Your back puts up with a great many indignities. Ordinarily, your spine adopts three natural curves that help distribute your body’s weight. They go by the names cervical, thoracic, and lumbar curves.
No matter how good your posture, when you stay in a single position for a long time (for example, when you stand in line, sit at a desk, or drive across the country), you put increased pressure on parts of your spine, and you usually end up flattening or excessively curving these parts of your back. Your muscles strain to compensate, causing problems that range from a stiff neck to an aching lower back.
There’s no ironclad defense against back problems. However, you can do a lot by observing good posture, practicing backstrengthening exercises, and avoiding back-straining experiences.
The first rule of good posture is to vary your position. Contrary to what you might think, good posture doesn’t involve locking your back into an ideal arrangement for hours at a time. In fact, the human back hasn’t quite caught up to the strains and stresses of modern-day activities like sitting in motor vehicles and doing office work. You’ll spare your spine a lot of pain by interrupting long, tedious activities (driving, typing, and so on) to take a quick stretch and have a walk around. If you can, limit yourself to 20 minutes of focused office work or an hour of uninterrupted driving before you take a quick refresher.
Source of Information : Oreilly - Your Body Missing Manual