Lumbrical muscles (Hand)

The lumbrical muscles are a set of four small muscles in each hand that act to flex the fingers at the metacarpophalangeal joints, and extend them at the interphalangeal joints. The result is that the ends of the fingers are straightened, but bent at the first knuckle.

The foot has equivalent muscles, also called lumbrical muscles.

Origin
The lumbrical muscles of the hand originate from the four tendons of the flexor digitorum profundus muscle as they enter the hand. This is unusual as most muscles originate from a bone.

Insertion
The lumbrical muscles of the hand insert on each of the four fingers on the radial side of the extensor expansion at the dorsum of each proximal phalanx.

Innervation
The lumbrical muscles of the hand are innervated by the median nerve and the deep branch of ulnar nerve (C8 and T1). The median nerve innervates the two muscles on the lateral side while the deep branch of ulnar nerve innervates the two on the medial side.