Due to mentioned changes in this chapter of the book, "you are bent over by your lung", which leads to this outward appearance.
Did you know?
Between an "upright position" and the "COPD-position" (posture decay) a difference of about 30% of vital capacity exists. The vital capacity is the amount of air (volume) in between a maximum inhalation and a maximum exhalation: everything that one can actively move.
For your daily life activities you should put yourself as much as possible for you into an outright position. Than more air (volume) can be ventilated. Therefore learning how to place yourself into upright position takes an important place in learning and practicing respiratory muscle training.