Lumbrical muscles (hand, anatomy)

The four lumbrical muscles arise from the tendons of flexor digitorum profundus. They have different origins:

• first and second lumbricals from the radial side of the tendons of flexor digitorum profundus to the index and middle fingers respectively
• third lumbrical from both sides of the tendons to the middle and ring fingers
• fourth lumbrical from both sides of the tendons to the ring and little fingers

Each passes distally to the radial side of its nearest metacarpophalangeal joint of the fingers to be inserted into the dorsal extensor expansion of digits two to five.

Their actions on these digits are to:

• extend the interphalangeal joints
• flex the metacarpophalangeal joints

The innervation of the lumbricals is dual:

• the radial first and second lumbricals are supplied by the median nerve (C8, T1)
• the ulnar third and fourth lumbricals are supplied by the deep branch of the ulnar nerve (C8, T1)

Occasionally, the third lumbrical can receive its innervation from the median nerve.