Hand Muscle Anatomy: Interossei, Lumbricals, and Thenar

Dorsal Interossei Muscles

The Dorsal Interossei are intrinsic muscles of the hand. Their origin (O) is located on the adjacent sides of the metacarpal bones, specifically the dorsal half of the metacarpals looking toward the axis of the hand. Their insertion (I) is at the first phalanx and the dorsal aponeurosis of the fingers. Their primary action (A) includes flexion of the metacarpophalangeal joints and acting as separators (abductors) for the fingers around the middle finger axis. They are innervated (IN) by the ulnar nerve (C8-T1). Related structures (R) include the abductor pollicis and lumbricals.

Palmar (Ventral) Interossei Muscles

The Palmar (Ventral) Interossei have their origin (O) on the ventral half of the 2nd, 4th, and 5th metacarpals. Their insertion (I) is the dorsal aponeurosis and the base of the proximal phalanx of the 2nd, 4th, and 5th fingers. Their action (A) involves the metacarpophalangeal joints of the 2nd, 4th, and 5th fingers, facilitating interphalangeal flexion, extension, and adduction. Their innervation (IN) is the same as the dorsal interossei (ulnar nerve).

Lumbrical Muscles of the Hand

The Lumbricals have their origin (O) in the tendons of the flexor digitorum profundus. Their insertion (I) is the dorsal aponeurosis of digits 2 through 5. Specifically, the 1st lumbrical inserts into the 2nd digit, while the 3rd, 4th, and 5th lumbricals follow a similar pattern for the remaining digits. Their action (A) includes flexion of the metacarpophalangeal joints and extension of the interphalangeal joints for fingers 2-5. They are innervated (IN) by the median and ulnar nerves (C8-T1). Relations (R) include the abductor pollicis and the palmar interossei.

Adductor Pollicis Muscle

The Adductor Pollicis consists of two heads. The oblique origin (O) is from the anterior carpal bones, the capitate bone, the base of the 2nd metacarpal, and the radiated carpal ligaments. The transverse origin (O) is the lower edge of the 3rd metacarpal. Its insertion (I) is the base of the proximal phalanx of the thumb. The action (A) is the adduction of the thumb toward the index finger and slight flexion. It is innervated (IN) by the ulnar nerve (C8-T1). It relates (R) to the ventral and dorsal interossei, as well as the lumbricals, and crosses the flexor pollicis longus and opponens pollicis.

Thenar Muscles: Abductor, Opponens, and Flexor

Abductor Pollicis Brevis

The Abductor Pollicis Brevis has its origin (O) at the scaphoid bone and the flexor retinaculum. Its insertion (I) is the base of the proximal phalanx of the thumb. Its action (A) is to split and extend the thumb and phalanges. It is innervated (IN) by the median nerve (C6-C7). It covers the opponens pollicis and relates to the short flexor and adductor pollicis.

Opponens Pollicis

The Opponens Pollicis has its origin (O) at the trapezium bone and its insertion (I) at the radial border of the 1st metacarpal. Its action (A) is the opposition of the thumb. It is innervated (IN) by the median nerve (C6-C7). It is covered by the short flexor and abductor pollicis brevis.

Flexor Pollicis Brevis

The Flexor Pollicis Brevis has its origin (O) at the capitate bone, flexor retinaculum, and trapezium. Its insertion (I) is the base of the proximal phalanx of the thumb. Its action (A) includes flexion and opposition of the thumb at the metacarpophalangeal joint. It is innervated (IN) by both the median and ulnar nerves. It relates (R) to the adductor (dorsal plane) and abductor (ventral plane) and covers the opponens pollicis.