Flexor Carpi Ulnaris: Origin, Insertion, Nerve, Action & Cubital Tunnel

The Flexor Carpi Ulnaris (FCU) is the most medial muscle of the superficial anterior forearm. It is a powerful wrist flexor and acts as a crucial anatomical guide for the ulnar nerve. It is unique among the anterior forearm muscles because it is not innervated by the Median Nerve.

[Image of Flexor Carpi Ulnaris muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	1. Humeral Head: Medial Epicondyle (Common Flexor Tendon). 2. Ulnar Head: Olecranon and posterior border of the Ulna (via aponeurosis).
Insertion (Distal)	Pisiform bone (primary), then via ligaments to the Hook of Hamate and base of the 5th Metacarpal.
Nerve Supply	Ulnar Nerve (C7, C8) - *One of the few flexors not supplied by the Median Nerve.*
Blood Supply	Ulnar Artery.
Primary Actions	Flexion: Flexes the wrist joint. Ulnar Deviation: Adducts the wrist (moves it towards the pinky side).

Deep Dive: The Ulnar Nerve's Roof

The FCU is defined by its relationship with the Ulnar Nerve ("Funny Bone" nerve).

1. The Cubital Tunnel

The Ulnar Nerve passes behind the medial epicondyle and enters the forearm by passing between the two heads (Humeral and Ulnar) of the Flexor Carpi Ulnaris. The tendinous arch connecting these two heads is called Osborne's Ligament (or the Cubital Tunnel Retinaculum). Tightness here causes Cubital Tunnel Syndrome.

2. The Sesamoid Bone

The Pisiform is actually a sesamoid bone embedded directly inside the tendon of the Flexor Carpi Ulnaris. This increases the muscle's leverage, making it a very powerful flexor.

Physio Corner: Clinical Relevance

💪 Functional Fact: The FCU is essential for chopping motions (like using an axe or hammering). It works with the Extensor Carpi Ulnaris to stabilize the wrist in ulnar deviation, preventing the wrist from buckling upon impact.

Palpation

Ask the patient to flex their wrist and deviate toward the pinky against resistance. The FCU tendon is the most medial tendon on the anterior wrist, inserting directly into the Pisiform bone. It stands out sharply—you can't miss it!

⚠️ Clinical Pathology: Cubital Tunnel Syndrome
Compression of the ulnar nerve as it passes under the FCU arch at the elbow leads to numbness and tingling in the ring and little fingers ("sleeping hand"). Advanced cases lead to atrophy of the hand intrinsics (Claw Hand).

Manual Muscle Testing (MMT)

We combine flexion and ulnar deviation to isolate the FCU.

Testing Tip: Ensure the forearm is supinated (palm up) and resting on a table to eliminate gravity for Grade 2 testing.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Sitting. Forearm supinated (palm up) on table. Wrist hanging off edge. Action: Patient flexes the wrist and deviates toward the pinky (up and in). Resistance: Applied at the hypothenar eminence (base of 5th metacarpal), pushing down (extension) and toward the thumb (radial deviation). Grade 3: Full range against gravity. Grade 4/5: Holds against resistance.
Grade 2 (Gravity Eliminated)	Position: Forearm in neutral (thumb up/mid-position) resting on table. Action: Patient flexes the wrist (sliding across table). Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate the tendon just proximal to the Pisiform bone. Cue: "Try to bend your wrist toward your pinky." Grade 1: Tendon tension felt. Grade 0: No activity.

Frequently Asked Questions

Why isn't FCU supplied by the Median Nerve?

Embryologically, the medial side of the forearm flexor mass is supplied by the Ulnar nerve. The FCU and the medial half of the Flexor Digitorum Profundus are the only two muscles in the anterior forearm not supplied by the Median nerve.

Does FCU flex the elbow?

Weakly. Because its humeral head originates from the medial epicondyle, it crosses the elbow joint. However, its primary leverage is at the wrist.

What ligaments continue from the FCU tendon?

The Pisohamate Ligament (to the Hamate) and the Pisometacarpal Ligament (to the 5th Metacarpal). These ligaments effectively extend the insertion of the FCU beyond the Pisiform.

Test Your Knowledge: FCU Quiz

1. Which nerve supplies the Flexor Carpi Ulnaris?

2. The FCU inserts primarily onto which bone?

3. The Ulnar Nerve enters the forearm by passing:

4. Besides flexion, what is the action of the FCU?

5. Which muscle works with FCU to produce pure Ulnar Deviation?

6. Where does the Ulnar head of the FCU originate?

7. Cubital Tunnel Syndrome involves compression of the nerve at the:

8. During MMT, resistance is applied towards:

9. Which artery runs just lateral to the FCU tendon at the wrist?

10. True or False: FCU tendon passes through the Carpal Tunnel.

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Magee, D. J. (2014). Orthopedic Physical Assessment. 6th ed. Elsevier.