Muscular system · Upper limb

Lumbricals !

The lumbricals are four small palm muscles running from the flexor tendons to the extensor mechanism; they bend the knuckles while straightening the fingers.

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What it is

Four small worm-shaped muscles in the palm, starting on the FDP tendons.

What it does

Bend the knuckles while straightening the finger joints (the 'tabletop' position).

Key facts

Origin

Tendons of FDP

Insertion

Lateral sides of the extensor expansions of digits 2–5

Action

Flex the MCP joints and extend the IP joints

Nerve

1st–2nd: median nerve; 3rd–4th: deep ulnar nerve (C8–T1)

Blood supply

Superficial and deep palmar arches

Relations

Superficial

FDS tendons, the superficial palmar arch and the common palmar digital nerves and arteries, under the palmar aponeurosis

Deep

The interossei, the adductor pollicis (for the first two), the deep palmar arch and the deep branch of the ulnar nerve

Course

From the FDP tendons in the palm each runs along the radial side of an MCP joint, palmar to the deep transverse metacarpal ligament, to the radial lateral band of the extensor mechanism

How to find it

Not palpable. In the palm they are found along the radial side of each FDP tendon beyond the carpal tunnel; their combined action is the tabletop position (knuckles bent, finger joints straight).

Examination

• Tabletop hold: the fingers are held with the knuckles bent and the IP joints straight against pressure; weakness of the ring and little fingers with index and middle preserved fits an ulnar nerve lesion

On imaging

Too small to assess routinely. On high-resolution ultrasound and MRI they are thin bellies on the radial side of the FDP tendons in the palm, beside the common digital nerves. They are rarely imaged for their own sake: imaging assesses the FDP tendons and the median and ulnar nerves instead, and MRI shows denervation of the hand muscles as a group.

Clinical notes

• Ulnar paradox: clawing is worse after an ulnar lesion at the wrist than at the elbow, because the ring and little FDP still pull the fingertips into flexion
• Lumbrical-plus finger: after an FDP cut beyond the lumbrical origin, trying to bend the finger pulls through the lumbrical and paradoxically straightens the IP joints
• The hand is splinted in the intrinsic-plus (safe) position, knuckles bent and finger joints straight, to keep the collateral ligaments and volar plates from shortening

Anatomical variants

• The textbook split (median for the first two, ulnar for the last two) is found in only about half to three-fifths of hands; other patterns, often involving the 3rd lumbrical, occur in a sizeable minority

Memory aid

Median nerve muscles of the hand: LOAF (Lateral two Lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis).

Related conditions

• Cubital tunnel syndrome (ulnar neuropathy at the elbow)

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2)

musculi lumbricales manus TA2 2532

Official Spanish (DeCS)

Músculos lumbricales de la mano

English (TA2)

Lumbricals

Related

• Median nerve
• Ulnar nerve in the hand
• Superficial palmar arch
• 2nd metacarpal
• 3rd metacarpal
• 4th metacarpal
• Proximal phalanx, index finger
• Proximal phalanx, middle finger

Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Hand Lumbrical Muscles.

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Representative model, for educational purposes only.