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Lumbricals I, II, III, IV

OVERVIEW
Notations:

I – Index Finger (2nd Digit) Lumbrical
II – Long Finger (3rd Digit) Lumbrical
III – Ring Finger (4th Digit) Lumbrical
IV– Little Finger (5th Digit) Lumbrical

Key Points:

• Flexes the metacarpal-phalangeal joint.
• Extends the interphalangeal joint.
• Intrinsic hand muscles.
• With wrist and finger flexion, the lumbricals may move proximally into the carpal tunnel and increase pressure on the median nerve.

EXAMINATION
Muscle Function:
(for each corresponding finger)
Primary:
• Metacarpal-phalangeal joint flexion.
• Proximal and distal interphalangeal joint extension.
Unique Attributes:
• Links the finger flexors to the finger extensors.
• Origin is a tendon (flexor digitorum profundus), so the function may vary depending on the status of the tendon.
Palpation: Muscle is too deep to palpate.
 
Strength Testing: Test each lumbrical individually with both of the following tests.
• Position – interphalangeal joints in extension.  Stabilize – the wrist. Resist – metacarpal-phalangeal joint flexion by applying pressure over palmar proximal phalanges (in the direction of extension).
• Position – metacarpal-phalangeal joint in flexion or extension. Stabilize – the wrist. Resist – finger extension at the interphalangeal joints by applying pressure to the middle and distal phalanx respectively (in the direction of flexion).
Relevant Anatomy:
Innervation:
• Roots: C8, T1.
• Nerve:
• Lumbrical I, II – Median nerve, proper (I) and common (II) palmar digital nerves.
• Lumbrical III, VI – Ulnar nerve, deep motor branch.
• Innervation Route: C8, T1 →
• Median nerve → proper palmar digital nerve → lumbrical I branch.
• Median nerve → common palmar digital nerve → lumbrical II branch.
• Ulnar nerve → deep motor branch → lumbrical III, IV branches.
• Innervation Variations:
• Lumbrical I can be innervated by ulnar nerve.
• Lumbrical II can be innervated by ulnar, or by median and ulnar nerves.
• Lumbrical III can be innervated by median nerve.
Origin:
• Index and long finger lumbricals (unipennate) – radial side of flexor digitorum profundus tendons to index and long fingers.
• Ring finger lumbrical (bipennate) – ulnar aspect of long finger flexor digitorum profundus tendon and radial aspect of ring finger flexor digitorum profundus tendon.
• Little finger lumbrical (bipennate) – ulnar aspect of ring finger flexor digitorum profundus tendon and radial aspect of little finger flexor digitorum profundus tendon.
Insertion:
• Radial side of the extensor hood of the respective fingers.
• Note that at the metacarpal-phalangeal joint level (except at index finger), the lumbrical is palmar to the deep transverse metacarpal ligament (therefore provides flexion at metacarpal phalangeal joint).
• Then the lumbrical passes dorsal to the axis of the finger to insert on the extensor hood and contributes to extension at the interphalangeal joints.