Rigging with Blendshapes
 

Hello all.
I've just finished creating a rigg for a character but i have a problem. It was deforming weirdly in areas like the stomach and arms so i wanted to try to make a simple muscle system to fix it. I managed to work it out by using a Nurbs sphere with a sculpt deformer for the stomach but i need some help connecting that to the rigg.
Is there any way to make it so that when the spine rotates along the Z axis it makes the blend shape for the sphere go up or down?
Any help is appreciated :)