What is the primary action of extensor digitorum?

• A. Flexes the fingers at the MCP joints
• B. Extends the fingers at the MCP joints (and assists in IP extension)
• C. Extends the wrist
• D. Abducts the fingers

Answer: (B)

Extensor digitorum is primarily responsible for straightening the fingers at the metacarpophalangeal joints. When it contracts, it pulls the proximal phalanges back from a flexed position, extending the MCP joints to open the fingers. Because its tendon splits into the extensor mechanism that also connects with the joints of the finger bones, it can assist in extending the interphalangeal joints as well, though that IP extension relies on the extensor hood working with other muscles like the lumbricals and interossei. It does not flex the MCP joints, extend the wrist as its main action, or abduct the fingers (those roles belong to the flexors, wrist extensor group, and dorsal interossei, respectively).