Now let’s look at the muscles which produce wrist movement. There are three flexors and three extensors.
We’ll look at the flexors first. The two important ones are flexor carpi radialis, and flexor carpi ulnaris. They both arise from the medial epicondyle, where they share a massive tendon of origin, the common flexor tendon, with two other flexor muscles. In addition, flexor carpi ulnaris has an extensive ulnar head, which arises from this border of the ulna.
The ulnar nerve, as we’ll see, passes between the two heads of flexor carpi ulnaris as it enters the forearm. The two wrist flexors diverge, to arrive at the radial and ulnar sides of the wrist. Flexor carpi radialis passes through a deep ligamentous tunnel, and ends up inserting on the base of the second metacarpal.
Flexor carpi ulnaris inserts on the pisiform bone. From the pisiform, the pull of flexor carpi ulnaris is transmitted to the hamate bone, and to the base of the fifth metacarpal, by these strong ligaments, the piso-hamate and piso-metacarpal ligaments.
The two wrist flexors, acting together, produce flexion of the wrist. Acting separately, the ulnar and radial flexors contribute to ulnar abduction, and radial abduction respectively.
Lying between these two main wrist flexors is a third small one, palmaris longus. It arises from the medial epicondyle, like the other two. Its tendon, seen here in the intact forearm, lies superficial to all its neighbors, and inserts not into bone, but into this dense layer of fascia, the palmar aponeurosis, which covers the palm of the hand. Through this soft tissue insertion, palmaris longus helps to flex the wrist. It’s frequently absent.
Now let’s go round to the other side of the forearm and see the wrist extensors. Here they are: extensor carpi radialis longus, and brevis, and extensor carpi ulnaris. Brachioradialis, which you’ll remember goes from here to here, has been removed in this dissection.
Extensor carpi radialis longus arises from the lateral epicondylar ridge, just below brachioradialis. Extensor radialis brevis arises from the lateral epicondyle, an origin which it shares with several other extensor muscles. They all arise together from the epicondyle and from this common extensor tendon.
Extensor carpi ulnaris arises from the lateral epicondyle, and it also has an ulnar head, which arises from this border of the ulna.
As the extensor tendons cross the back of the wrist they pass under this structure, the extensor retinaculum, which acts as a pulley. Extensor carpi radialis longus and brevis are inserted on the bases of the second and third metacarpals, extensor ulnaris on the base of the fifth metacarpal.
When the wrist extensors act together, they extend the wrist. That’s an important part of the action we make when we go to grip something. The powerful gripping muscles, whose tendons run over the front of the wrist, are slack and feeble when the wrist is flexed, but they become tight and powerful when it’s extended.
When the radial extensors, or the ulnar extensor contract separately, they help to produce radial or ulnar abduction of the wrist. They do this in conjunction with the corresponding wrist flexor muscle, either radial or ulnar.