Quote from cstfx:
care to explain, then, how a kneeling chair or exercise ball is recommended more for long sitting then your desk chair, if back support is supposed to be so important? In a wood chair, you do not rely on the back for support - it encourages you to lean slightly forward, as do these.
Yes forcing yourself to sit upright away from the backrest is excellent and will help your spine.
The exercise ball requires you to support your spine and engage the abs, which strengthens the abs and in turn supports the spine.
The muscles of the back are also engaged and must work to maintain an upright posture increasing their strength.
Basically the core muscles will have to work active-dynamically to keep you upright. In a chair leaning against the backrest it is passive. You can lay back against the backrest and the chair will assist to keep you seated, which in turn over time can decrease the strength of the core leading to back problems.
These links have good information.
American College of Sports Medicine
has many papers in their journals covering these topics.
Also try the
National Academy of Sports Medicine
National Strength and Conditioning Association