Question: What's one clue that the ordinary-looking person sitting in that office chair might be a 'cello player?
Answer: There's room for napping kittens behind her on the chair.
For the non-cellist readers, the explanation is thus. When I play the cello I sit well forward on the chair, balanced on the bony protuberances at the bottom of the pelvis commonly called the "sit bones" or "sitz bones" and less commonly by their anatomical name, the ischial tuberosities. Some players, particularly bigger guys, can achieve a free and balanced sitting stance all the way back in the chair, but not I. It's evidently become a habit I don't even notice, until I discover that four Country Cousins are taking a nap behind me as I type.