Third Plantar Interosseous (Foot)
Foot · Lower Limb
The third plantar interosseous is the most lateral of the three plantar interossei, a unipennate muscle arising from the medial side and base of the fifth metatarsal and inserting on the medial side of the proximal phalanx of the fifth toe. It adducts the fifth toe, drawing it medially toward the axis of the second toe, and it also flexes the metatarsophalangeal joint and extends the interphalangeal joints of that toe. It is supplied by the lateral plantar nerve (S2, S3). Together with its two companions, it draws the lateral toes toward the midline of the foot, balancing the abductive pull of the dorsal interossei during gait.
Origin, Insertion, Action & Nerve
Origin
Medial side and base of the 5th metatarsal (unipennate)
Insertion
Medial side of proximal phalanx of the 5th toe
Action
Adducts the 5th toe toward the 2nd-toe axis; flexes MTP and extends IP joints
Nerve
Lateral plantar nerve (S2, S3)
Attachments explained
The Third Plantar Interosseous (Foot) is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.
Origin
Medial side and base of the 5th metatarsal (unipennate)
Insertion
Medial side of proximal phalanx of the 5th toe
Action & function
When the Third Plantar Interosseous (Foot) contracts, it produces the following movements: Adducts the 5th toe toward the 2nd-toe axis; flexes MTP and extends IP joints.
As part of the Foot group of the Lower Limb, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.
Nerve supply & clinical relevance
The Third Plantar Interosseous (Foot) receives its nerve supply from the Lateral plantar nerve (S2, S3). This nerve carries fibres from spinal segment(s) S2, S3.
Because a muscle can only contract when its nerve is intact, injury to the Lateral plantar nerve or to its spinal roots (S2, S3) can weaken or paralyse the Third Plantar Interosseous (Foot), impairing the movements it normally produces (adducts the 5th toe toward the 2nd-toe axis). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Third Plantar Interosseous (Foot) (the OIANS method)
Fix the third plantar interosseous as the little-toe adductor. It arises from the fifth metatarsal and inserts medially on the fifth toe, so it drags that toe toward the second-toe axis, completing the trio of plantar adductors serving the outer three toes.
Use it to close the interosseous framework: dorsal interossei abduct four toes from the second-toe axis and are bipennate, while plantar interossei adduct three toes toward that axis and are unipennate, all under the lateral plantar nerve. Recognising the third plantar as the last adductor of the lateral toe lets you reconstruct the entire interosseous group from first principles.
Frequently asked questions
Where is the Third Plantar Interosseous (Foot) located?
The Third Plantar Interosseous (Foot) is a muscle of the Foot group, located in the Lower Limb.
What is the origin of the Third Plantar Interosseous (Foot)?
Medial side and base of the 5th metatarsal (unipennate)
What is the insertion of the Third Plantar Interosseous (Foot)?
Medial side of proximal phalanx of the 5th toe
What movements does the Third Plantar Interosseous (Foot) produce?
Adducts the 5th toe toward the 2nd-toe axis; flexes MTP and extends IP joints
What nerve supplies the Third Plantar Interosseous (Foot)?
Lateral plantar nerve (S2, S3)
Is the Third Plantar Interosseous (Foot) free to study in OIANS?
The Third Plantar Interosseous (Foot) is always free to browse. Its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory. Quiz and Flashcard practice for the Lower Limb is part of the one-time Lifetime upgrade, though; only the Upper Limb decks are free to test yourself on.
Related muscles
Study the Third Plantar Interosseous (Foot)
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