Computer stress is part of our lives these days. Whether sitting or standing at your desk, the fact is that it just isn't good for our bodies.
When computers first came out, I saw that they were developing chairs like out of Star Trek that would have the person sitting in a sort of recliner chair position with their arms/backs and necks totally supported and resting. I think people would have fallen asleep!
When I first started as a massage therapist in 1989, most of the injuries were low back issues from lifting or just sitting. Now the added stress of working staring at the computer and typing/using the mouse has brought us a whole new set of work related conditions.
The other issue really is just being so still all day. Our muscles were meant to move and they just don't like sitting there all day doing nothing. Well they aren't doing nothing really. They are actually doing a lot just sitting there. Your back and neck muscles are holding you upright. When you push your head forward to look at the computer, it puts more strain on your back. The mouse and keyboard work, make you do things like lean on your elbow or forearms which can create more issues with your neck and shoulders.
Laptops have made it even worse with their compact keyboards and always being attached to the monitor - it is impossible to position them properly to work without strain.
Most of the problem is that we are actually not strong enough to sit there all day typing. What??? I know that doesn't make sense, but typing all day is a repetitive motion and requires more strength than you might think. Muscles get tight when they are not strong enough to do the job they are being asked to do.
See also: The health hazards of sitting from the Washington Post