Lumbrical muscle
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The lumbrical muscles are intrinsic muscles in the fingers that allow flexion at the metacarpophalangeal joints, while maintaining extension at the interphalangeal joints.
There are four of these small worm-like muscles on each hand. This muscles are unusual in that they do not attach to bone, instead attaching proximally to the tendons of flexor digitorum profundus and distally to extensor expansions on the dorsal surface (back of) the hand.
The medial two lumbricals, those of the ring and the little finger, are bifurcated (attach proximally to two tendons each). Their motor supply comes from the ulnar nerve.
The lumbrical muscles of the index and middle finger are innervated by the median nerve.
There are also lumbrical muscles of the foot that have a similar action, though these are of less clinical concern.