Flexor Digitorum Superficialis: Origin, Insertion, Action & The "Split" Tendon

The Flexor Digitorum Superficialis (FDS), formerly known as the Flexor Digitorum Sublimis, is the sole muscle of the intermediate layer of the anterior forearm. It allows for speed and dexterity in finger flexion and is famous for its unique "split" tendon mechanism.

[Image of Flexor Digitorum Superficialis muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Humeroulnar Head: Medial Epicondyle of Humerus and Coronoid Process of Ulna. Radial Head: Upper half of the anterior border of the Radius.
Insertion (Distal)	Splits into 4 tendons which attach to the sides of the Middle Phalanges of the medial four digits (2-5).
Nerve Supply	Median Nerve (C7, C8, T1).
Blood Supply	Ulnar and Radial Arteries.
Primary Actions	PIP Flexion: Flexes the Proximal Interphalangeal joints (primary). Secondary: Flexes the MCP joints and the Wrist.

Deep Dive: Camper's Chiasm

The defining anatomical feature of the FDS is how its tendons interact with the deeper muscle, the Flexor Digitorum Profundus (FDP).

1. The Split (Chiasm)

The FDS is "Superficial," meaning it sits on top of the "Profundus." However, the FDS inserts on the Middle Phalanx, while the Profundus must reach the Distal Phalanx.

To solve this traffic jam, the FDS tendon splits into two legs (like a snake's tongue) just before insertion. The FDP tendon shoots through this gap (called Camper's Chiasm) to reach the fingertip.

2. The Median Nerve Bridge

The two heads of origin (Humeroulnar and Radial) form a fibrous arch. The Median Nerve and Ulnar Artery pass underneath this arch to travel down the forearm. Compression here acts as a potential site for "Pronator Syndrome" (high median nerve compression).

Physio Corner: Clinical Relevance

💪 Functional Fact: The FDS is used for forceful grip when the wrist is flexed, but also for delicate movements like typing or playing piano. Unlike the FDP (which works en masse), the FDS has more independent control over individual fingers.

Palpation

Ask the patient to flex their fingers rapidly. Palpate the anterior forearm just medial to the midline. You can feel the individual muscle bellies of the FDS contracting. At the wrist, the tendons pass through the Carpal Tunnel superficial to the FDP tendons.

⚠️ Clinical Pathology: Carpal Tunnel Syndrome
The Carpal Tunnel contains 9 tendons and 1 nerve. The 4 tendons of the FDS and the 4 tendons of the FDP make up the bulk of the tunnel's contents. Inflammation (tenosynovitis) of the FDS sheaths is a common cause of Median nerve compression.

Manual Muscle Testing (MMT)

To test the FDS, you must mechanically inhibit the Flexor Digitorum Profundus (FDP).

Testing Tip: The FDP shares a common muscle belly for the middle, ring, and little fingers. If you hold these fingers in extension, you pull the FDP tendons taut, rendering the FDP ineffective on the free finger. This isolates the FDS.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Supine or sitting. Hand supinated on table. Stabilization: The therapist holds all fingers except the test finger in full extension (flat against the table). This blocks the FDP. Action: Patient flexes the PIP joint of the free finger. Resistance: Applied to the head of the Middle Phalanx, pushing down into extension. Grade 3: Full PIP flexion. Grade 4/5: Holds against resistance.
Grade 0, 1 (Palpation)	Action: Palpate the tendon at the wrist or the muscle belly in the forearm. Cue: "Try to bend your middle knuckle." Grade 1: Trace contraction. Grade 0: No activity.

Frequently Asked Questions

Does FDS flex the tip of the finger?

No. The FDS stops at the Middle Phalanx. Therefore, it acts on the PIP joint. The tip (DIP joint) is flexed exclusively by the Flexor Digitorum Profundus (FDP).

Can some people lack an FDS tendon?

Yes. It is relatively common to have a weak or absent FDS tendon to the Little Finger (5th digit). In these individuals, pinky flexion is driven entirely by the FDP.

What is the difference between "Superficialis" and "Sublimis"?

They are the same muscle. "Sublimis" is the older Latin term meaning "high" or "superficial." "Superficialis" is the modern anatomical term. You may hear older clinicians use "Sublimis."

Test Your Knowledge: FDS Quiz

1. Where does the Flexor Digitorum Superficialis insert?

2. Which nerve supplies the FDS?

3. What is Camper's Chiasm?

4. Which joint is the primary target of the FDS?

5. To isolate FDS during testing, you must:

6. FDS is located in which compartment of the forearm?

7. How many tendons does FDS send through the Carpal Tunnel?

8. Does the FDS flex the thumb?

9. Which head of the FDS originates from the radius?

10. "Swan Neck Deformity" involves hyperextension of which joint?

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.