What are the 4 lumbrical muscles?

The Four Lumbrical Muscles of the Hand

The four lumbrical muscles of the hand are intrinsic muscles that originate from the flexor digitorum profundus tendons and insert into the extensor expansions on the radial side of each finger.

Anatomy and Identification

The four lumbrical muscles are:

1. First Lumbrical - Unipennate muscle originating from the radial side of the first flexor digitorum profundus tendon (index finger)
2. Second Lumbrical - Unipennate muscle originating from the radial side of the second flexor digitorum profundus tendon (middle finger)
3. Third Lumbrical - Bipennate muscle originating from adjacent sides of the second and third flexor digitorum profundus tendons (middle and ring fingers)
4. Fourth Lumbrical - Bipennate muscle originating from adjacent sides of the third and fourth flexor digitorum profundus tendons (ring and small fingers)

Innervation

• First and Second Lumbricals: Innervated by the median nerve 1
• Third and Fourth Lumbricals: Innervated by the deep branch of the ulnar nerve 1

Function

The lumbrical muscles have unique biomechanical properties that allow them to:

• Flex the metacarpophalangeal joints (MCPJ)
• Extend the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints 2
• Contribute to coordinated finger movements by:
  • Unloading elastic tension across the interphalangeal joints
  • Facilitating extension of these joints
  • Adding a small but significant flexor force at the metacarpophalangeal joint 3
  • Potentially contributing to radial deviation and rotation of the fingers

Clinical Significance

1. Injury Patterns: Lumbrical muscles can be injured during activities requiring asymmetric hand postures, particularly in climbers. The injury mechanism often involves shear forces between adjacent flexor digitorum profundus tendons when they glide in opposite directions 4.

2. Anatomical Variations: Variations in lumbrical muscle anatomy have been documented, including:

   • Absence of muscles
   • Reduction in number
   • Presence of accessory slips
   • Variations in insertion patterns 5

3. Mechanical Advantage: Research suggests that the anatomical mechanics for motor function of the lumbrical muscles are similar across all fingers, which may contribute to coordinated finger movements and adequate hand function 2.

4. Proprioceptive Role: Lumbricals contain numerous muscle spindles and may play a role in controlling finger movement by monitoring the rate of hand closing during grasp 3.

Surgical Considerations

When performing hand surgery, preservation of the lumbrical muscles and their innervation is important for maintaining fine motor control of the fingers. The complex innervation patterns and branching to different parts of the muscle bellies should be considered when dealing with deep injuries in the palmar region, including transmetacarpal amputations 1.

Understanding the anatomy and function of these small but important muscles is crucial for diagnosing and treating hand injuries and disorders affecting finger coordination and dexterity.