow flexion and this is by no means a main action of the flexor carpi ulnaris ^{[4] [5]}.

This image shows wrist flexion, which is the movement of bending the wrist in the direction of the underneath of the forearm (anterior surface of the forearm). The flexor carpi ulnaris is a wrist flexor. The opposite of wrist flexion would be wrist extension which involves extending the wrist in the direction of the top of the forearm (posterior surface of the forearm).

This image shows wrist adduction, which is the movement of bending the wrist to the side. The flexor carpi ulnaris is a wrist adductor. During wrist adduction, the wrist bends away from the thumb, which would be towards the body when the wrist is in the anatomical position. The opposite movement of wrist adduction is wrist abduction.

This image demonstrates elbow flexion, which involves bending elbow. The opposite movement of elbow flexion is elbow extension. The flexor carpi ulnaris can assist very weakly in elbow flexion.

Innervation

The flexor carpi ulnaris is innervated by the muscular branch of the ulnar nerve (C7, C8, T1). The ulnar nerve passes between the two heads of the flexor carpi ulnaris ^{[6] [7] [8] [9]}.

Pictured here we can see the ulnar nerve which innervates the flexor carpi ulnaris. The ulnar nerve is highlighted in red on the left of the picture.

Blood Supply

Blood is supplied to the flexor carpi ulnaris primarily via the ulnar artery (86% of cases) and the recurrent ulnar artery (14% of cases). The ulnar artery arises from the brachial artery ^{[10] [11]}.