Hand functions

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Here is a comprehensive overview of hand functions — the muscles, tendons, nerves, and movements that enable the hand's remarkable capabilities.

Hand Functions

Overview

The hand performs two broad categories of function:

Prehensile (grasping) — power grip, precision pinch, lateral pinch, opposition
Non-prehensile — pushing, tapping, pressing

These are executed through a coordinated interplay of extrinsic muscles (originating in the forearm) and intrinsic muscles (wholly within the hand).

1. Intrinsic Muscles of the Hand

Intrinsic muscles have both their origin and insertion within the hand. They are organized into three groups:

A. Thenar Muscles (thumb eminence)

All act on the thumb at the carpometacarpal (CMC) and metacarpophalangeal (MCP) joints.

Muscle	Action	Nerve
Abductor pollicis brevis	Thumb abduction	Median (C8, T1)
Opponens pollicis	Thumb opposition (CMC)	Median (C8, T1)
Flexor pollicis brevis (superficial head)	Thumb flexion (MCP)	Median (C8, T1)
Flexor pollicis brevis (deep head)	Thumb flexion	Ulnar (C8, T1)
Adductor pollicis	Thumb adduction, MCP flexion	Ulnar (C8, T1)

Thenar and hypothenar muscles — palmar schematic

The ability to grasp an object between the pad of the thumb and a finger depends entirely on the thenar muscles (recurrent branch of the median nerve, C8–T1). — Gray's Anatomy for Students

B. Hypothenar Muscles (little-finger eminence)

All innervated by the ulnar nerve (C8, T1).

Muscle	Action
Abductor digiti minimi	Abduction + MCP flexion of little finger; PIP/DIP extension
Flexor digiti minimi brevis	MCP flexion of little finger
Opponens digiti minimi	Opposition of little finger

C. Metacarpal Muscles (Lumbricals and Interossei)

Lumbricals (×4)

Origin: radial sides of the flexor digitorum profundus (FDP) tendons
Insertion: dorsal digital expansion (extensor hood) of digits 2–5
Action: Flex MCP joints + extend PIP and DIP joints simultaneously — the key "intrinsic-plus" position
Innervation: 1st & 2nd → Median nerve; 3rd & 4th → Ulnar nerve (both C8, T1)

Lumbrical and interossei muscles — palmar view

Dorsal Interossei (×4)

Origin: adjacent sides of metacarpals 1–5 (bipennate)
Action: MCP flexion + abduction of fingers away from the middle finger axis; PIP/DIP extension
Innervation: Ulnar nerve (C8, T1)

Palmar Interossei (×3)

Origin: ulnar side of MC2; radial sides of MC4 and MC5
Action: MCP flexion + adduction of fingers toward the middle finger; PIP/DIP extension
Innervation: Ulnar nerve (C8, T1)

Dorsal vs. palmar interossei — abduction and adduction actions

Flexing the MCP joints while simultaneously extending the PIP and DIP joints is entirely dependent on the intrinsic muscles, mainly innervated by the deep branch of the ulnar nerve (C8–T1). — Gray's Anatomy for Students

2. Extrinsic Tendons

Flexor Tendons

Nine tendons pass through the carpal tunnel:

Flexor digitorum superficialis (FDS) ×4 — inserts on the volar proximal middle phalanx → flexes PIP and MCP joints
Flexor digitorum profundus (FDP) ×4 — inserts at the volar base of the distal phalanx → primarily flexes DIP joint, also PIP and MCP
Flexor pollicis longus (FPL) ×1 — inserts at the base of the thumb distal phalanx → flexes thumb IP joint

The FDS and FDP run together in synovial sheaths within fibro-osseous tunnels (flexor pulleys), making them susceptible to infectious tenosynovitis.

Extensor Tendons

Nine extensor tendons course under the extensor retinaculum through 6 compartments. In the finger, the extensor mechanism divides into:

Central slip → attaches to the middle phalanx → extends PIP
Two lateral bands (joined by lumbrical and interosseous contributions) → attach to the dorsal base of the distal phalanx as the terminal tendon → extend DIP

Adjacent extensor digitorum communis (EDC) tendons are connected by juncturae tendinum — so a complete EDC laceration proximal to a juncture may still allow some finger extension via the neighboring tendon.

3. Functional Movements of the Hand

Movement	Primary Muscles	Nerve
Finger flexion (DIP)	FDP	Anterior interosseous (median) to digits 2–3; ulnar to digits 4–5
Finger flexion (PIP)	FDS	Median nerve
Finger flexion (MCP)	Interossei, lumbricals	Ulnar (± median for 1st/2nd lumbricals)
Finger extension	EDC + EIP + EDM	Radial/posterior interosseous nerve
Finger abduction	Dorsal interossei + abductor digiti minimi	Ulnar nerve
Finger adduction	Palmar interossei	Ulnar nerve
Thumb opposition	Opponens pollicis, abductor pollicis brevis, FPB	Median nerve (recurrent branch)
Thumb adduction	Adductor pollicis	Ulnar nerve
Thumb IP flexion	FPL	Anterior interosseous nerve (median)

4. Nerve Contributions — Summary

Nerve	Key Hand Functions
Median	Thenar opposition/abduction; 1st & 2nd lumbrical; FDS & FDP (digits 2–3); FPL
Ulnar	All interossei; 3rd & 4th lumbricals; hypothenar muscles; adductor pollicis; FDP (digits 4–5)
Radial (PIN)	All finger and wrist extensors; no intrinsic function

5. Clinical Correlates

Ulnar nerve palsy → loss of interossei and most intrinsics → "claw hand" (MCP hyperextension, PIP/DIP flexion) most pronounced in ring and little fingers; loss of finger abduction/adduction
Median nerve palsy → loss of thenar muscles → "ape hand" (inability to oppose thumb); loss of FDS, FDP digits 2–3
Radial nerve palsy → wrist drop; no intrinsic motor loss
Boutonnière deformity — central slip rupture → PIP flexion + DIP hyperextension
Mallet finger — terminal tendon avulsion → isolated DIP flexion deformity

Sources: Gray's Anatomy for Students; THIEME Atlas of General Anatomy and Musculoskeletal System; Tintinalli's Emergency Medicine

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