I did Callanetics years ago (I still have the book and video).
At the time, I really liked it and found that my bottom and thighs shaped up and I became much more flexible. However, now I know more about exercise, I don't think that Callanetics provides a balanced routine - there is almost nothing for the upper body. And she works quads but not hamstrings - she does an unusual exercise that is sort of like very slow sissy squats where you are up on your toes. You use only your body weight, but it is slow, and I have never done any other exercise that made my quads burn as much! I can only liken it to the burn I get from calf raises. My quads developed a really nice shape (but not my hamstrings!)
The bottom/hip exercises also work.... I don't really understand how though, because the movements are very small, not full range movements that you do in traditional exercises. And, the ab work is very advanced and difficult.
The stretches are definitley worth doing - she has a background in ballet, and she holds the stretches for a long time. I remember finding the whole routine quite relaxing.
Overall, I think it would be good to learn the Callentics exercises that you are interested in, and incorporate them into a more balanced schedule.
Sharon
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