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Lumbrical muscles

The lumbrical muscles of the foot are four small, slender intrinsic muscles located in the second muscular layer of the sole, deep to the flexor digitorum brevis and superficial to the quadratus plantae. They resemble worm-like structures (from Latin lumbricus, meaning worm) and play an essential role in toe movement coordination, allowing simultaneous flexion at the metatarsophalangeal (MTP) joints and extension at the interphalangeal (IP) joints).

Their fine motor function stabilizes the toes during walking, running, and balance adjustments, contributing to smooth digital control and maintaining the integrity of the longitudinal arches.

Synonyms

  • Plantar lumbricals

  • Lumbricales pedis

Origin, Course, and Insertion

  • Origin: Tendons of the flexor digitorum longus (FDL) in the sole of the foot

    • First lumbrical: From the medial side of the FDL tendon to the second toe

    • Lateral three lumbricals: From the adjacent sides of the FDL tendons for the 2nd–5th toes

  • Course: Each lumbrical passes distally on the medial side of the corresponding toe

  • Insertion: Dorsal digital expansion (extensor hood) on the medial side of the extensor digitorum longus tendons of the 2nd–5th toes

Relations

  • Superficial: Tendons of flexor digitorum longus

  • Deep: Interosseous muscles

  • Medial: Medial plantar neurovascular bundle (for the first lumbrical)

  • Lateral: Lateral plantar nerve and vessels

  • Inferior: Plantar aponeurosis

Nerve Supply

  • First lumbrical: Medial plantar nerve (S1–S2)

  • Second to fourth lumbricals: Deep branch of the lateral plantar nerve (S2–S3)

Arterial Supply

  • Plantar metatarsal arteries (from the deep plantar arch)

  • Small contributions from medial and lateral plantar arteries

Venous Drainage

  • Plantar venous arch and deep plantar veins, draining into the posterior tibial vein

Function

  • Flexion of MTP joints: Flexes the toes at the metatarsophalangeal joints

  • Extension of IP joints: Extends toes at the interphalangeal joints

  • Digital balance: Maintains coordinated movement between long and short toe flexors/extensors

  • Postural stability: Helps stabilize the foot arches and toes during propulsion and balance

Clinical Significance

  • Overuse injuries: Common in dancers and athletes due to repetitive toe flexion–extension

  • Entrapment or weakness: May occur with medial or lateral plantar neuropathies, causing toe clawing or imbalance

  • Diabetic neuropathy: Leads to intrinsic muscle wasting and claw-toe deformities

  • Post-surgical relevance: Important in tendon transfer or correction of digital deformities

  • Imaging utility: MRI aids in diagnosing muscle atrophy, denervation, and plantar neuropathies

MRI Appearance

  • T1-weighted images:

    • Normal lumbricals: intermediate signal intensity (similar to skeletal muscle)

    • Surrounding fat: bright signal, enhancing contrast

    • Denervation atrophy: reduced bulk with increased intramuscular fat signal (bright on T1)

  • T2-weighted images:

    • Normal muscle: intermediate-to-low signal, slightly darker than on T1

    • Pathology: bright hyperintense signal in cases of edema, myositis, or acute denervation

    • Chronic atrophy: overall low signal due to fibrotic replacement

  • STIR:

    • Normal lumbricals: intermediate-to-dark signal

    • Pathology: bright signal indicating edema, inflammation, or ischemia

    • Excellent for detecting early muscle denervation or overuse injury

  • Proton Density Fat-Saturated (PD FS):

    • Normal: intermediate-to-dark signal, homogeneous muscle texture

    • Pathologic: focal or diffuse hyperintensity within muscle belly representing edema or strain

    • Highlights subtle perimuscular fluid collections or fascial inflammation

  • T1 Fat-Sat Post-Contrast:

    • Normal lumbricals: uniform mild enhancement

    • Myositis or synovitis: patchy or diffuse enhancement

    • Chronic atrophy: minimal or peripheral enhancement with central low signal fibrosis

CT Appearance

Non-Contrast CT:

  • Lumbricals appear as small soft-tissue densities between the tendons of flexor digitorum longus and interossei

  • Normal appearance: thin, symmetric, low-attenuation bands

  • Chronic changes: fatty infiltration or atrophy causing reduced volume

  • Useful for identifying calcifications or ossifications after trauma or surgery

Post-Contrast CT (standard):

  • Normal: homogeneous mild enhancement

  • Pathology: localized enhancement in inflammation or fibrosis

  • May demonstrate muscle edema, abscess, or postoperative scarring

MRI images

Lumbrical muscles axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI images

Lumbrical muscles axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

MRI images

Lumbrical muscles axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00002

MRI images

Lumbrical muscles axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00003

MRI image

Lumbrical muscles CORONAL CT IMAGE