You more than likely spend most of your time throughout the day sitting at a computer. It's today's way of life. Few people realize just how much pressure sitting has on the intervertebral discs though. Believe it or not, there have been studies on that very thing though, and the results might surprise you. Not only is sitting hard on the spine, it's one of the hardest things on your spine.
In between your vertebrae are discs. These discs act as shock absorbers. As we age, the discs change. If the discs in between the vertebrae get damaged, this can lead to a host of problems. Sometimes you hear about people having herniated discs or ruptured discs. To keep it simple, let's just say that the more we abuse a disc, the more likely it is to get damaged. So we want to try to limit the amount of damage done to it. And that's where sitting comes in. I know what you are saying, sitting doesn't do anything. I'm just sitting there! But what you may not realize is that when you move into different positions, your spine places different amounts of pressure on the disc. And sitting believe it or not, is one of the hardest things on the spine. It adds a lot of compression forces on the intervertebral discs.
Take a look at this graph, and look below for the explanation
In the diagram, you can see the model of someone standing with the 100 below him. That is the baseline for the amount of pressure that is put on your vertebral disk. From that point, different positions were tested to see if more pressure or less pressure was put on the vertebral disk. You will notice that when the person is laying down, the amount of pressure decreases. That makes sense. However, there are some interesting things about the other positions that you need to understand.
When the individual leans forward while standing, the pressure on the discs increases to 150%, and while holding a small weight (about 5 LBS) that pressure increases to 220%. But that's not the worst of it. Look at the sitting position. Sitting up straight equals 140%, leaning forward while sitting equals 185% (that would be the position you are more than likely sitting in right now while sitting slumped at your computer), and adding a small weight increases that pressure to 275%. And while you probably aren't holding weight in your hands, you have to consider the likelihood that as your own bodyweight increases, this will have an impact.
I know I can't convince everyone to switch to a stand-up desk, but what would I suggest? You have to take into account that you are almost doubling the amount of pressure on your discs when you sit at your desk. And you have to realize you are sitting at your desk, in your car, and on your couch for 8, 10, maybe even 12 hours a day. So even though you may feel that you aren't doing anything all day, you actually are. You are loading your lumbar spine's intervertebral discs. Take this into account when you do your workout.
Your should really be careful with flexion type exercises when you do your workout routine. Not every exercise you see in the magazine is going to be good for you. Take for instance, sit-ups. A lot of people do situps, but it doesn't mean it is the best exercise for you, especially after you've been sitting in the same position all day long. Take the time to focus on other exercises in your routine. 
Sometimes a plank (iso-abs) just makes more sense for someone that's been sitting all day. It forces contraction of the transverse abdominis and forces the body to work as a whole. There are a lot of variations of this exercise. If you are a beginner, you may have to start off on your knees. No matter what level you are though, you need to make sure you keep your back flat in order to minimize pressure on the lumbar spine.
Take the time to think about your routine and why you are doing the exercises you are doing. If you have a personal trainer, ask them why they are choosing your exercises. If they can't explain to you why you need to be doing that particular exercise, well...I'll leave that one alone. Just because particular exercises are popular, doesn't mean they are good for you though. Take the time to learn about your body and take care of it. Afterall, it's the only one you've got.
If you have any questions or comments, feel free to reply below.