Dunno, but here's one of my favorite exercises from PT:

Start by sitting on the ball, feet about hip-shoulder width apart.

Walk/roll forward to a lying down position, so that the ball is now under your middle back or shoulder blades. Arms can be out at sides to help maintain balance.

Lift/straighten one leg and hold. (Build up to 10 seconds)
Switch to other leg.
3 reps, then roll back up to sitting position.

If this is confusing, let me know, I'll draw some pictures or have DH take some pictures.