"Sit up straight!" – That's what my mother used to say when I was little… but can we really sit up straight???
The neutral spine is not straight, it has natural curves that help us move, be flexible and still keep us strong and balanced. When we sit, our lumbar curve, the lower back, ‘disappears’ or bends forward. Losing the natural spinal curves creates stress and puts pressure on the spine, especially in the lumbar area. That, in the long run, can cause us pain and discomfort.
When we sit, and it doesn't matter which chair we use, we want to keep our neutral spine’s curves. We want to support our back and keep that "bowl" in our lower back as natural as possible, so we use a back rest and keep an upright posture.
Also, we want to keep our arms close to our body. Our elbows should be at about 90° to use the keyboard and mouse. The angle between the back and the hips are at about 100°-110°. This angle keeps the load on the back minimal, but still enables us to be functional (and not fall asleep). Legs should be supported on the floor or a footrest.
Maintaining a healthy posture will help reduce the stress and load. But it's not enough. It is important to be dynamic while sitting, to move around, to get up, stand and stretch! Combine sitting and moving and you feel the difference at the end of the day. :)