Military posture is not good posture. Our spine is naturally curved, not ramrod straight.
The human head weighs fifteen pounds, give or take. It is supported by a complexity of ligament and bone. It rests, like a ball on a pole, on the uppermost point of the cervical spine.
When we consult a cellphone, the head falls forward. Repetitive slumping turns us C-shaped. The neck develops a compensatory musculature, a pronounced thickness extending from the base of the skull to the top of the back. We lose the ability to breathe diaphragmatically. Our internal organs are forced to function in an ever-diminishing space.
See how children walk, how regal they are. Good posture requires no effort. Old age is bad posture.