Tendon of first dorsal interosseous muscle

The tendon of first dorsal interosseous muscle inserts into the radial side of the base of the proximal phalanx of the index finger and its extensor hood. Functionally, the primary role of the first dorsal interosseous muscle is to abduct (spread) the index finger away from the midline of the hand (which runs through the middle finger). Furthermore, due to its connection to the dorsal aponeurosis, it also assists in flexion at the metacarpophalangeal (MCP) joint and extension at the interphalangeal (IP) joints of the index finger.