I don't know which muscles they are technically and I don't even know how to describe them, but this is what I did to find them.
Get one of those big excersize balls and sit on it (not right in the middle, but slightly forward so your legs must keep you balanced). Now straightend up your back and take dance position with your arms, and then finally extend your head to the left. When done correctly you should engage the muscles in the middle of the back and right next to your spine. These are the muscles that must be used to maintain correct posture for the lady. Coincidentally they are also the muscles needed to support your neck muscles from getting weak and giving out in the middle of a competition. (That happened to me and was one of the scariest things I've felt. You literally can't hold your head up. I went to a dance trainer to find out how I could strengthen my neck muscles and he told me it was the back and gave me this excersize to do.)
Hope that helps.

I read an advice on the right posture for the one who dances. I believe such advice helps me a lot in my future social dancing activities. Thank you so much. I hope to hear some more couple of advice.

Dancer, Thanks for your very instructive explanation .
ylchen

Yichin. For the lady there is more shaping than the man who just stands up straight with the head to the left. This I have been told. Imagine you are on a railway train which is coming into the station. You are at the door with the window 1/4 open from the top. There is somebody who you do not want to see you on the platform. So facing the way the train is moving. Stretch your body so that your left eyes only is looking out of the window at the platform. Both sides equally stretched. That's about how you should be in the man's arm. It will put the right amount of weight on the man's right wrist. In the film "Shall We Dance ", look at the poise of Jenfifer Lopez. Happy Dancing

Ylchen,
Please do not refer to Jennifer Lopez for any ballroom dance help. Yes she trained with some of the best dancers in the country for about 2 months, but as we all know it takes much more time than that to develop any sense of ballroom technique... and I'm sorry to say she needed more help than she obviously had time for. I have a lot of issues with that movie, it was great entertainment but terrible dancing on JLo's part. It's pretty laughable to anyone who has ever competed at Blackpool that the producers tried to pass her off as a semi finalist at Blackpool!

My little daughter and I had a sense of that too many things did not make sense in the film " Shall we dance ." Unlike the early series of ER, at least it is a combination of reality and art . I familiar with the environment of E & R , although it is too excited and too concentrated. If every minute working in such situation, it will be terrible hazard to both doctors and patients!
Lately, there were some programs about ballroom dance provided by " travelling and living ": 3 lucky persons selected had a chance to be trained by top dancers , (sorry , I prefer to enjoy the 2 male instructors , especially the Russian ,than the female)) for 5 weeks then they attended competition in U.S. The purpose I followed these programs was to find out how the fundamental basis were introduced by those structors. Unfortunately, it was hard to see.

ylchen

Don, I have a stupid question, I heard someone said : lady should keep longer right stretching line or a little bit higher right hip . If so, what is the length of a little bit ?
I am puzzled on it .
ylchen

Yichen. First to Dancer. The close up shot of Jennifer was as good as it gets.
Yichin. Your right hip will be a little higher that the left. To quote Richard Gleave from my tape he says the lady has two curves, a banana shape to the left and very slightly back. One shape without the other is no good. The head will be naturally to the left on a tilt. Keep the chin up without shortening the back of the neck. The stretch to the sides does not have a collapsed side. Think of a piece of cane which is bent. Each segment bends equally with the inside measurement still the same size as the outside. Hope this helps. Its much easier with a video to look at and to listen to.

Thank you very much . Don.

Don--
I'm not sure which close up of Jennifer Lopez you are referring to not that it really matters because all of her dance scenes were mediocre and screamed beginner. Sure she was on the good side of beginner, but by no means a champion dancer in any stretch of the imagination. If you want a comparison just watch any dance scene with JLo, then watch the small part Karina Smirnoff (a real professional dancer) has dancing with the crazy long haired guy. She has the poise and air or a pro (unfortunately not the same acting skills as JLo though).