While watching her boyfriend split wood, a teenager was struck on the back of her carpels by a sharp- edged flying wedge. Her extensor digitorum tendons were exposed, though not severed, indicating that the surrounding synol sheath had been opened. What other muscle has its tendon surrounded by the same synol sheath?

Extensor Indicis is a deep forearm extensor that extends the index finger only. Its tendon joins the tendon of the extensor digitorum which goes to the second digit, and both tendons inse into the extensor expansion. Since these tendons inse together, it would make sense that they would be contained in a common synol sheath. All of the other tendons listed have their own synol sheaths.