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Third Lumbrical of the Foot

musculus lumbricalis tertius pedis

The third lumbrical controls the fourth toe MTP joint and extensor mechanism. It is bipennate and laterally placed, working with the third plantar interosseous to stabilise the fourth ray. Loss of the third lumbrical contributes to fourth toe claw deformity in intrinsic minus conditions.

Nerve: Lateral plantar nerve (deep branch, S2, S3) Blood Supply: Third plantar metatarsal artery Region: Foot & Ankle

Anatomical Data

Origin, Insertion & Supply

Origin	Adjacent borders of the second and third FDL tendon slips (bipennate)
Insertion	Medial base of the proximal phalanx of the fourth toe and extensor hood
Nerve Supply	Lateral plantar nerve (deep branch, S2, S3)
Blood Supply	Third plantar metatarsal artery

Biomechanics

Function & Actions

Actions	Flexes the MTP joint of the fourth toe; Extends the IP joints of the fourth toe

Clinical Relevance

Clinical Notes

The third lumbrical is included in comprehensive EMG studies of the foot intrinsic muscles for peripheral neuropathy staging. Its selective assessment helps localise lateral plantar nerve deep branch lesions within the foot. Claw deformity at the fourth toe with intact second and third toe intrinsic function suggests a selective injury distal to the first lumbrical's medial plantar branch.

Palpation

Not directly palpable. Functionally assessed by fourth MTP flexion with IP extension resistance.

Pathology

Common Injuries & Conditions

Fourth Toe Intrinsic Minus

Claw deformity at the fourth toe from third lumbrical loss producing MTP hyperextension and IP flexion, managed with tendon transfer or arthrodesis depending on flexibility.