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.