I have always been told that in many bodybuilding exercises (for instance chest and back exercises, but also squat, deadlift etc) it's very important to protect the back through the scapular adduction (retraction) and the abdomen contraction. The first one in order to stabilize the upper back and protect shoulders, the second back in order to protect lower back, by preventing it from arching too much or experiencing too much stress.
I have always applied that useful advise, and all people have followed me have always told me I did it correctly. However, I'm not sure to do it 100% correctly, so I need some specific advices.
Precisely, I find it difficult to keep my abdomen tight and simultaneously perform scapular retraction. If I only contract my abs, without thinking about the shoulders, it's ok. But if I also retract my shoulder blades, I feel that my abs are not perfectly contracting and my low back tends to arch a bit more. If I try to force more my abs to be tight, I feel my shoulder blades are not perfectly retracted.
Do you have some advices or can you explain me something about this fact?