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First lumbrical muscle of foot

The first lumbrical muscle is one of the four small lumbricals located in the second muscular layer of the sole of the foot. It is the most medial and largest of the lumbricals and plays a key role in coordinating flexion of the metatarsophalangeal (MTP) joint and extension of the interphalangeal (IP) joints of the second toe.

Unlike the other lumbricals, the first lumbrical arises from a single tendon of the flexor digitorum longus (FDL), while the remaining lumbricals arise from adjacent tendons. It has a unipennate structure, lying medial to the second metatarsal, and its fine motor control contributes to the stability and balance of the forefoot during walking.

Synonyms

  • Medial lumbrical of the foot

  • First plantar lumbrical

  • Lumbrical to the second toe

Origin, Course, and Insertion

  • Origin: Medial side of the tendon of the flexor digitorum longus for the second toe

  • Course: Passes anteriorly and medially along the medial side of the second toe, crossing the deep transverse metatarsal ligament

  • Insertion: Medial side of the extensor expansion (extensor hood) of the second toe on the dorsal aspect

Relations

  • Superiorly: Tendon of flexor digitorum longus

  • Inferiorly: Plantar aponeurosis and flexor digitorum brevis tendons

  • Medially: First metatarsal and medial plantar vessels

  • Laterally: Second lumbrical muscle

  • Posteriorly: Quadratus plantae muscle

  • Anteriorly: Base of the proximal phalanx and extensor hood of the second toe

Nerve Supply

  • Medial plantar nerve (branch of the tibial nerve, roots L4–L5)

Arterial Supply

  • Medial plantar artery and first plantar metatarsal artery

Venous Drainage

  • Medial plantar vein draining into the posterior tibial vein

Function

  • Flexion at MTP joint: Flexes the second toe at the metatarsophalangeal joint

  • Extension at IP joints: Extends the proximal and distal interphalangeal joints via the extensor expansion

  • Forefoot stability: Balances flexor and extensor forces during stance and toe-off

  • Fine motor control: Contributes to smooth digital motion and toe alignment during gait

Clinical Significance

  • Overuse strain: Seen in dancers or runners due to repetitive MTP flexion

  • Nerve entrapment: Medial plantar nerve compression leads to weakness of first lumbrical and impaired toe control

  • Diabetic neuropathy: May show early atrophy or fatty replacement detectable on MRI

  • Muscle imbalance: Weakness can contribute to claw toe deformity

  • Surgical importance: Acts as a landmark for medial plantar nerve and vessel identification during foot surgeries

MRI Appearance

  • T1-weighted images:

    • Normal muscle: intermediate signal intensity with clear fascicular texture

    • Fat planes: bright, well-defined around muscle belly

    • Pathology (atrophy): muscle replaced by bright fatty signal

    • Tendon insertions appear as low-signal bands blending with extensor hood

  • T2-weighted images:

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

    • Pathology: focal bright hyperintensity indicating edema, strain, or denervation changes

    • Normal tendon insertion remains low signal

  • STIR:

    • Normal muscle: intermediate-to-dark signal intensity

    • Pathology: bright hyperintensity in acute injury, edema, or nerve-related inflammation

    • Useful for detecting early denervation edema in medial plantar nerve compression

  • Proton Density Fat-Saturated (PD FS):

    • Normal muscle: intermediate-to-dark, uniform signal intensity

    • Pathology: focal or diffuse bright signal representing muscle strain, myositis, or partial tear

    • Clearly shows perimuscular fluid or subtle inflammatory change

  • T1 Fat-Sat Post-Contrast:

    • Normal: mild homogeneous enhancement

    • Acute myositis or denervation: diffuse enhancement

    • Chronic denervation: minimal enhancement with fatty infiltration

CT Appearance

Non-Contrast CT:

  • Muscle: small, soft-tissue density along medial plantar aspect between FDL tendons

  • Difficult to distinguish from adjacent lumbricals due to size

  • Chronic changes: fatty atrophy appears as low-attenuation regions

  • Denervated muscle: reduced bulk with increased perimuscular fat

Post-Contrast CT (standard):

  • Normal muscle: mild uniform enhancement

  • Pathology: enhancement along muscle belly or sheath in myositis or post-traumatic inflammation

  • Useful for evaluating calcifications, fibrosis, or space-occupying lesions compressing the medial plantar nerve

MRI image

CT image

first lumbrical muscle of foot ct axial image