I'm not a lady, but I know YMCA is doable. Had a spinner SO years ago that was tiny and tight. She also was a very flexible martial artist. She enjoyed this position very much. Me, it's was just OK. I was seated and couldn't thrust much. She was on her head and couldn't bob much. But it was a nice DATY experience.
I regret that I have never been able to do a head stand. But I am fairly flexible in other ways. **giggles***
My twat does stay fairly tight since I don't have sex that often and I do exercise. Oh and did I mention the pitbull and chiweenie that take me for a walk everyday? LOL!