Hand Muscles and Nerve Supply — Intrinsic Muscles, LOAF, Claw Hand, Ape Hand and Wrist Drop

Hand muscles are split into extrinsic muscles, whose bellies lie in the forearm and send long tendons into the hand, and intrinsic muscles, which begin and end within the hand. The intrinsic muscles give fine motor control and contribute a large share of pinch and grip strength — weakness of the intrinsics costs roughly 60–80% of pinch and grip strength (StatPearls — Intrinsic Hand Deformity).

• Thenar group (ball of the thumb): abductor pollicis brevis, flexor pollicis brevis, opponens pollicis.
• Adductor pollicis: lies next to the thenar group but is not part of the thenar eminence and has a different nerve.
• Hypothenar group (ball of the little finger): abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi — plus the small subcutaneous palmaris brevis.
• Lumbricals: four slender muscles arising from the flexor digitorum profundus (FDP) tendons.
• Interossei: dorsal (abductors) and palmar (adductors), between the metacarpals.

Only two nerves supply them: the median nerve (enters through the carpal tunnel) and the ulnar nerve (enters through Guyon's canal). The radial nerve supplies no intrinsic hand muscle — its role in the hand is the forearm extensors and sensation over the dorsum.

The thenar muscles (APB, FPB, opponens) receive the recurrent motor branch of the median nerve, given off just distal to the carpal tunnel. This branch lies superficially near the middle of the thenar mass and carries no sensory fibres, so a knife or glass laceration can paralyse opposition while sensation stays normal — easy to miss in the emergency department (StatPearls — Median Nerve).

Both groups insert into the extensor expansion (dorsal hood), so they share one combined action: flex the metacarpophalangeal (MCP) joints and extend the interphalangeal (IP) joints — the 'writing' or 'L' position of the fingers. This is the key to understanding claw hand: lose them and the MCP joints hyperextend while the IP joints flex.

Some texts count a fourth palmar interosseous to the thumb; StatPearls lists 4 dorsal and 3 palmar in its intrinsic-muscle chapter. Either way, every interosseous is supplied by the deep branch of the ulnar nerve — which is why finger-spreading and finger-gripping tests examine the ulnar nerve.

The ulnar nerve carries C8 and T1 fibres, and the intrinsic muscles as a whole depend on the C8–T1 segments. That is why a lower trunk (C8–T1) brachial plexus injury — Klumpke palsy — paralyses every intrinsic muscle at once, producing a 'total claw hand' with palmar wasting, unlike an isolated ulnar lesion that spares the LOAF muscles.

Two branches localise lesions neatly. The palmar cutaneous branch of the median nerve arises proximal to the wrist and runs over the flexor retinaculum, so carpal tunnel syndrome spares sensation over the thenar eminence. The dorsal cutaneous branch of the ulnar nerve leaves the main trunk in the forearm, so wrist (Guyon canal) lesions spare the dorsal ulnar hand while elbow (cubital tunnel) lesions involve it.

Claw hand is an intrinsic-minus posture: the paralysed interossei and lumbricals can no longer flex the MCP joints or extend the IP joints, so the intact extrinsic muscles win — the long extensors hyperextend the MCP joints and FDP/FDS flex the IP joints. In a pure ulnar lesion the ring and little fingers claw most, because the median-supplied 1st and 2nd lumbricals still hold the index and middle fingers (StatPearls — Claw Hand).

• Froment sign — pinching paper between thumb and index finger, the thumb IP joint flexes because flexor pollicis longus (median) substitutes for a weak adductor pollicis (ulnar).
• Jeanne sign — reciprocal hyperextension of the thumb MCP joint during the same pinch.
• Wartenberg sign — the little finger rests abducted: the third palmar interosseous is weak and the radial-supplied extensor digiti minimi pulls unopposed.
• Duchenne sign — clawing of the ring and little fingers (MCP hyperextension, PIP flexion).
• Egawa (Pitres-Testut) sign — cannot abduct the middle finger side to side with the palm flat (2nd and 3rd dorsal interossei).
• Masse sign — flattened palmar metacarpal arch from hypothenar and interosseous wasting.

After carpal tunnel syndrome, ulnar entrapment is the second commonest compressive neuropathy of the upper limb, and the cubital tunnel is its commonest site. Leprosy is an important systemic cause of ulnar claw hand in Indian practice.

Ape hand follows loss of the thenar muscles — classically a low median lesion (wrist laceration, long-standing carpal tunnel syndrome) or injury to the recurrent motor branch. The thenar eminence flattens and the thumb lies adducted, in the plane of the palm, unable to abduct or oppose (StatPearls — Median Nerve Injury).

Hand of benediction (papal benediction sign) is taught in Indian exams as the sign of a high (elbow or above) median lesion: when the patient tries to make a fist, the index and middle fingers stay extended because FDS and the lateral half of FDP are paralysed, while the ring and little fingers flex through the ulnar-supplied FDP. Flexor pollicis longus is also lost, so the thumb tip cannot flex.

The radial nerve supplies no intrinsic muscle, but it supplies the wrist and finger extensors, so its injury produces wrist drop. The spiral groove of the humerus is the most common site — mid-shaft humerus fracture or prolonged compression ('Saturday night palsy'). Triceps strength is typically preserved at this level; brachioradialis and the extensors are weak, and sensation is lost over the dorsum of the hand, especially the first web space (StatPearls — Wrist Drop).

In carpal tunnel syndrome, numbness and paresthesia start in the thumb, index, middle and radial half of the ring finger, later come weak abduction and opposition and thenar atrophy. Phalen test: both wrists held fully flexed back-to-back for one minute reproduces the symptoms. Tinel test: tapping over the carpal tunnel — StatPearls quotes a sensitivity of 50% and specificity of 77%. Surgical release divides the transverse carpal ligament; the commonest complication is a neuroma of the palmar cutaneous branch (StatPearls — Carpal Tunnel Syndrome).

• Nerve supply one-liners — adductor pollicis (ulnar, deep branch), deep head of FPB (ulnar), palmaris brevis (ulnar, superficial branch), 1st/2nd lumbricals (median).
• Action questions — abduction of fingers (dorsal interossei), adduction (palmar interossei), MCP flexion with IP extension (lumbricals and interossei).
• Clinical signs — Froment (adductor pollicis vs FPL), Wartenberg (third palmar interosseous), Egawa (dorsal interossei), inability to make an 'O' in anterior interosseous nerve palsy.
• Image-based — photograph of a claw, ape thumb, benediction posture or wrist drop: name the nerve and the level.
• Paradox questions — ulnar paradox (distal lesion claws more); Klumpke palsy = total claw.
• Tunnels — contents of the carpal tunnel, boundaries of Guyon's canal, which sensory branch is spared.

For more patterns across years, see most repeated topics and solve recalled papers on the NEET PG PYQ and INI-CET PYQ pages. Related upper-limb topics: Erb's palsy and the brachial plexus and Volkmann's ischaemic contracture.