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First Plantar Interosseous of the Foot

musculus interosseus plantaris primus pedis

The first plantar interosseous adducts the third toe toward the midline axis of the second ray. As the most medial plantar interosseous, it acts in opposition to the second dorsal interosseous and maintains third toe alignment. The three plantar interossei together act as PAD muscles (Plantar ADduct) in the foot, in contrast to the dorsal interossei (DAB, Dorsal ABduct).

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

Anatomical Data

Origin, Insertion & Supply

Origin	Medial side and base of the third metatarsal (unipennate)
Insertion	Medial base of the proximal phalanx of the third toe and extensor hood
Nerve Supply	Lateral plantar nerve (deep branch)
Blood Supply	Third plantar metatarsal artery

Biomechanics

Function & Actions

Actions	Adducts the third toe toward the second toe axis; Assists MTP flexion and IP extension of the third toe via extensor hood

Clinical Relevance

Clinical Notes

The first plantar interosseous is the largest and most assessable of the three plantar interossei. Its loss in intrinsic minus foot allows the long extensors to dominate, producing third toe clawing with MTP hyperextension and PIP flexion. Tendon transfer procedures targeting intrinsic rebalancing must consider all three plantar interossei as a functional group.

Palpation

Not reliably palpable in isolation due to plantar soft tissue depth. Functional assessment by active third toe adduction testing.

Pathology

Common Injuries & Conditions

Third Toe Clawing

Functional loss of first plantar interosseous producing MTP hyperextension and IP joint flexion at the third toe from unopposed long extensor dominance, managed with intrinsic rebalancing procedures.