Got to say, I agree with both these posts. My old man had two, grandmother and grandson. Grandson still alive, 9. Grandmother died 6 months ago aged 15. Had to be careful with them, and watch they didn't jump up/down on the settee/steps etc.
Although not bow backed like you say fd, their legs are that short, and backs so long, it definitely does place a lot of unnecessary strain etc on all their joints and particularly their vertebrae.
Five years ago, at the age of four, the grandson jumped down off a very low wall, about 14-15"", in the old man's garden. Result, some sort of nerve/