Hello, and welcome to the board,
I had a one level lurabar fusion in January and recently began physical therapy at a clinic that specializes in spinal issues. The woman I am seeing is a specialist in using biofeedback to work on pelvic floor issues. There is a direct connection between weak pelvic floor muscles, lower lurabar issues, and SI joint problems.
She pulled out a model today and was explaining how in my case, now with a fusion at L4-L5, since I can no longer bend there, the SI joints take the brunt of those movements. In order to regain balance in all these muscles, I particularly need to strengthen the pelvic floor muscles and other muscles in that vicinity. She believes that if all women understood how to do kegel exercises correctly (working the "right" muscles), there would be fewer complaints about back pain.
Apparently many surgeons have not made the connection, so you may have to do some research to find someone that will work with you.