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.