Palmar interossei adduct the fingers toward the middle finger and assist MCP flexion and IP extension.
The palmar interossei muscles are small, deep intrinsic muscles of the hand that lie between the metacarpal bones. Their primary function is to adduct the fingers toward the axial line (middle finger) and assist in flexion at the metacarpophalangeal joints and extension at the interphalangeal joints. These muscles are critical for precise finger movements and coordinated grip.
The palmar interossei are located in the palmar side of the hand, lying between the metacarpal bones of the fingers (excluding the middle finger). They are positioned deep to the flexor tendons and lumbricals.
There are typically three palmar interossei muscles, though some individuals may have a fourth (usually associated with the thumb or little finger). Each muscle arises from a single metacarpal bone and inserts onto the base and extensor expansion of the proximal phalanx of the corresponding digit.
The palmar interossei muscles perform the following actions:
All palmar interossei are innervated by the deep branch of the ulnar nerve (C8–T1).
The palmar interossei lie deep to the lumbrical muscles and flexor tendons. They run alongside the sides of the metacarpals and blend into the lateral bands of the extensor expansions.
Due to their deep location, the palmar interossei are not visible or palpable on the surface. However, their function can be demonstrated by adduction of the fingers toward the midline of the hand.
These muscles arise from mesenchymal condensations in the hand's dorsal and palmar muscle masses. Their ulnar nerve innervation corresponds with their deep location and intrinsic function in the hand.
In non-human primates and other mammals, the interossei are generally less differentiated. In humans, their development supports the evolution of fine motor skills, precision pinch, and complex hand gestures necessary for tool use and communication.