First Lumbrical (Hand)
Hand · Upper Limb
The first lumbrical is a small, worm-like muscle of the palm, unusual in arising from tendon rather than bone, specifically the radial side of the flexor digitorum profundus tendon to the index finger. It inserts onto the radial side of that finger's extensor expansion. This arrangement lets it flex the metacarpophalangeal joint while simultaneously extending the interphalangeal joints, producing the delicate writing posture of the fingers. The two radial lumbricals, including this one, are supplied by the median nerve (C8, T1).
Origin, Insertion, Action & Nerve
Origin
Radial side of the FDP tendon to the index finger (unipennate)
Insertion
Radial side of the extensor expansion of the index finger
Action
Flexes the MCP joint and extends the IP joints of the index finger
Nerve
Median nerve (C8, T1)
Attachments explained
The First 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
Radial side of the FDP tendon to the index finger (unipennate)
Insertion
Radial side of the extensor expansion of the index finger
Action & function
When the First Lumbrical (Hand) contracts, it produces the following movement: Flexes the MCP joint and extends the IP joints of the index 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 First Lumbrical (Hand) receives its nerve supply from the Median 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 Median nerve or to its spinal roots (C8, T1) can weaken or paralyse the First Lumbrical (Hand), impairing the movements it normally produces (flexes the MCP joint and extends the IP joints of the index finger). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the First Lumbrical (Hand) (the OIANS method)
The defining oddity of the lumbricals is that they arise from tendon, not bone, bridging a flexor tendon to an extensor expansion. That bridge explains their signature action: they flex the knuckle while straightening the finger, the exact posture of holding a pen. Learn this concept once and it applies to all four.
For nerve supply, use the rule of halves: the two lateral (radial) lumbricals, including this first one to the index finger, follow the median nerve, matching the median-supplied lateral half of flexor digitorum profundus. The two medial lumbricals follow the ulnar nerve, a symmetry that is easy to remember and often tested.
Frequently asked questions
Where is the First Lumbrical (Hand) located?
The First Lumbrical (Hand) is a muscle of the Hand group, located in the Upper Limb.
What is the origin of the First Lumbrical (Hand)?
Radial side of the FDP tendon to the index finger (unipennate)
What is the insertion of the First Lumbrical (Hand)?
Radial side of the extensor expansion of the index finger
What movements does the First Lumbrical (Hand) produce?
Flexes the MCP joint and extends the IP joints of the index finger
What nerve supplies the First Lumbrical (Hand)?
Median nerve (C8, T1)
Is the First Lumbrical (Hand) free to study in OIANS?
Yes. The First 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 First Lumbrical (Hand)
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