The flexor carpi ulnaris (FCU) is a long, superficial muscle located along the inner border of the forearm. It is the primary muscle responsible for bending the wrist and tilting it toward the little finger, and it also serves as a key landmark for surgeons and therapists. This article explains its anatomy, function, blood and nerve supply, clinical importance, and how to test it in practice.
The flexor carpi ulnaris runs along the ulnar side of the forearm, forming the medial boundary of the anterior compartment. It is a flat, fusiform muscle that becomes a distinct tendon near the wrist, making it easy to identify on physical examination.
The flexor carpi ulnaris has two heads of origin. The humeral head arises from the medial epicondyle of the humerus, while the ulnar head arises from the olecranon and the upper two-thirds of the posterior border of the ulna.
The two heads converge into a single muscle belly and then form a strong tendon that attaches to several carpal and metacarpal structures:
Because the tendon attaches to the pisiform, which acts like a sesamoid bone, the force of the FCU is transferred through ligaments to the hamate and the fifth metacarpal. This arrangement helps stabilize the ulnar side of the wrist during gripping and pushing movements.
The flexor carpi ulnaris is innervated by muscular branches of the ulnar nerve, with fibers from the C7, C8, and T1 spinal nerve roots. This makes it one of the few muscles in the forearm that is supplied by the ulnar nerve without any contribution from the median nerve.
The blood supply comes mainly from muscular branches of the ulnar artery, with additional contributions from the recurrent ulnar artery near the elbow. Because the ulnar nerve passes between the two heads of the FCU and then travels beneath its tendon, the muscle is a surgical landmark for exposing the neurovascular bundle in the distal forearm.
The flexor carpi ulnaris performs three primary actions at the wrist:
During pure wrist flexion, the FCU works together with the flexor carpi radialis. During ulnar deviation, it acts alongside the extensor carpi ulnaris. When both muscles on the ulnar side contract together, they produce a strong adduction movement that is important in activities such as hammering, throwing, and racquet sports.
"The flexor carpi ulnaris is one of the few muscles in the forearm that gives clinicians a direct window to the ulnar neurovascular bundle. Its dual origin and distal bone attachments make it a key structure in wrist surgery."
The flexor carpi ulnaris is clinically important for several reasons. It is a frequent source of wrist pain, it can be involved in nerve compression syndromes, and it often becomes tight or weak in people who perform repetitive hand tasks.
Palpating the flexor carpi ulnaris is straightforward if you know where to look. Ask the patient to flex the wrist and tilt it toward the little finger while you provide resistance. You will feel the muscle belly contract along the medial forearm, and its tendon will become prominent just above the pisiform.
For a formal manual muscle test:
If the patient shows weakness or pain during this test, the FCU and its distal attachment should be examined more closely with ultrasound to rule out tears or tendinopathy.
"When a patient reports medial wrist pain that worsens with resisted wrist flexion, the flexor carpi ulnaris and its distal tendon should be among the first structures you assess."
Rehabilitation of the flexor carpi ulnaris depends on the underlying diagnosis. For acute strains, a short period of rest, ice, and activity modification is usually enough. For chronic tendinopathy, progressive loading and eccentric strengthening are more effective than passive rest alone.
When the condition is related to ulnar nerve or artery compression, rehabilitation alone is not enough. Refer the patient for nerve conduction studies or vascular imaging and consider surgical consultation if conservative management fails.
| Feature | Details |
|---|---|
| Muscle type | Skeletal, fusiform |
| Compartment | Superficial anterior forearm |
| Origin | Medial epicondyle of humerus; olecranon and posterior ulna |
| Insertion | Pisiform; hook of hamate; base of fifth metacarpal |
| Nerve supply | Ulnar nerve (C7, C8, T1) |
| Blood supply | Ulnar artery and recurrent ulnar artery |
| Main actions | Wrist flexion; ulnar deviation; pisiform stabilization |
| Antagonist | Extensor carpi radialis longus and brevis |
The flexor carpi ulnaris is a compact but powerful muscle that runs from the medial elbow to the pisiform bone at the wrist. Knowing its origin, insertion, innervation, and actions is essential for diagnosing wrist pain, performing manual muscle tests, and planning rehabilitation programs. Its close relationship with the ulnar nerve and artery also makes it a critical landmark in hand surgery and in the assessment of ulnar-sided wrist complaints.
The flexor carpi ulnaris is a superficial muscle of the anterior forearm that flexes the wrist and deviates the hand toward the little finger. It has two heads, one from the medial epicondyle of the humerus and one from the ulna, and it attaches distally to the pisiform, hamate, and fifth metacarpal.
The flexor carpi ulnaris is innervated by muscular branches of the ulnar nerve, carrying fibers from the C7, C8, and T1 spinal roots. This is notable because most other superficial forearm flexors are supplied by the median nerve.
It originates from two sites: the medial epicondyle of the humerus via the common flexor tendon, and the olecranon and upper two-thirds of the posterior border of the ulna. These two heads unite to form the muscle belly.
The muscle inserts onto the pisiform bone, and then continues through ligaments to the hook of the hamate and the base of the fifth metacarpal. This indirect attachment allows the muscle to stabilize the ulnar column of the wrist.
It produces wrist flexion and ulnar deviation, and it helps stabilize the pisiform during forceful gripping. When acting alone, it tilts the hand toward the little finger while also bending the wrist forward.
It is a superficial muscle of the anterior forearm, located in the outermost layer. You can see its tendon at the wrist, especially when you flex and ulnar-deviate the hand against resistance.
The ulnar artery runs beneath the flexor carpi ulnaris muscle in the distal forearm. The muscle provides a protective cover for the artery and the ulnar nerve, which is why it is often used as a surgical landmark when approaching these structures.
To test it, ask the patient to flex the wrist and tilt it toward the little finger while you push against the palm in the opposite direction. You can also palpate the tendon proximal to the pisiform to confirm that the muscle is contracting.
Common conditions include FCU tendinopathy, acute muscle strain, ulnar nerve entrapment at Guyon's canal, and hypothenar hammer syndrome involving the ulnar artery. These cause medial wrist pain, weakness, or tingling in the little finger.
