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Third lumbrical muscle of foot

The third lumbrical muscle is one of the four small, spindle-shaped lumbrical muscles located in the second layer of the sole of the foot. It arises from the tendons of the flexor digitorum longus (FDL) and plays an essential role in coordinating toe flexion at the metatarsophalangeal (MTP) joint and extension at the interphalangeal (IP) joints of the lateral toes.

The lumbricals serve as fine-tuning stabilizers that balance flexion and extension forces during gait. The third lumbrical, specifically, acts on the third toe (middle toe), maintaining digital alignment and supporting the longitudinal and transverse arches of the foot during stance and propulsion.

Synonyms

  • Third plantar lumbrical

  • Lumbrical of the third toe

Origin, Course, and Insertion

  • Origin: Arises from the adjacent sides of the tendons of the flexor digitorum longus corresponding to the third and fourth digits (between the FDL tendons of the middle and ring toes)

  • Course: Small fusiform belly passes forward between the flexor digitorum longus and flexor accessorius (quadratus plantae), then medially to the toe it serves

  • Insertion: Onto the medial side of the base of the proximal phalanx of the third toe, blending with the extensor expansion (dorsal digital aponeurosis)

Relations

  • Superficial: Tendons of flexor digitorum longus and the flexor accessorius muscle

  • Deep: Plantar interossei muscles

  • Medially: Second lumbrical muscle

  • Laterally: Fourth lumbrical muscle

  • Superiorly: Tendons of flexor digitorum longus

  • Inferiorly: Plantar aponeurosis and plantar vessels

Nerve Supply

  • Lateral plantar nerve (superficial branch) (root value S2–S3)

Arterial Supply

  • Plantar metatarsal arteries (branches of the deep plantar arch)

  • Small contributions from the lateral plantar artery

Venous Drainage

  • Plantar venous plexus, draining into the posterior tibial veins

Function

  • Flexion: Flexes the proximal phalanx of the third toe at the metatarsophalangeal (MTP) joint

  • Extension: Extends the middle and distal phalanges at the interphalangeal (IP) joints

  • Stabilization: Maintains alignment of the toes during stance and gait

  • Arch support: Assists in maintaining the longitudinal and transverse arches of the foot

  • Coordination: Provides fine motor control and balance during toe-off phase of walking

Clinical Significance

  • Muscle strain: Rare but may occur in dancers or athletes due to repetitive toe flexion and push-off

  • Lumbrical imbalance: Can contribute to digital deformities such as claw toe or hammer toe, especially with neuropathy

  • Entrapment neuropathy: Lateral plantar nerve compression affects the third and fourth lumbricals

  • Surgical importance: Important landmark in plantar dissections and digital deformity correction procedures

  • Imaging role: MRI helps assess denervation, atrophy, or fibrofatty replacement in neuropathic or post-surgical feet

MRI Appearance

  • T1-weighted images:

    • Normal muscle: intermediate signal intensity with fine fascicular pattern

    • Fat around the muscle: high signal contrast outlining its slender belly

    • Pathology (denervation or atrophy): increased T1 signal due to fatty infiltration

  • T2-weighted images:

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

    • Acute denervation or strain: hyperintense signal indicating edema or inflammation

    • Chronic neuropathy: mixed intermediate signal with fatty streaks

  • STIR:

    • Normal muscle: intermediate-to-dark signal intensity

    • Pathology: bright hyperintensity indicates edema, strain, or denervation change

    • Best sequence to identify early muscular edema and subtle fascicular disruption

  • Proton Density Fat-Saturated (PD FS):

    • Normal: intermediate-to-dark homogeneous signal

    • Pathologic: focal bright signal within the muscle belly or at origin/insertion

    • Excellent for differentiating edema, partial tear, or denervation pattern

  • T1 Fat-Sat Post-Contrast:

    • Normal: mild homogeneous enhancement of muscle belly

    • Active inflammation or infection: intense or patchy enhancement

    • Chronic denervation: reduced or absent enhancement with volume loss

CT Appearance

Non-Contrast CT:

  • Muscle: thin soft-tissue structure along second layer of plantar foot

  • Distinguishable from adjacent tendons and interossei by location and density

  • Chronic atrophy: appears as volume loss with surrounding fat density

Post-Contrast CT (standard):

  • Normal muscle: homogeneous mild enhancement

  • Inflamed or denervated muscle: shows increased enhancement and swelling

  • Useful for detecting calcification, fibrosis, or foreign body in post-surgical or chronic cases

MRI image

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

MRI image

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

MRI image

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

CT image

third Lumbrical Muscle of Foot axial ct image