A topic that comes up quite allot in clinic is posture sitting at a desk. My favourite thing to sit on at a desk is a stool. In my opinion a stool is better than a chair because you are less tempted to role back off your sitting bones into a slouch as you get tired and begin to use the backrest. The sitting bones (Ischial Tuberosities), which are the hard bony bits you will feel if you sit on your hands, are meant for sitting on and if you role the pelvis back and off them you will fall into a slouch which puts allot of strain on the lower back. Most chairs have the seat tipping slightly back, which will naturally encourage you to slouch!
Another good thing about sitting on a stool is that you are much more likely to get up and have a break as you start getting tired, instead of using the backrest to compensate for the muscles in the back getting tired; This is great because sitting there for any longer than 45-60 minutes without a walking or lying break is not great on the back.
You don’t have to pay allot of money for an ergonomic stool. All you need is an office chair that has three leavers on it; the first two leavers will control the height and back rest, but if it has a third leaver then it allows you to adjust the tilt of the seat, and that is the crucial ingredient. The first thing to do is to dismantle and discard the back rest of the chair (for the reasons mentioned above). Then adjust the seat so that it tilts about 10 degrees forwards. This tilts your pelvis slightly forwards which will naturally maintain a healthy lower back posture (Lumbar Lordosis), by keeping you on your sitting bones. Another thing that will help you to sit upright effortlessly is to adjust the seat so that it is slightly higher than your knees, so that you have just over a 90 degree angle on the knees (see diagram). The best way to get hold of an office desk with three leavers is to keep an eye out for them in Charity shops; they do come up every now and again and are usually not more that £30-50. Otherwise have a look at places that sell office furniture, they tend to cost around £150 or so.
The next thing to consider is the height of your desk. It should be high enough so that you can easily slide your knees underneath it and have your arms in the position shown in the picture above. If your desk needs to be raised a little you can do so by making little wooden feet that slot into the bottom of the desk legs and bring it to the right height for you (or saw off a portion of the legs if it needs lowering). Adjustable desks (also called standing desks) are great because they allow you to vary between standing and sitting at your desk, without the lower back getting locked into either (backs love movement). They are not always the cheapest of options, but well worth it if you spend allot of time at your desk. It is possible to get a manually operated desk for less than £200.