Humans aren't designed to sit down all day. We are designed to move. We have core muscles because we stand up and move around; because we support ourselves against the force of gravity. If you sit against the back of a chair all day, you risk weakening your abdominals, lumbo-pelvic control and your posterior chain. This means that you will have less than adequate support when you stand up.
We are designed to survive. Your body is amazing. It adapts so that you may have a better chance of survival. It will adapt to you moving more – this is really all that fitness is. Your body adapts to your regular requests to move more, move further or faster, or to lift weights. It will also adapt to moving less, to supporting itself less, to doing less.
But you will still want to stand up at the end of the working day. You may want to walk around all weekend shopping or sightseeing. You may after sitting all day, decide to go to the gym and go to a class where you are jumping around, or lifting weights. Add to these extremes the postural compensations, muscles weaknesses and imbalances caused by hours and days being supported by a chair, and you are fast heading towards injury.
So if you do have to sit for long periods of time, try sitting up without using the back of the chair for support. And I mean sit UP, don’t slouch. Best of all, move around regularly. Get up whenever you can even if only for a few minutes. Look after your body, it IS more important than your home or your car. Treat it with the respect it deserves.