Flexor Digitorum Profundus: Origin, Insertion, Dual Nerve & Jersey Finger

The Flexor Digitorum Profundus (FDP) is a deep muscle of the anterior forearm. The word Profundus means "deep." It is the only muscle capable of flexing the distal interphalangeal (DIP) joints—in other words, it is the only muscle that can curl your fingertips.

[Image of Flexor Digitorum Profundus muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Proximal 3/4 of the anterior and medial surfaces of the Ulna and Interosseous Membrane.
Insertion (Distal)	Bases of the Distal Phalanges of the medial four digits (2nd-5th fingers).
Nerve Supply	Lateral Half (Index/Middle): Anterior Interosseous Nerve (Median branch). Medial Half (Ring/Little): Ulnar Nerve.
Blood Supply	Anterior Interosseous Artery.
Primary Actions	Primary: Flexes the DIP joints. Secondary: Assists in flexing the PIP, MCP, and Wrist joints.

Deep Dive: The "Hybrid" Muscle

The FDP is anatomically significant because of its dual innervation and its relationship with the superficial flexor (FDS).

1. Dual Innervation

The FDP is one of the few "hybrid" muscles in the body (supplied by two different nerves):
• The part moving the Index & Middle fingers is powered by the Median Nerve (specifically the Anterior Interosseous Nerve).
• The part moving the Ring & Little fingers is powered by the Ulnar Nerve.
(This is why in Ulnar nerve palsy, you lose the grip strength of the pinky and ring fingers).

2. The "Camper's Chiasm"

To reach the fingertip, the FDP tendon must pass through the tendon of the Flexor Digitorum Superficialis (FDS). At the proximal phalanx, the FDS tendon splits into two legs, creating a tunnel (Camper's Chiasm) for the FDP to pass through and insert distally.

Physio Corner: Clinical Relevance

💪 Functional Fact: While FDS is used for speed and general grasping, the FDP is essential for Power Grip (like holding a heavy suitcase or doing a pull-up). The curl of the fingertips locks the object into the hand.

Palpation

It is difficult to palpate the belly as it lies deep to the FDS and Flexor Carpi Ulnaris. However, you can palpate the tendons on the volar surface of the middle phalanx of the fingers, especially when the FDS is relaxed.

⚠️ Clinical Pathology: Jersey Finger
This is an avulsion of the FDP tendon from the distal phalanx. It typically happens in football/rugby when a player grabs an opponent's jersey, and the finger is forcefully extended while actively flexing. It most commonly affects the Ring Finger. The patient will be unable to flex the DIP joint (fingertip) in isolation.

Manual Muscle Testing (MMT)

To test the FDP, you must isolate the DIP joint.

Testing Tip: You must stabilize the Middle Phalanx. If you don't, the FDS will flex the PIP joint, and it will look like the patient is strong even if their FDP is paralyzed.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Forearm supinated, hand flat on table. Stabilization: Therapist firmly holds the middle phalanx of the finger being tested. Action: Patient flexes the tip of the finger (DIP joint). Resistance: Applied to the pulps of the distal phalanx, pushing down into extension. Grade 3: Full range of motion. Grade 4/5: Holds against strong pressure.
Grade 0, 1 (Palpation)	Action: Palpate the tendon on the palmar aspect of the middle phalanx. Cue: "Try to scratch the table with your fingertip." Grade 1: Trace tension felt. Grade 0: No activity.

Frequently Asked Questions

What is the Lumbrical connection?

The four Lumbrical muscles of the hand originate directly from the tendons of the FDP in the palm. This means the FDP acts as the "bone" of origin for the lumbricals.

Why is the Ring Finger most prone to Jersey Finger?

The ring finger fingertip is anatomically tethered. When gripping, the ring finger is often the last to release, taking the full force of extension against a contracted muscle.

How is it different from FDS?

FDS (Superficialis): Inserts on Middle Phalanx, flexes PIP joint, split tendon.
FDP (Profundus): Inserts on Distal Phalanx, flexes DIP joint, passes through the split.

Test Your Knowledge: FDP Quiz

1. Where does the FDP insert?

2. Which nerve supplies the FDP to the index finger?

3. What is the specific action of FDP that FDS cannot do?

4. Jersey Finger is an avulsion of which tendon?

5. The FDP tendons pass through the split in which other tendons?

6. Where does the FDP originate?

7. Which muscles originate from the FDP tendons in the palm?

8. Which nerve supplies the FDP to the Ring and Little fingers?

9. To isolate FDP during testing, you must stabilize the:

10. "Profundus" translates to:

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.