my physiotherapist told me that it was important that one had core stability.
she said that it has been noticed that people with back problems also have transversus abdominis (TA) muscles which have relaxed. efforts made to retrain these muscles but were to no avail. my opinion is that if chest breathing is a symptom of TOS and TOS sufferers need to lose weight if they are to improve their condition. then quite possibly the relaxing of the abdominals will decrease the pressure on the diaphragm and therefore the impact of chest breathing will be less.
if i am chest breathing (which is bad) and I have a large meal(which is bad) and then try and pull my stomach in, the consequences of the chest breathing are worsened. i maintain that the relaxation of the abdominals is a consequence not a cause of TOS. since i have been taking medication to control my chest breathing, pressure on my diaphragm does not have a significant affect as it it did previously. my view is out of step with most others.