The four lumbricals are unique muscles that originate from tendons (the FDP tendons) rather than bone and insert into the extensor expansion, connecting the flexor and extensor systems. Their remarkable action of simultaneously flexing the MCP and extending the IP joints produces the intrinsic plus posture that is the foundation of precision grip. Their dual innervation reflects the median-ulnar boundary at the midhand.
| Origin | Radial sides of the FDP tendons of the index and middle fingers; Adjacent sides of the FDP tendons of the middle/ring and ring/little fingers |
|---|---|
| Insertion | Radial side of the extensor expansion of each finger |
| Nerve Supply | Radial two: median nerve (C8, T1); Ulnar two: deep branch of the ulnar nerve (C8, T1) |
| Blood Supply | Palmar digital arteries |
| Actions | Flexion of the MCP joints; Extension of the PIP and DIP joints simultaneously — the intrinsic plus action; Fine positioning of the fingers for precision grip |
|---|
The intrinsic plus action of MCP flexion with IP extension is what allows the fingers to form the precise cupped position for holding a pen, picking up small objects, and all dexterous manipulation. Without lumbrical function, the hand loses the ability to position the fingers in the intrinsic plus posture and falls into the intrinsic minus claw deformity.
Lumbrical plus deformity occurs paradoxically when the FDP tendon is cut — advancing the lumbrical origin proximal to normal produces paradoxical IP extension on attempted finger flexion. This complication of flexor tendon repair in the finger is avoided by proper tendon management. Lumbrical canal syndrome from lumbrical muscle hypertrophy in manual workers or weight lifters can produce carpal tunnel-like symptoms from proximal displacement of the hypertrophied muscle belly into the carpal tunnel with finger flexion.
The lumbricals are not individually palpable as they are small intramuscular structures within the palm, but their function is demonstrated by the intrinsic plus position and tested by resisting simultaneous MCP flexion with IP extension.
Paradoxical IP extension on attempted finger flexion from FDP tendon shortening relative to the lumbrical after flexor tendon repair, requiring surgical correction of tendon length imbalance.
