Sitting well is not complicated. It requires common sense and some intuition. When you sit well, your back feels good because your vertebrae have adequate space and circulation. Your breath is slow because your diaphragm elevates and depresses without restriction. Your neck and head feel level and without tension. You are able to concentrate because your eyes are not straining.
You know what it is: one hip pushed out to the side when standing, usually to hold a child. In the physical therapy world, it is "Hip Adduction Syndrome." Daddies can get it too, and so can non-parents, but women who are mothers are most susceptible. Over time, standing this way puts pressure on the hip and predisposes the lower back, knees, neck and shoulders to arthritis.
This post is for you, my running friends! And anyone else who has short, tight hip flexors...
The stress and activities of our industrialized world have not been kind to our bodies. Slouching over the computer all day leads to difficulty straightening up. Lifting, pushing and pulling with poor body mechanics leads to lower back wear and tear.
When attempting good posture, we often "pull the shoulder blades back." This usually feels good, because it counteracts our forward bending and slouching tendencies.