re:Not obvious!
Scott bends one knee in this famous picture:
Here.
It's almost impossible to find astronauts kneeling in pictures; they kneeled during Apollo 15, 16 and 17 because their spacesuits were better, but then it never lasted long enough for the other to take a picture. Pictures from videos, maybe.
Now, today astronauts can bend their knees in 0G, while their boots are attached to the MFR
There's one of Bruce Mc Candless from STS51B, I think.
Raoul