Bob, it seems you really want to know it

In terms of flexibility and strength, first knowing how the body works and what one can expect from it will help tremendously in achieving your goals. If you believe any of the following, then I have to send you to the library first:
People bend at the waist
Not true. The waist is not an anatomical entity. It may be important for fashion, but the concept of a waist is entirely useless in terms of how the body works. One bends from the hip joints. Bending at the waist will twist the spine. The spine is not a joint. So, for any back exercise, make sure you are always bending from the hip joints. This is also true for playing the piano. Most people bend at the waist.
The arm begins at the shoulder joint
Not true. The arm begins where the clavicle joins the breast bone, in the middle of the torso just under the throat. The shoulder is not a rigid structure that is afixed to the torso. It is connected to the clavicle which is afixed to the torso.
Can you point to where your hip joints are?
They are about at the same hight as the pubic bone.
Do you know where the center of your head is?
It is halfway between the ears.
Only if know where flexibility can reasonably be expected and only after you have adjusted your posture correctly can you aspire to gain the highest degree of flexibility. Flexibility is primarily a muscular aspect. Posture is also a muscular aspect, but not how most people believe it is (see below). So, fix your muscular system first, before you start stretching it.
Correct posture does not require special muscle action. The skeleton is designed to be in perfect balance when there is the minimum amount of muscle action, i.e. all muscles, particularly those that act on the neck, should be released. Strong abdominal muscles may help in attaining a different posture, but that is really an illusion. Posture should not be forced by muscle action. On the contrary. It is attained automatically by releasing all muscle action. Lock at toddlers or native people who have never seen a couch, and you'll immediately realize this. (Re-)learning how to release unnecessary tension in the muscles is a real struggle if one has acquired a bunch of bad habits.
As pointed out in your other thread, refer to my favorite book for more details

Good luck!