exercises for S1 that jams

Hi Esther

Just back from seeing my PT - I had a jammed S1 again...- this is a re-occuring thing for me - its jams to the left and then my L5 gets involved as my ligament pulls on it from the off kilter S1.  Vacuuming sets this off the worst - so its particularly related to torsion.  I went through several months of avoiding torsion, bending knees, and trying to build up core strength..i tried vacuuming two weeks ago,only to have this happen again as if I had never done any of that work.

For the past 9 months, I ve been doing many exercises on the ball for ab work and trying to strengthen my whole core - unfortunately i havent been able to do much Plank work as I also have a bad shoulder.  Can you suggest some of the best exercises for me to do for this particular problem? 

thanks