Oh yeah, I know what you mean. To be honest, I’m not completely sure how exactly it’s supposed to work, but I’ve just tried to stick to the principle of maintaining a braced and neutral a position as is possible. In this regard, I guess I would say don’t think of it as keeping your back flat, but rather as neutral as you can, and keeping your pelvis as even, and as close to the ground as you can, while doing the stretches. Don’t push it further than you can without breaking into a faulty position. Not being able to achieve the full stretch in a stable position is likely just another indication of mobility restrictions that need to be worked on. Maybe do this hip opener exercise, and test to see if it improves your ability to keep your lumbar neutral while in flexion?

Hi BKIllini,
I had the same problem for the better part of four years, seeing various MDs (primary care, Neurologists, Orthopedic doctors, and Surgeons), physical therapists, and chiropractors. The MD’s had said my left SI Joint was hypermobile. I have had only one chiropractor along the way who would give me an adjustment that would provide instantaneous relief, however it would not last more than a day or two.

Finally, a year ago, I found a physical therapist, who gave me the most thorough evaluation I have ever had. He said that the joint was actually Hypomobile, and was stuck in a misaligned state due to the jagged nature of the iliac and sacral surfaces, which comprise the joint. He then used Muscle Energy techniques to realign my SI joints, my T and L spines (apparently having on SI joint out of whack for years messes a bunch of other stuff up upstream), and correct a leg length discrepancy. The technique uses your own muscle contractions with the legs positioned in various ways, sometimes against light resistance from the therapist, to realign and restore mobility to joints. Right after my first visit, my SI joints maintained their alignment and left me pain-free and feeling like I had felt immediately before I had done that last deadlift rep, which injured me 4 years ago. Now I still had some mobility issues that weren’t addressed, but I used Starrett’s videos and his book to help correct them, and I feel I have made major strides in that area as well.

I’m assuming the therapists that you have seen were like those I saw (before the last one that fixed me up) and only gave you a bunch of stretching and/or strengthening exercises, without paying much attention to alignment or gait issues. I’d encourage you to find one who utilizes Manual Osteopathic Techniques and Muscle Energy to correct low back/pelvic girdle dysfunctions.