With few muscles in the hand, the majority of the movement of the fingers originates in the muscles of the wrist and forearm. These muscles act on ligaments and tendons in the hand to extend, or lift, the fingers. As it is often the case that many muscles act on the same joint and some muscles act on multiple joints in different fingers, the extension of the fingers is a complicated process.
Terminology -- Dorsal, Ventral, Proximal, Distal
With anatomy common to or similar across all fingers, shared terminology is used in descriptions of the muscles involved in extension of the fingers. Locations of the muscles, bone segments and joints are described using dorsal, ventral, proximal and distal sides. Dorsal is the upper and ventral is the lower side of the arm and fingers when the arms are extended, palm-side down. Distal and proximal refer to joints and bone segments further away from and closer to the palm, respectively.
Terminology -- Joints and Phalanges
A phalanx is a bone segment, with all fingers having a proximal and distal phalanx and only the thumb lacking a middle phalanx. The joints of the fingers refer to these phalanges, with the thumb having only an interphalangeal -- IP -- joint and all other fingers having both distal and proximal interphalangeal joints -- DIP and PIP, respectively. All fingers are connected to the hand by metacarpophalangeal -- MCP -- joints.
Thumb
Extension of the thumb's IP joint primarily arises from the extensor pollicis longus muscle, located in the dorsal side of the forearm. Extension of the thumb's MCP joint is controlled by the extensor pollicis brevis, also located on the dorsal side of the forearm. In addition to extension, this muscle is involved with abduction of the thumb, which occurs when spreading the fingers.
Index Finger
Extension of both the MCP and PIP joints of the index finger is controlled by the extensor indicis muscle, located deep in the dorsal side of the forearm. Extending the same two joints is the extensor digitorum index, originating in the shared digital extensor mechanism located across the MCP joint of all fingers but the thumb. This tendon is controlled by the extensor digitorum communis muscle, located in the dorsal forearm, and extends to the dorsal side of the middle phalanx of the index finger.
Middle Finger
Extension of the middle finger's MCP and PIP joints is controlled by the extensor digitorum middle tendon, originating in the extensor digitorum communis and extending to the middle phalanx of the middle finger.
Ring Finger
Extension of the MCP and PIP joints of the ring finger is controlled by the extensor digitorum ring tendon. Like the index and middle fingers, this muscle is grounded in the extensor digitorum communis muscle and extends to the middle phalanx of the ring finger.
Pinkie Finger
Like the index, middle and ring fingers, extension of the MCP and PIP joints of the pinkiel finger is controlled by the extensor digitorum tendon. In addition to extension of these joints, the extensor digiti minimi is also involved with extension of the carpometacarpal joint, located near the wrist on the small finger side of the hand.
