Flexor Digitorum Longus: Origin, Insertion, Nerve & The "Gripping" Muscle

The Flexor Digitorum Longus (FDL) is a deep muscle of the calf, located on the medial side of the lower leg. It is the foot's equivalent of the Flexor Digitorum Profundus in the hand. It is responsible for curling the toes to grip the ground, providing balance and propulsion during walking.

Quick Anatomy Snapshot

Origin (Proximal)	Posterior surface of the Tibia (medial side, below the soleal line).
Insertion (Distal)	Bases of the Distal Phalanges of the lateral four toes (2, 3, 4, 5).
Nerve Supply	Tibial Nerve (L5, S1, S2).
Blood Supply	Posterior Tibial Artery.
Primary Actions	Toe Flexion: Flexes the DIP, PIP, and MTP joints of the lateral four toes. Plantarflexion: Assists in pointing the foot down. Inversion: Assists in turning the sole of the foot inward.

Deep Dive: The Medial Pathway

The path of the FDL tendon is complex and involves crossing other structures to reach the toes.

1. "Tom, Dick, and Harry"

This mnemonic describes the order of tendons passing behind the Medial Malleolus (from Anterior to Posterior):
Tom: Tibialis Posterior
Dick: Flexor Digitorum Longus
And: Posterior Tibial Artery & Nerve
Harry: Flexor Hallucis Longus

2. The Master Knot of Henry

In the sole of the foot, the FDL tendon crosses over the tendon of the Flexor Hallucis Longus. This crossing point is known as the Master Knot of Henry. It is a common site for adhesions or tendon entrapment.

3. The "Perforating" Muscle

Just like in the hand, the Longus tendon passes through a split in the Brevis tendon (Flexor Digitorum Brevis) to reach the tip of the toe. This allows the Longus to act on the Distal Phalanx, while the Brevis acts on the Middle Phalanx.

Physio Corner: Clinical Relevance

💪 Functional Fact: While walking barefoot on sand, the FDL grips the surface to propel you forward. In modern shoes, its gripping function is less utilized, often leading to weakness and instability of the longitudinal arch.

Palpation

The muscle belly is deep and hard to palpate directly. However, the tendon is palpable just behind the medial malleolus (ankle bone), immediately posterior to the Tibialis Posterior tendon. Ask the patient to curl their toes while you feel for the tendon moving behind the ankle bone.

⚠️ Clinical Pathology: Shin Splints
Medial Tibial Stress Syndrome (Shin Splints) often involves the FDL and Tibialis Posterior. Since the FDL originates from the posterior tibia, excessive traction from running (especially with over-pronation) can cause periostitis (inflammation of the bone lining) along the inner shin.

Manual Muscle Testing (MMT)

To test the FDL, we focus on the distal interphalangeal (DIP) joints.

Testing Tip: You must stabilize the metatarsals and proximal phalanges to isolate the tip of the toe.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Supine or Sitting. Ankle in neutral (90°). Action: Patient curls the toes, specifically flexing the tips. Resistance: Applied to the plantar surface of the Distal Phalanges of toes 2-5, pushing up into extension. Grade 3: Full range of motion. Grade 4/5: Holds against moderate/strong resistance.
Grade 0, 1 (Palpation)	Action: Palpate posterior to the medial malleolus. Cue: "Try to grip the floor with your toes." Grade 1: Tendon tension felt. Grade 0: No activity.

Frequently Asked Questions

Does it flex the big toe?

No. The Big Toe (Hallux) has its own separate flexor: the Flexor Hallucis Longus. The FDL only acts on the lateral four toes.

How is it different from Flexor Digitorum Brevis?

FDL (Longus): Originates in the leg, inserts on Distal Phalanx, flexes the tip of the toe.
FDB (Brevis): Originates in the foot (heel), inserts on Middle Phalanx, flexes the middle of the toe.

What muscle corrects the pull of FDL?

Because the FDL enters the foot from the medial side, it pulls the toes diagonally. The Quadratus Plantae muscle attaches to the FDL tendon and pulls it laterally, straightening the line of pull so the toes curl straight down.

Test Your Knowledge: FDL Quiz

1. Where does the Flexor Digitorum Longus originate?

2. In the "Tom, Dick, and Harry" mnemonic, the FDL corresponds to:

3. Which nerve supplies the FDL?

4. Where does the FDL insert?

5. The crossing of FDL over FHL in the foot is called:

6. Which muscle attaches to the FDL tendon to correct its angle of pull?

7. Besides toe flexion, the FDL assists in:

8. The FDL is located in which compartment of the leg?

9. What joint does FDL flex that FDB does not?

10. The tendons of FDL pass through the tendons of which muscle?

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.
• Brukner, P., & Khan, K. (2017). Clinical Sports Medicine. 5th ed. McGraw-Hill Education.