There seems to be a divergence of opinion on what the BEST POSTURE at Address and Impact should ideally be. One model says that the spine, when viewed from down the line should be nearly straight... from the neck all the way to the tail bone. Some recommend a slight cup at the base of the spine. I believe this is called the LORDONIC CURVE. I have heard it recommended that the shoulders be slightly pulled back, in order to give a straight appearance of the back.
It is usually also said that in order to maintain this posture, the core muscle strength must be good, as well as the abdominal muscle strength in general.
Now, it is interesting that few, if any tour players that do strive to stand similar to this, can maintain this posture on the downswing. The tour player does not usually pull the tailbone in radically, as many amateurs do, but they do not keep the back straight, nor the head up. At least I have not seen one that does.
Mr. Jack Nicklaus, for example, is sometimes criticized for his posture. However, his posture is probably similar at Address, and Impact.
Annikan, some time back, posted a picture of Sam Snead. He did not have even close to the MEDICAL MODEL POSTURE, however, he was a very effective striker of the ball, and lasted for many years without much injury that I know of.
Let's talk about this some. We don't want to be too slumped over, but artifically rigid is not good either.