Lumbricals

The lumbricals are four small, intrinsic muscles located in the hand. They are unique in that they do not attach directly to bone, but rather originate from the tendons of the flexor digitorum profundus and insert into the extensor expansions of the fingers. These muscles play a crucial role in the fine motor movements of the hand, particularly in the flexion of the metacarpophalangeal joints and the extension of the interphalangeal joints.

Intrinsic Muscles of the Hand

Anatomy

Origin

The lumbricals originate from the tendons of the flexor digitorum profundus muscle. Specifically, the first and second lumbricals arise from the radial sides of the tendons of the index and middle fingers, respectively. The third and fourth lumbricals originate from the adjacent sides of the tendons of the middle, ring, and little fingers.

Insertion

The lumbricals insert into the extensor expansions on the dorsal aspect of the proximal phalanges of the fingers. This insertion allows them to influence both the flexion and extension of the fingers.

Innervation

The lumbricals are innervated by two different nerves:

• The first and second lumbricals (lateral two) are innervated by the median nerve.
• The third and fourth lumbricals (medial two) are innervated by the ulnar nerve.

Blood Supply

The blood supply to the lumbricals is provided by the superficial palmar arch, the common palmar digital arteries, and the deep palmar arch.

Function

The primary function of the lumbricals is to flex the metacarpophalangeal joints while extending the interphalangeal joints of the fingers. This action is essential for the "upstroke" of writing and for many other fine motor tasks that require precision and dexterity.

Clinical Significance

Lumbrical Plus Finger

A condition known as "lumbrical plus finger" can occur when there is an imbalance in the function of the lumbricals, often due to injury or surgical intervention. This condition results in paradoxical extension of the interphalangeal joints when attempting to flex the fingers.

Injury and Dysfunction

Injury to the nerves supplying the lumbricals, such as in carpal tunnel syndrome (affecting the median nerve) or ulnar nerve entrapment, can lead to weakness or paralysis of these muscles. This can result in difficulty with finger movements and a characteristic "claw hand" deformity.

See Also

• Flexor digitorum profundus
• Extensor expansion
• Median nerve
• Ulnar nerve

External Links

• Lumbricals of the Hand - NCBI Bookshelf
• Anatomy.tv - Lumbricals