Hey buddy,

Do you have a back problem that you are trying to work on, or are you just trying to strengthen your core?
Also how do you know your firing your TrA - are you using a real time ultrasound to view it?
DMA pilates believe that you can acitvely work one side more than the other by biasing the exercises and I have certainly tried in the past.
More recently Stuart McGill and Susan Mercer (as well as others) have been asking physio's why we still even teach core exercises. So there is definately debate as to their validity. Some people love them, and others feel the research isn't there to back it up...

My thoughts on strengthening your core... only do it if you have a problem that needs working on.
It's very hard to selectively fire the TrA and multifidus and most people who think they can do it are mistaken.

Hopefully I haven't sparked a major debate... But some more info in regards to your pains and so forth perhaps I can offer advice... I've done DMA pilates and although I'm using it less and less I could advise you on the exercises and how to bias them to try to recruit one side more.

Have a good day
Pudding