As a singer, the job is to get as much air into your lungs with as little rigidity as possible. The more rigid you are, the less the sound can resonate, the harder you have to try to make more sound. What you want to try to do is to breath at the bottom of your abdomen, allowing your abdomen to expand to the front, side and back. What you try not to do is to expand your upper chest and more importantly not to allow your shoulders to rise. Having the shoulders rise causes tightness in the neck and around the throat. So, here I am, trying to breath in the correct manner. I’m getting lots of expansion out front and side but just not getting much expansion in the back. I discovered that my posture was very much contributing to this. I have been a victim of the “swayback” posture, and this compresses your lower back making it very difficult to expand. As soon as I tried a bit of a pelvic tilt and was able to expand my lower back much easier.
The irony is singing is that the less “intense” you are physically, the better, more resonant sound you will get.