Fourth Lumbrical (Hand)
Hand · Upper Limb
The fourth lumbrical is a bipennate muscle arising from the adjacent sides of the flexor digitorum profundus tendons to the ring and little fingers, and inserting onto the radial side of the extensor expansion of the little finger. It flexes the metacarpophalangeal joint and extends the interphalangeal joints of the little finger. As a medial (ulnar) lumbrical, it is supplied by the deep branch of the ulnar nerve (C8, T1).
Origin, Insertion, Action & Nerve
Origin
Adjacent sides of the FDP tendons to the ring and little fingers (bipennate)
Insertion
Radial side of the extensor expansion of the little finger
Action
Flexes the MCP joint and extends the IP joints of the little finger
Nerve
Deep branch of ulnar nerve (C8, T1)
Attachments explained
The Fourth Lumbrical (Hand) 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
Adjacent sides of the FDP tendons to the ring and little fingers (bipennate)
Insertion
Radial side of the extensor expansion of the little finger
Action & function
When the Fourth Lumbrical (Hand) contracts, it produces the following movement: Flexes the MCP joint and extends the IP joints of the little finger.
As part of the Hand group of the Upper 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 (Hand) receives its nerve supply from the Deep branch of ulnar nerve (C8, T1). This nerve carries fibres from spinal segment(s) C8, T1.
Because a muscle can only contract when its nerve is intact, injury to the Deep branch of ulnar nerve or to its spinal roots (C8, T1) can weaken or paralyse the Fourth Lumbrical (Hand), impairing the movements it normally produces (flexes the MCP joint and extends the IP joints of the little finger). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Fourth Lumbrical (Hand) (the OIANS method)
Close the lumbrical set with the fourth, the little-finger muscle, bipennate like the third and arising from two adjacent profundus tendons. Its action repeats the family pattern: it flexes the metacarpophalangeal joint while extending the interphalangeal joints.
Confirm the rule of halves at its endpoint: this is the second of the ulnar-supplied medial lumbricals. Having learned all four, you can now recite the pattern, two lateral lumbricals on the median nerve and two medial ones on the ulnar nerve, which mirrors the split supply of the deep finger flexor.
Frequently asked questions
Where is the Fourth Lumbrical (Hand) located?
The Fourth Lumbrical (Hand) is a muscle of the Hand group, located in the Upper Limb.
What is the origin of the Fourth Lumbrical (Hand)?
Adjacent sides of the FDP tendons to the ring and little fingers (bipennate)
What is the insertion of the Fourth Lumbrical (Hand)?
Radial side of the extensor expansion of the little finger
What movements does the Fourth Lumbrical (Hand) produce?
Flexes the MCP joint and extends the IP joints of the little finger
What nerve supplies the Fourth Lumbrical (Hand)?
Deep branch of ulnar nerve (C8, T1)
Is the Fourth Lumbrical (Hand) free to study in OIANS?
Yes. The Fourth Lumbrical (Hand) is free in OIANS: its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory, and because it belongs to the Upper Limb it is also covered by the free Quiz and Flashcard decks. The other regions, the "All Muscles" deck, and the Progress and Explore tools are unlocked with a one-time Lifetime purchase.
Related muscles
Study the Fourth Lumbrical (Hand)
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