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First Lumbrical of the Foot

musculus lumbricalis primus pedis

The first lumbrical is the only foot lumbrical innervated by the medial plantar nerve. It arises from the medial aspect of the first FDL tendon and inserts into the extensor hood of the second toe, producing the intrinsic-plus position. It is the most medially placed and most reliably present of the four lumbricals.

Nerve: Medial plantar nerve (S2, S3) Blood Supply: First plantar metatarsal artery Region: Foot & Ankle

Anatomical Data

Origin, Insertion & Supply

Origin	Medial border of the first FDL tendon slip (unipennate)
Insertion	Medial base of the proximal phalanx of the second toe and extensor hood
Nerve Supply	Medial plantar nerve (S2, S3)
Blood Supply	First plantar metatarsal artery

Biomechanics

Function & Actions

Actions	Flexes the MTP joint of the second toe; Extends the PIP and DIP joints of the second toe via the extensor hood

Clinical Relevance

Clinical Notes

The first lumbrical is preserved in lateral plantar nerve injuries but lost with medial plantar nerve damage. Its function is assessed by the ability to flex the second MTP joint while extending the PIP joint (intrinsic-plus test). Transfer of the first lumbrical is used in second toe crossover deformity correction. In claw toe surgery, lumbrical advancement restores the intrinsic-plus position at the second MTP joint.

Palpation

Not directly palpable due to deep plantar position. Assessed functionally by second MTP flexion with PIP extension testing.

Pathology

Common Injuries & Conditions

Second Toe Crossover from Lumbrical Loss

Medial deviation and dorsal subluxation of the second toe from loss of first lumbrical MTP flexion and extensor hood stability, requiring lumbrical transfer or plantar plate repair.