The first lumbrical is the most radial and is median-nerve innervated (unlike lumbricals 3 and 4 which are ulnar). Its unique origin from the FDP tendon means that when the finger is flexed, the lumbrical origin moves distally with FDP, slackening the lumbrical and allowing smooth coordinate flexion and extension.
| Origin | Radial side of the FDP tendon to the index finger |
|---|---|
| Insertion | Radial side of the extensor expansion of the index finger |
| Nerve Supply | Median nerve — digital branch (C8, T1) |
| Blood Supply | Digital arteries |
| Actions | Flexes the index finger MCP joint; Extends the index finger IP joints — via the lateral band of the extensor expansion; Stabilises the MCP joint during precision grip |
|---|
The lumbrical-plus finger is a paradoxical extension of the IP joints when the patient tries to flex the finger — it occurs when the FDP tendon is cut distal to the lumbrical origin, causing the lumbrical to be pulled proximally with flexion attempts. This produces IP extension via the extensor expansion instead of flexion.
Tested by resisted MCP flexion with IP joints held extended.
Paradoxical IP extension on attempted finger flexion from a distal FDP graft that is too short, pulling the first lumbrical proximally and transmitting the flexion force through the lateral band to produce IP extension.
