To have good posture, it is essential that your back, muscles, and joints be in tip-top shape.
Your Back
A healthy back has three natural curves: a slight forward curve in the neck (cervical curve), a slight backward curve in the upper back (thoracic curve), and a slight forward curve in the low back (lumbar curve). Good posture actually means keeping these three curves in balanced alignment.
Your Muscles
Strong and flexible muscles also are essential to good posture. Abdominal, hip, and leg muscles that are weak and inflexible cannot support your back's natural curves.
Your Joints
Hip, knee, and ankle joints balance your back's natural curves when you move, making it possible to maintain good posture in any position.
Good posture - when you are standing - is straight vertical alignment of your body form the top of your head, through your body's center, to the bottom of your feet. From a side view, good posture can be seen as an imaginary vertical line through the ear, shoulder, hip, knee and ankle. In addition, the three natural curves in your back can be seen.
From a back view, the spine and head are straight, not curved to the right or left. The front view of good posture shows equal heights of shoulders, hips and knees. The head is held straight, not tilted or turned to one side.
*Sourced from: American Physical Therapy Association (www.APTA.org). "The Secret of GOOD POSTURE. A Physical Therapist's Perspective." 20M/08-07/PR-9