First Lumbrical (Foot)
Foot · Lower Limb
The first lumbrical of the foot is the most medial of the four small worm-like muscles of the second plantar layer. Unlike the others it is unipennate, arising from the medial side of the first tendon of flexor digitorum longus, the tendon that passes to the second toe. It inserts on the medial side of the extensor expansion of the second toe. By pulling on the extensor hood while the long flexor pulls on the phalanx, it flexes the metatarsophalangeal joint and extends the interphalangeal joints of the second toe. It is supplied by the medial plantar nerve (S2, S3), which distinguishes it from the more lateral lumbricals. This combination of flexion at one joint and extension at the next allows the toes to press flat against the ground.
Origin, Insertion, Action & Nerve
Origin
Medial side of the 1st flexor digitorum longus tendon (to the 2nd toe)
Insertion
Medial side of the extensor expansion of the 2nd toe
Action
Flexes the MTP joint and extends the IP joints of the 2nd toe
Nerve
Medial plantar nerve (S2, S3)
Attachments explained
The First Lumbrical (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 of the 1st flexor digitorum longus tendon (to the 2nd toe)
Insertion
Medial side of the extensor expansion of the 2nd toe
Action & function
When the First Lumbrical (Foot) contracts, it produces the following movement: Flexes the MTP joint and extends the IP joints of the 2nd toe.
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 First Lumbrical (Foot) receives its nerve supply from the Medial 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 Medial plantar nerve or to its spinal roots (S2, S3) can weaken or paralyse the First Lumbrical (Foot), impairing the movements it normally produces (flexes the MTP joint and extends the IP joints of the 2nd toe). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the First Lumbrical (Foot) (the OIANS method)
Understand the lumbricals by their paradoxical action, which is identical in principle to the hand: they arise from a flexor tendon yet insert into the extensor expansion, so they flex the MTP joint and extend the IP joints. For the first lumbrical, apply that logic to the second toe, whose extensor hood it reaches from the medial side.
Use innervation to separate the first lumbrical from the rest. Only the first is supplied by the medial plantar nerve; the second, third, and fourth take the lateral plantar nerve. Its unipennate origin from a single flexor tendon reinforces its status as the odd one out, mirroring how the two medial lumbricals of the hand are the median-nerve exceptions.
Frequently asked questions
Where is the First Lumbrical (Foot) located?
The First Lumbrical (Foot) is a muscle of the Foot group, located in the Lower Limb.
What is the origin of the First Lumbrical (Foot)?
Medial side of the 1st flexor digitorum longus tendon (to the 2nd toe)
What is the insertion of the First Lumbrical (Foot)?
Medial side of the extensor expansion of the 2nd toe
What movements does the First Lumbrical (Foot) produce?
Flexes the MTP joint and extends the IP joints of the 2nd toe
What nerve supplies the First Lumbrical (Foot)?
Medial plantar nerve (S2, S3)
Is the First Lumbrical (Foot) free to study in OIANS?
The First Lumbrical (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 First Lumbrical (Foot)
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