Sitting posture and your back
The spine has has three natural curves: (front to back, that is – a sideways curve is called a scoliosis and is not welcome) The neck should curve gently forward (this is called a lordosis); the mid back, where the ribs are should curve gently back (this is called a kyphosis); and the lower back should have a lordosis also. The articulations between the vertebrae work best when these three curves are optimal. Think of the three curves together as the body’s natural spring.
But not only do the vertebral articulations work best when the spinal curves are optimal, so do the support or 'core' muscles, a great many of which are anchored to the spine.
Our spines were not really designed to sit for 8 or more hours per day while peering at a screen in front of us. Commonly gravity tends to take over and the lower back 'collapses' losing its forward curve. The body slumps, the shoulders roll forward and down and you have to lift your chin to look at your screen or at people around you. At this point there is an increase in pressure on the back of the lumbar vertebral discs (especially the lowest ones) there is too much increase in the kyphosis (forward curve) through the mid back and the lifted chin focuses pressure on the base of the neck.
The deep abdominal core muscles now have less space to draw upwards as a front body support so the lower back tires and the shoulder/neck muscles, normally used for movement, not posture, also tire easily as they are trying to compensate for lack of core muscle support. Its a vicious circle. The lower back aches and the shoulder muscles become like bricks.
See the illustration above– these are actually do's and dont's of yoga. The first two poses show the slumped lower back, rounded shoudlers and raised chin.
The third image (on the right) is a yoga pose that is really good for building core strength and for working/stretching the postural muscles. Notice the subject is sitting on a block. This makes it all easier, the lower back has its lordosis maintained, the sternum (breast bone) is lifted, the chin is level and lifting the arms will elongate the spine and loosen all the postural muscles.
Try the pose for a few minutes a day and try to take the feeling of it to your office chair!