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First lumbrical muscle of hand

The first lumbrical muscle is one of four slender intrinsic muscles located in the palm of the hand. It lies in the mid-palmar region, lateral to the second metacarpal, and is associated with the index finger. The lumbricals are unique because they originate from tendons rather than bone. The first lumbrical is the most lateral and smallest of the group and plays a crucial role in the complex flexion–extension mechanics of the fingers, coordinating metacarpophalangeal (MCP) flexion and interphalangeal (IP) extension.

Synonyms

  • Radial lumbrical of the index finger

  • First palmar lumbrical

Origin, Course, and Insertion

Origin: Arises from the radial (lateral) side of the tendon of the flexor digitorum profundus for the index finger
Course: Passes distally and radially along the palm, superficial to the deep palmar arch, crossing the MCP joint of the index finger
Insertion: Into the lateral side of the extensor expansion (dorsal digital expansion) of the index finger, blending with the tendons of the extensor digitorum

Relations

Anteriorly: Palmar aponeurosis and superficial palmar arch
Posteriorly: Deep palmar arch and interossei muscles
Medially: Second lumbrical muscle
Laterally: First dorsal interosseous muscle
Distally: Extensor expansion of the index finger

Tendon Attachments

  • The tendon passes superficially to the deep transverse metacarpal ligament

  • Inserts into the radial side of the extensor expansion, aiding coordinated finger movements

  • Contributes to the lumbrical canal, through which neurovascular structures pass

Nerve Supply

  • Median nerve (digital branch, roots C8–T1)

Arterial Supply

  • Superficial and deep palmar arches via branches of the radial and ulnar arteries

Function

  • Flexion of the metacarpophalangeal (MCP) joint of the index finger

  • Extension of the interphalangeal (IP) joints via the extensor expansion

  • Fine motor control: Facilitates precision grip and delicate movements (e.g., writing, pinching)

  • Synergistic action: Works with interossei and long flexors/extensors for coordinated finger motion

Clinical Significance

  • Nerve injury: Median nerve lesions (carpal tunnel syndrome) cause weakness or paralysis of the first lumbrical, affecting fine motor control of the index finger

  • Overuse strain: May occur in repetitive finger flexion or prolonged grasping

  • Muscle atrophy: Seen in chronic neuropathies; clinically tested by flexion of MCP with extended IP joints

  • Surgical importance: Important landmark in carpal tunnel release and tendon transfer procedures

  • Imaging relevance: Evaluated in cases of denervation, traumatic laceration, and compressive neuropathies

MRI Appearance

T1-weighted images:

  • Muscle belly: intermediate signal intensity with clear fascicular architecture

  • Surrounding fat: bright signal, delineating muscle margins

  • Tendinous portions: low signal (dark linear bands)

  • Denervation: increased T1 signal (fatty infiltration) in chronic cases

T2-weighted images:

  • Normal muscle: intermediate-to-low signal, slightly darker than on T1

  • Pathologic muscle: hyperintense signal in acute denervation, strain, or edema

  • Tendons: dark low-signal structures within the lumbrical canal

STIR:

  • Normal muscle: intermediate-to-dark signal

  • Abnormal: bright hyperintense areas indicating edema, inflammation, or acute nerve compression

Proton Density Fat-Saturated (PD FS):

  • Normal: intermediate-to-dark, homogeneous muscle signal

  • Pathologic: bright focal or diffuse signal in strain, partial tear, or neuritis-related changes

T1 Fat-Sat Post-Contrast:

  • Normal: mild homogeneous enhancement

  • Inflamed or ischemic muscle: patchy or diffuse enhancement

  • Denervation or fibrosis: minimal or peripheral enhancement pattern

CT Appearance

Non-Contrast CT:

  • Muscle: uniform soft-tissue density, seen between flexor tendons and interossei

  • Tendons: slender, linear soft-tissue densities crossing MCP joint

  • Calcification or chronic fibrosis: focal hyperdensities along tendon path

Post-Contrast CT (standard):

  • Normal muscle: homogeneous enhancement

  • Inflammation or postoperative scarring: focal or diffuse enhancement

  • Useful for evaluating traumatic lacerations, tendon injuries, or foreign bodies in the palmar region

MRI image

First lumbrical muscle of hand of hand axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

First lumbrical muscle of hand of hand axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

MRI image

First lumbrical muscle of hand of hand axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00002

MRI image

First lumbrical muscle of hand of hand coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image

First lumbrical muscle of hand ct axial image