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Fourth lumbrical muscle of foot

The fourth lumbrical muscle is one of the four small intrinsic muscles of the foot located in the second muscular layer. It is the most lateral of the lumbricals, arising from the tendon of the flexor digitorum longus (FDL) and inserting on the extensor expansion of the fifth toe.

It plays an essential role in coordinated toe flexion and extension, particularly during the stance and push-off phases of gait. Despite its small size, the fourth lumbrical contributes to metatarsophalangeal joint flexion and interphalangeal joint extension, maintaining balance and precision in toe movements.

Synonyms

  • Lateral lumbrical of the foot

  • Lumbrical IV

  • Fifth toe lumbrical

Origin, Course, and Insertion

  • Origin: From the adjacent sides of the tendons of the flexor digitorum longus corresponding to the fourth and fifth toes

  • Course: Runs obliquely forward from the tendon origin, deep to the flexor digitorum brevis, along the lateral aspect of the fourth metatarsal

  • Insertion: Inserts into the medial side of the extensor expansion on the dorsal surface of the proximal phalanx of the fifth toe

Tendon Attachments

  • The tendon of the fourth lumbrical passes plantar to the deep transverse metatarsal ligament

  • Joins the extensor hood on the medial aspect of the fifth toe

  • Works in coordination with the flexor digitorum longus and extensor digitorum longus tendons for balanced toe movement

Relations

  • Superiorly: Tendons of flexor digitorum longus

  • Inferiorly: Plantar aponeurosis

  • Medially: Third lumbrical muscle

  • Laterally: Lateral plantar artery and nerve branches

  • Posteriorly: Quadratus plantae muscle

  • Anteriorly: Extensor expansion of the fifth toe

Nerve Supply

  • Lateral plantar nerve (deep branch) — branch of tibial nerve (roots S2–S3)

Arterial Supply

  • Lateral plantar artery, with contribution from the plantar metatarsal arteries

Venous Drainage

  • Plantar venous plexus, draining into the lateral plantar vein → posterior tibial vein

Function

  • Flexes the metatarsophalangeal joint of the fifth toe

  • Extends the interphalangeal joints via the extensor expansion

  • Assists in gait stabilization, especially during propulsion phase

  • Fine motor control: Maintains toe alignment and prevents clawing deformity

Clinical Significance

  • Lumbrical weakness: May contribute to digital imbalance and claw toe deformities

  • Entrapment neuropathy: Lateral plantar nerve lesions can impair fourth lumbrical function

  • Atrophy: Seen in chronic neuropathies or compartment syndrome of the foot

  • Tendon injury or scarring: May occur after trauma or plantar surgery

  • Imaging relevance: Identification helps differentiate intrinsic muscular pathology from tendon or nerve disorders

MRI Appearance

  • T1-weighted images:

    • Muscle: intermediate signal intensity, appearing as a small fusiform structure lateral to the third lumbrical

    • Fat planes: bright, delineating borders clearly

    • Tendon attachments: low signal (dark), continuous with extensor hood on the fifth toe

  • T2-weighted images:

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

    • Pathology: focal or diffuse hyperintense signal with enlargement or loss of fascicular detail in strain, denervation, or edema

    • Chronic atrophy: low-to-intermediate signal with fatty replacement

  • STIR:

    • Normal muscle: intermediate-to-dark signal intensity

    • Acute pathology: bright hyperintense regions representing edema or inflammation

    • Useful for detecting early denervation edema before atrophy develops

  • Proton Density Fat-Saturated (PD FS):

    • Normal: intermediate-to-dark homogeneous signal

    • Pathologic: focal bright hyperintense signal indicating myositis, strain, or fluid accumulation

    • Excellent for evaluating subtle intramuscular changes and perimuscular edema

  • T1 Fat-Sat Post-Contrast:

    • Normal: uniform mild enhancement

    • Inflamed or denervated muscle: focal or patchy enhancement

    • Chronic atrophy: minimal enhancement with high fat replacement signal

CT Appearance

Non-Contrast CT:

  • Muscle: small soft-tissue density lateral to third lumbrical, indistinct without contrast

  • Loss of normal contour or volume may indicate atrophy

  • Indirect changes: fatty infiltration or surrounding edema in chronic denervation

Post-Contrast CT (standard):

  • Normal muscle: homogeneous enhancement

  • Pathologic enhancement: localized increased vascularity in inflammatory myositis or nerve compression

  • Useful for identifying postoperative fibrosis, scarring, or chronic muscle wasting

MRI image

fourth Lumbrical Muscle of Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

fourth Lumbrical Muscle of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

fourth Lumbrical Muscle of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

CT image

fourth lumbrical muscle of foot ct axial image