Lumbrical muscles of the hand
The lumbrical muscles, which get their name due to their worm-like appearance (lumbricidae - Latin = earthworm), are four short intrinsic muscles of the hand located between the metacarpal bones, deep to the palmar fascia. Along with the dorsal interossei and the palmar interossei, the lumbrical muscles belong to the short muscles of the hand.
The lumbrical muscles of the hand flex the fingers at the metacarpophalangeal (MCP) joints, and extend them at the interphalangeal (IP) joints. These actions are important for many functions of the hand, such as gripping movements. Their counterparts, the lumbricals of the foot, have a similar action on the toes.
This article will discuss the anatomy and function of the lumbrical muscles of the hand.
|Tendons of flexor digitorum profundus muscle
|Extensor expansion of hand
Lumbricals 1-2: Median nerve (C8-T1)
Lumbricals 3-4: Ulnar nerve (C8-T1)
Mnemonic: '1 2 me, 3 4 u' (One to me, three for you)
|Dorsal carpal arch (dorsal metacarpal and dorsal digital arteries), superficial palmar arch (common palmar metacarpal arteries)
Metacarpophalangeal joints 2-5: Finger flexion
Interphalangeal joints 2-5: Finger extension
Origin and insertion
The lumbrical muscles of the hand are numbered 1-4 from the most radial/lateral to the most ulnar/medial. Each lumbrical muscle originates from one or two adjacent tendons of the flexor digitorum profundus muscle. Lumbricals 1-2 are usually unipennate, meaning their fibers arise obliquely from one tendon, while lumbricals 3-4 are bipennate, arising from two adjacent tendons.
- The first lumbrical arises from the radial side and palmar surface of the tendon of the index finger.
- The second lumbrical arises from the radial side and palmar surface of the tendon of the middle finger.
- The third lumbrical arises from the ulnar side of the middle finger rendon and the radial side of ring finger tendon.
- The fourth lumbrical arises from the ulnar side of the ring finger tendon and the radial side of the little finger tendon.
From their points of origin, each of the lumbrical muscles passes distally along the radial side of the corresponding finger, anterior to the deep transverse metacarpal ligament. The lumbricals then extend obliquely to attach on to the lateral margin of the extensor expansion of hand (dorsal digital expansion) of the extensor digitorum muscle at the proximal phalanges of digits 2-5.
Want an easy and effective way to revise the origins, insertions, innervations and functions of the muscles? Check out our muscle anatomy charts, which contain all this information in four convenient PDFs.
Here's a mnemonic to help you learn the innervation of the lumbricals in no time at all!
'1 2 me, 3 4 u' (One to me, three for you)
- 1st and 2nd lumbricals: median nerve
- 3rd and 4th lumbricals: ulnar nerve
Learn more about the anatomy of hand muscles, including the lumbricals, using the articles, videos, illustrations and quizzes included in the following study units:
The lumbricals receive most of their arterial supply from the anastomotic network on the dorsal surface of the hand known as the dorsal carpal arch. Its branches, first and second dorsal metacarpal arteries and dorsal digital arteries, supply the first and second lumbricals.
The third and fourth dorsal digital arteries, as well as second and third common palmar digital arteries (branches of the superficial palmar arch) supply the third and fourth lumbricals.
The fact that the lumbrical muscles of the hand originate from tendons and insert into the extensor expansions, instead of bony structures, makes both of their attachment points quite mobile. That means the muscles are capable of two different actions. These are flexion at the metacarpophalangeal joints and extension in both the proximal (PIP) and distal interphalangeal joints (DIP). The reason for the opposite actions is that the tendons cross the MCP on the palmar side but distally insert at the dorsal side of the finger. These combined movements play a role in complex movement of the hand (e.g. for holding a pen), and contribute to the general dexterity of the hand.
In addition, it is found that the lumbrical muscles of the hand contain many muscle spindles and have a large fiber length, which indicates that they likely play a role in proprioception.
Don't forget to quiz yourself on the lumbrical and other muscles of the hand to consolidate your knowledge!
When the tendons of the flexor digitorum profundus detach distal to the origin points of the lumbricals an interesting phenomenon occurs: when trying to close the fist, the fingers strangely extend instead.
After detachment of the distal tendons, the lumbricals now serve as the new insertion surface of the flexor digitorum profundus. This means, even though the person consciously activates the flexor muscle, they actually move the lumbricals instead. And since the flexor digitorum profundus and the lumbricals are antagonists in the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints, the intended fist closure paradoxically leads to an extension of the fingers. This oddity is clinically referred to as the lumbrical-plus finger and can occur after injuries or amputations.
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