To maintain a good posture during sitting or standing still, one has to use spinal and abdominal muscles (in a perfectly healthy individual it is supposed to happen unconsciously) quite a lot.
However if one has to stand still or sit for a long time, these muscles start to experience fatigue (even if they are well trained). I have heard from a doctor once that in order to help this, normally these muscles relax and contract back following the breathing cycle. In my case apparently it does not happen automatically so I have a muscle fatigue very fast when I sit or stand for longer times.
Can somebody suggest me some references on this coupling between the breathing and spinal/abdominal muscles relaxation? I have not managed to find anything on the web. The only things I was able to find were about how difficult it is to breathe normally if you have a bad posture, which is of course a useful thing to know, but it is not at all what I am interested in.
P.S. I am not quite sure if I have to place this question here or to health.se