Now let’s look at the muscles that extend the fingers. There are three, a large one, that extends all four fingers, and two small ones, for the index and little fingers.
The large one is extensor digitorum, sometimes called extensor digitorum communis. It arises from the common extensor tendon, and thereby from the lateral epicondyle. As it passes distally it divides into four slips, which pass together under the extensor retinaculum. We’ll follow them beyond there in a minute.
The extensor muscle to the little finger, extensor digiti minimi, arises from the ulnar side of extensor digitorum, and passes under the retinaculum by itself. The extensor muscle to the index finger, extensor indicis, lies deep to extensor digitorum. It arises from the ulna and the interosseous membrane. Its tendon passes under the retinaculum along with extensor digitorum.
Emerging from beneath the extensor retinaculum, the extensor tendons fan out. As they approach the MP joints they branch and rejoin in an irregular fashion. Extensor indicis and extensor digiti minimi join the respective extensor digtorum tendons as they reach the MP joint. Here at the MP joint each extensor tendon gives rise to the extensor hood, then divides into three parts, as we saw when we looked at the extensor mechanism a little while back. The extensor muscles produce extension at all three joints of the finger. Their main effect is at the MP joint.
As we’ll see later, the interosseous muscles and the lumbricals also have major roles in extending the interphalangeal joints.