The first lumbrical is the most radially placed hand lumbrical, arising from the radial border of the first FDP tendon as a unipennate muscle. It is the most consistent median nerve lumbrical, preserved in ulnar nerve injuries, allowing the index finger to maintain the intrinsic-plus position when the ring and little fingers are clawed. It passes volar to the deep transverse metacarpal ligament to reach the extensor hood.
| Origin | Radial (lateral) side of the first FDP tendon, unipennate |
|---|---|
| Insertion | Radial side of the extensor expansion (dorsal hood) of the index finger |
| Nerve Supply | Median nerve (C8, T1), the only lumbrical consistently innervated by the median nerve |
| Blood Supply | First palmar metacarpal artery |
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It flexes the MCP joint and extends the interphalangeal joints of the index finger, producing the tabletop position used in writing and precision grip.
The first lumbrical is the key muscle distinguishing median from ulnar nerve territory in intrinsic hand testing. In ulnar nerve palsy, the first and second lumbricals (median) prevent clawing of the index and middle fingers while the ring and little fingers claw from loss of the third and fourth lumbricals (ulnar). Lumbrical plus finger, paradoxical IP extension with FDP activation, occurs when the lumbrical origin retracts proximally after FDP rupture, creating a reverse bowstring effect.
Not directly palpable. Assessed by index finger MCP flexion with IP extension (intrinsic-plus test).
Paradoxical extension of the index finger IP joints when attempting to flex the finger, caused by FDP tendon disruption and proximal migration of the first lumbrical origin, producing the characteristic lumbrical-plus deformity.
Fixed intrinsic-plus deformity from first lumbrical fibrosis after crush injury or compartment syndrome, limiting passive IP flexion with MCP in extension, treated by intrinsic release.
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