Well... its been mentioned before about the mechanical aspects of this. Any large object pushing into the sphincter is also going to deform areas on the adjacent left & right buttocks as well. So, just sub-dividing a portion of the hip area is not a complete solution. But its a start.

TrekkieGrrl's reference photos would be useful to map out where and how far those deforms take place, where extra sub-divisions on the left & right buttocks would be needed.

 Don't sweat the petty things, and don't pet the sweaty things!  ; )