Fourth Lumbrical (Foot)

Origin, Insertion, Action & Nerve

Attachments explained

The Fourth 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.

Action & function

When the Fourth Lumbrical (Foot) contracts, it produces the following movement: Flexes the MTP joint and extends the IP joints of the 5th 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 Fourth Lumbrical (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 Fourth Lumbrical (Foot), impairing the movements it normally produces (flexes the MTP joint and extends the IP joints of the 5th toe). This is why knowing the innervation is central to localising nerve lesions in clinical practice.

How to study the Fourth Lumbrical (Foot) (the OIANS method)

Place the fourth lumbrical as the lateral end of the row, acting on the fifth toe from origin tendons shared with the fourth. Finishing the count here confirms the pattern that each lumbrical inserts one toe lateral to its lower origin tendon, so the series runs cleanly from the second toe to the fifth.

Anchor its lateral plantar nerve supply by remembering the whole distribution at once: first lumbrical medial plantar, the rest lateral plantar. Since the fourth is unambiguously in the lateral group, use it to double-check that only a single lumbrical, the first, breaks from the lateral plantar pattern.

Frequently asked questions

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