Now we get into the discussion of my least favorite part of class - the spine strengthening series. As you've heard, the postures you like the least are the ones you need the most and all four of the spine strengthening series fit that description for me. As much as I don't look forward to these four postures, they are the ones that completely handled my back pain that existed prior to starting Bikram. They are fantastic for working all the different parts of the spine.
The first of these postures is Cobra pose, which works the lower back, and you can absolutely feel it in your lower back. Once in a while (not very often) my lower back is a little sore going into class but after this posture any soreness goes away immediately. Lower back pain is such a common occurrence that literally millions of people consider it a normal fact of existence and there are innumerable "cures" out there for lower back pain. Special chairs, special beds, special keyboards, pills, chiropractors, you name it, but this posture can put your spine in such good shape that you won't have to worry about lower back pain again.
It took me a while to get the form of this posture and the set up is extremely important. At first I didn't really feel like I was getting much of the posture, and I probably wasn't, but then I had some teachers check out my form and made a few corrections and I noticed a drastic improvement after that. As usual, the dialogue is excellent about what to do, but I would suggest having a teacher closely check out the position of your hands in the set up. It can make a world of difference.