The lumbricals are four thin muscles (the name means worm-like) which are each associated with a finger.
The lumbricals arise from the sides of the tendons of the flexor digitorum profundus.
The 2 medial lumbricals are bipennate and are innervated by the deep branch of the ulnar nerve
The 2 lateral lumbricals are unipennate and are innervated by digital branches of the median nerve
Use the mnemonic MEAT LOAF for remembering innervation of the intrinsic hand muscles. Meat = Median nerve innervates the LOAF muscles: Lateral 2 lumbricals, opponens pollicis, abductor pollicis brevis, flexor pollicis brevis. The rest of the intrinsic hand muscles are innervated by the ulnar nerve.
- Sides of the flexor digitorum profundus tendon
- Lumbricals 1 and 2 – radial sides and palmar surface of tendons of index and middle finger (unipennate muscles)
- Lumbricals 3 and 4 – adjacent sides of middle and ring, and ring and little fingers (respectively)
- Extensor hoods of associated digits (radial side, insert lateral extensor expansion)
The lumbricals are interesting because they form a link between the flexor tendons and the extensor tendons through their origin on the flexor digitorum profundus and their insertion onto the extensor expansion arising from the extensor digitorum tendon.
This unique anatomy enables the two main functions:
- Flexion at metacarpophalangeal joints
- Extension at interphalangeal joints
- Lateral 2 lumbricals – median nerve
- Medial 2 lumbricals – deep branch of ulnar nerve
- Lateral 2 lumbricals: first and second dorsal metacarpal arteries and dorsal digital arteries
- Medial 2 lumbricals: second and third common palmar digital arteries, 3rd and 4th dorsal digital arteries and anastomoses with palmar digital arteries.