[peterwwillis]

Your forward-facing deltoids are probably engaged too, as well as your scalenes, while your lower spine is getting compressed and your core gets weaker. More is going on that meets the eye.

Not only are there more muscle groups associated with these problems, but if you try to correct these problems improperly you can develop a muscle imbalance which can exacerbate all these issues. You need the right balance of muscle supporting other parts of your body for everything to function safely.

So before you tell people just to stretch their pecs, please consult a physical therapist to evaluate you and give you specific exercises tailored to your physicality. There is no 'one weird trick' to improving your body.