By the time we reach adulthood, our daily activities have involved us bending our spine in mainly one direction forward.
If you think about it at home and at work, almost everything we do involves bending forward. Although we have the ability to move our spines left, right and backwards, we hardly ever do this. However, bending in all directions is actually very crucial to the health of your back.
Your spine has two concave arches, the cervical arch of your neck and the lumber arch of your lower back. Without these curves, your ligaments and discs become over-stressed, your muscles become overworked, and your back gets tired more easily, which then means you are more vulnerable to strain and injury.
It is important to maintain the natural curves of your spine in order to keep it flexible, and be able to bend backward with comfort and ease.
Posture and exercise are a way of keeping your back flexible into older age. This website has some examples of good posture for your back http://bhls.wordpress.com/2011/04/02/posture-for-a-healthy-back-what-is-good-posture/