Hi, slowcrawl! Letting your Dr know what you're doing can be a good thing...the Dr can let you know what modifications you may need (especially in the beginning)...
I had similar low back problem while doing a different exercise program a few years ago. It also required bending the knees and tucking the tailbone. I played around a bit & found that if I only bent my knees slightly (like 1/2") it helped. I also realized I was tucking my tailbone WAY too far! Just a slight/tiny tuck was all that was needed. Making those two adjustments eliminated my lower back pain.
Shawn's advice about visualization is a good alternative if you have problems...it still works. Or cut back to 1/2 hour active exercise & then do 1/2 hour of Small Universe meditation to help clear blockages.
Just my thoughts...
