Hi Tink,
I would say gym work is ok for people who have done it for years and there body is used to it. You can easily over do it and its far more beneficial for your AS to do more stretching exercise.Swimming, Badminton is a great way to get fit and stretch I used to play a lot and really got a lot from it. Tennis is another good one. Why push yourself to the point of misery when you can get as much with stretching and feel better not worse.