We do too much sitting. Even when sitting 'properly' there are downsides (weaker muscles, vascular pooling, stretched ligaments, poorer digestion). It's even worse when we don't sit properly. But, leaving aside the obvious point that we need to minimise total time spent sitting, and take regular mini breaks when we are sitting, what else should we bear in mind?
It's possible to sit properly in two quite different ways. These are active versus passive sitting.
When we sit actively, we use our back muscles to hold ourselves upright. We don't use the back of the chair to support our back. See the example of the kneeling chair below.
Note that you can replicate some of the good effects of a kneeling chair while using a conventional chair by sliding your bottom towards the front of the chair, having your feet behind you, resting on your toes, and your knees pointing down at an angle (this stops slumping). See the picture below for an example of this.
Advantages? Our back muscles don't get lazy and we can sway back and forward in the chair, keeping things moving.
Disadvantages? Prolonged sitting like this can get tiring, precisely because we are using our muscles! Also, kneeling chairs will gradually compress the knees, so you'll need a break after a while from that feeling.
When we sit passively, in a proper way, we rest our back against the back of the chair. See this picture below.
Advantages? We are nice and relaxed - it isn't tiring in that sense. A very good chair allows us to do this properly, but we must use the chair properly to take advantage of this (especially we must have our bottom right back against the chair, and we mustn't lean forward with our upper back).
Disadvantages? Our spinal muscles get lazy and weak from excessive sitting and we are very static and still compared to active sitting.
The worst sitting position?
Simple! The in-between proper active and proper passive sitting where you are doing neither correctly! Avoid like the plague, especially if you have a disc problem. See the picture below. There are no advantages to this type of sitting, and major problems can occur.
Effects of a bad desk / chair / computer set-up?
Even with the best of intentions a bad 'ergonomic' set-up can hamper us from sitting properly (either actively or passively). A bit of lateral thinking, advice and sometimes new equipment can often sort these problems out. Especially, bad ergonomics that encourage you to slump, like the lady above, must be sorted out.
Effects of a badly working or too-weak back?
Interestingly (and often un-appreciated), if you have a back problem then sitting can aggravate and compound the problem. Best to sort out your problem with an osteopath and make sure the spine itself is basically working OK, or - if it is just spinal muscle weakness - work on strengthening these muscles.
Summary / Advice
Ideally? If you have to sit a lot, get both types of chairs (a conventional chair and a kneeling chair) and mix and match between the two, using them properly. If you can only use a conventional chair, consciously alternate between proper active and proper passive sitting on a regular basis. Obvious point - if at the end of a long day sitting, you feel achy and sore - do something about it!
If you do have an active disc problem, then I am a fan of the kind of reclined sitting shown below. It's not ideal in many ways, but it can take the strain off the injured disc.