Lumbrical muscles of hand

The lumbrical muscles of hand are four slender, worm-like (resembling lumbricus) intrinsic muscles located deep within the midpalmar space. They are unique among hand muscles as they originate from a tendon (specifically the flexor digitorum profundus) rather than a bone.

Origin and Insertion

The lumbrical muscles originate from the tendons of the flexor digitorum profundus. They then extend to the dorsal aspect of the digits by passing around the lateral sides of the metacarpophalangeal joints and insert into the extensor expansions (dorsal digital apparatus) of the fingers.

• The first and second are unipennate and arise from the radial sides and volar surfaces of the tendons of the index and middle fingers, respectively

• The third is bipennate and arises from the contiguous sides of the flexor digitorum profundus tendons of the middle and ring fingers

• The fourth is bipennate and arises from the contiguous sides of the flexor digitorum profundus tendons of the ring and little fingers.

Innervation

The first and second lumbricals are typically innervated by the median nerve (common palmar digital branches), whereas the third and fourth lumbricals receive their innervation from the deep branch of the ulnar nerve.

Action

All four lumbricals muscles of hand share a common primary action: they flex the metacarpophalangeal (MCP) joints while simultaneously extending the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints. This coordinated action is crucial for the fine and delicate movements of the fingers, essential for tasks like gripping objects, typing, and other dexterous hand movements. Although they originate from the flexor digitorum profundus (FDP) tendons, the lumbricals' actions partially oppose the FDP, as the FDP flexes all three joints (MCP, PIP, and DIP).

Blood supply

The arterial circulation to the lumbricals arises from four different vessels, with the superficial palmar arch being the most important contributor. Other arteries forming the anastomotic network in this region include the deep palmar arch, dorsal digital arteries, and palmar digital arteries. Corresponding veins accompany these arteries, draining into the cephalic vein on the lateral side of the hand and the basilic vein on the medial aspect.