My bottom aches from sitting down all day.

[Nonverbis]

This is from Wordbuilder by Guw Wellman.




Could you tell me why "sitting down" rather than just "sitting"?

Let us consult a dictionary: https://www.oxfordlearnersdictionaries.com/definition/english/sit-down_1?q=sit+down

To sit down = to move from a standing position to a sitting position.

But he did not move from a standing position to a sitting one.

Is it Ok to say like shown in the textbook?

 

[jutfrank]

It's fine either way. The down is not necessary.