The fourth lumbrical is bipennate, the most ulnar of the four hand lumbricals, and is exclusively innervated by the ulnar nerve. Its loss in ulnar palsy produces the most severe clawing of the little finger — the intrinsic-minus deformity — because the little finger has no median nerve lumbrical overlap and depends entirely on the fourth lumbrical for extensor hood function.
| Origin | Adjacent sides of the third and fourth FDP tendons — bipennate |
|---|---|
| Insertion | Radial side of the extensor expansion of the little finger |
| Nerve Supply | Ulnar nerve deep branch (C8, T1) |
| Blood Supply | Fourth palmar metacarpal artery |
| Actions | Flexes the MCP joint of the little finger; Extends the PIP and DIP joints of the little finger |
|---|
The fourth lumbrical is the most affected in ulnar nerve palsy, and little finger clawing is the most visible sign of intrinsic minus. Its exclusive ulnar innervation makes it a reliable EMG target for ulnar nerve motor studies at the wrist. Paradoxically, low ulnar nerve lesions at the wrist produce more severe clawing than high lesions because FDP (which drives the deformity) is intact in low lesions but paralysed in high ones — the 'ulnar paradox'.
Not palpable directly. Tested by little finger MCP flexion with IP extension in the intrinsic-plus position.
More severe little finger claw deformity in low ulnar nerve lesions than high, because the fourth lumbrical loss creates IP extension failure while the intact FDP (in low lesions) drives the MCP into hyperextension more forcefully than the paralysed FDP of high lesions.
