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Fourth Lumbrical of the Foot

musculus lumbricalis quartus pedis

The fourth lumbrical is the most laterally placed foot lumbrical, controlling the fifth toe MTP joint and extensor hood. It is the smallest of the four lumbricals. The fifth toe is served by the fourth lumbrical, the abductor digiti minimi, and the flexor digiti minimi brevis, making it the most complex of the lesser toes in terms of intrinsic balance.

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

Anatomical Data

Origin, Insertion & Supply

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

Biomechanics

Function & Actions

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

Clinical Relevance

Clinical Notes

The fourth lumbrical is the most variable in its anatomy and may be fused with adjacent intrinsics in some individuals. Its atrophy is a late sign in severe peripheral neuropathy affecting the lateral plantar deep branch. Curly fifth toe deformity in children involves imbalance between the fourth lumbrical and FDL, managed with FDL tenotomy or transfer in symptomatic cases.

Palpation

Not directly palpable due to small size and deep plantar location adjacent to the fifth metatarsal.

Pathology

Common Injuries & Conditions

Curly Fifth Toe

Congenital or acquired flexion and medial rotation of the fifth toe from lumbrical-FDL imbalance producing toe-on-toe discomfort, managed with FDL tenotomy or derotation plasty.