Rashi: The limbs destined to descend to the grave.

Ibn Ezra: They are like branches of the grave - "va'Ta'as Badim." This is a metaphor, as if the grave is a tree below, and it makes branches. What is proper for the branches will descend. Alternatively, the branches will descend; it is as if man is on the grave's branches [and will descend when they fall].

Malbim: They are hinges on the doors of the grave; the door will seal the grave's opening, and no one will see what happens to the Mes there. I.e. even if the Nefesh would live forever, no one knows this in this world. Therefore, either people will establish me to be a Rasha, or they will perceive that Hashem acts improperly. Since Hashem's conduct causes one of these problems, how can you justify it? I do not find a Chacham among you!

Rashi: They will lodge there with earth. Nachas is like "va'Tanach Aleihem ha'Ru'ach."

Ibn Ezra: They will descend together on earth.

Devar Eliyahu: Will his acquisitions descend with him to the grave?!