Seems like a waste to try and practice anatomy when most of the body is obscured! But anyway um, the far buttock needs to be elongated downward. The back might be a bit too arched, but I don't even know what this woman is doing, so it's hard to know! I also can't tell how the shoulders are connected.
I highly recommend you pick a new composition with less of the body obscured if you really want to improve at anatomy!