Third Lumbrical (Hand)
Hand · Upper Limb
The third lumbrical is a bipennate muscle arising from the adjacent sides of the flexor digitorum profundus tendons to the middle and ring fingers, and inserting onto the radial side of the extensor expansion of the ring finger. Like all lumbricals it flexes the metacarpophalangeal joint while extending the interphalangeal joints. As one of the two medial (ulnar) lumbricals, 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 middle and ring fingers (bipennate)
Insertion
Radial side of the extensor expansion of the ring finger
Action
Flexes the MCP joint and extends the IP joints of the ring finger
Nerve
Deep branch of ulnar nerve (C8, T1)
Attachments explained
The Third 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 middle and ring fingers (bipennate)
Insertion
Radial side of the extensor expansion of the ring finger
Action & function
When the Third Lumbrical (Hand) contracts, it produces the following movement: Flexes the MCP joint and extends the IP joints of the ring 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 Third 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 Third Lumbrical (Hand), impairing the movements it normally produces (flexes the MCP joint and extends the IP joints of the ring finger). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Third Lumbrical (Hand) (the OIANS method)
Note the structural shift at the third lumbrical: unlike the first two, it is bipennate, arising from two adjacent profundus tendons rather than one. Its action, however, is identical, flexing the knuckle of the ring finger while extending its interphalangeal joints.
Use the rule of halves in reverse here: the third and fourth lumbricals are the medial pair and follow the ulnar nerve, matching the ulnar-supplied medial half of flexor digitorum profundus. Pairing this bipennate structure with the ulnar nerve marks the boundary between the median and ulnar lumbricals.
Frequently asked questions
Where is the Third Lumbrical (Hand) located?
The Third Lumbrical (Hand) is a muscle of the Hand group, located in the Upper Limb.
What is the origin of the Third Lumbrical (Hand)?
Adjacent sides of the FDP tendons to the middle and ring fingers (bipennate)
What is the insertion of the Third Lumbrical (Hand)?
Radial side of the extensor expansion of the ring finger
What movements does the Third Lumbrical (Hand) produce?
Flexes the MCP joint and extends the IP joints of the ring finger
What nerve supplies the Third Lumbrical (Hand)?
Deep branch of ulnar nerve (C8, T1)
Is the Third Lumbrical (Hand) free to study in OIANS?
Yes. The Third 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 Third Lumbrical (Hand)
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