Lumbricals (Foot): Origin, Insertion, Action & Hammer Toes

The Lumbricals of the foot are four small, worm-like muscles found in the second layer of the sole. Unlike most muscles that attach bone-to-bone, these unique muscles attach tendon-to-tendon. They play a crucial role in stabilizing the toes during walking and preventing deformities.

[Image of Lumbricals of the Foot anatomy]

Quick Anatomy Snapshot

Count	Four muscles (labeled 1st to 4th, from medial to lateral).
Origin (Proximal)	Tendons of the Flexor Digitorum Longus (FDL).
Insertion (Distal)	Medial side of the Extensor Expansion (Dorsal Hood) of the lateral four toes (2-5).
Nerve Supply	1st Lumbrical: Medial Plantar Nerve (S2, S3). 2nd, 3rd, 4th: Lateral Plantar Nerve (S2, S3).
Primary Actions	MTP Flexion: Flexes the Metatarsophalangeal joints. IP Extension: Extends the Interphalangeal joints.

Deep Dive: The "Worm" Muscles

The name "Lumbrical" comes from the Latin Lumbricus, meaning earthworm, describing their slender shape.

1. Unipennate vs. Bipennate

Just like in the hand, the structure differs between the first and the rest:
• 1st Lumbrical: Unipennate (arises from the medial side of the 2nd toe's FDL tendon).
• 2nd, 3rd, 4th Lumbricals: Bipennate (arise from adjacent sides of two FDL tendons).

2. The "Bye-Bye" Mechanism

The Lumbricals (along with the Interossei) put the toes in the "Intrinsic Plus" position (flexed knuckles, straight toes). This is essentially the position of a puppet hand waving "bye-bye." This balance forces the toes to grip the ground flatly rather than curling under.

Physio Corner: Clinical Relevance

💪 Functional Fact: During the propulsion phase of gait (push-off), the Lumbricals extend the IP joints to increase the rigidity of the toes, effectively lengthening the lever arm of the foot for a stronger push.

Palpation

Direct palpation is extremely difficult as they are small and located deep in the 2nd layer of the foot, beneath the Flexor Digitorum Brevis. They lie in the spaces between the metatarsal heads.

⚠️ Clinical Pathology: Hammer Toes
When the Lumbricals are weak or inhibited (Intrinsic Minus Foot), they lose the battle against the long flexors and extensors.
The Extensor Digitorum Longus hyperextends the MTP joint, and the Flexor Digitorum Longus curls the IP joints. This resulting deformity is a Hammer Toe.

Manual Muscle Testing (MMT)

Testing the "Intrinsic" motion of the toes.

Testing Tip: You must prevent the long flexors from curling the tips of the toes.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Supine or Sitting. Ankle neutral. Action: Patient flexes the MTP joints (knuckles) while keeping the IP joints (toes) straight. Resistance: Applied beneath the Proximal Phalanges of toes 2-5, pushing up into extension. Grade 3: Complete range. Grade 4/5: Holds against resistance.
Grade 0, 1 (Palpation)	Action: Palpation is unreliable due to depth. Observation of movement is key. Cue: "Bend your knuckles but keep your toes straight."

Frequently Asked Questions

Why do they have two different nerve supplies?

The foot is divided longitudinally for innervation. The medial side (1st Lumbrical) is supplied by the Medial Plantar Nerve, while the lateral side (2nd-4th) is supplied by the Lateral Plantar Nerve. This mimics the Median/Ulnar split in the hand.

Do they attach to bone at the origin?

No. They are unique because they originate from tendons (Flexor Digitorum Longus) and insert into tendons (Extensor Expansion). They are "floating" muscles.

Does the big toe have a Lumbrical?

No. The Lumbricals only attach to the tendons of the Flexor Digitorum Longus, which goes to toes 2-5. The Big Toe is controlled by its own specific muscles (Flexor Hallucis Longus/Brevis).

Test Your Knowledge: Lumbricals Quiz

1. What is the origin of the Lumbrical muscles?

2. Which nerve supplies the 1st Lumbrical?

3. What is the action of the Lumbricals?

4. Where do the Lumbricals insert?

5. Weakness of the Lumbricals leads to which deformity?

6. Which layer of the foot contains the Lumbricals?

7. How many Lumbrical muscles are there in one foot?

8. Which Lumbricals are bipennate (two heads)?

9. What does the name "Lumbrical" mean?

10. Which muscle is the antagonist to the Lumbricals at the MTP 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.
• Brukner, P., & Khan, K. (2017). Clinical Sports Medicine. 5th ed. McGraw-Hill Education.