Laptop use + survival to middle age = Back pain? Movement cure?

Problem: Back pain is almost inevitable for people who work like you and me.

Desired partial solution: Software/hardware that will let me set an interval (e.g., 20 minutes) after which my computer screen will go blank or display a reminder that I need to move around at least a little. I want the software to prevent me from using my computer for at least a minute!

[Above cartoon is from: "More Than 450 Diet, Fitness,Health and Medical Cartoons - A professional cartoon service" by Randy Glasbergen. See: http://www.selfhelpmagazine.com/
psychtoons/glasbergen/ergonomics.gif ]

In the last 3 months of 2006 I had both a computer crash and lower back pain bad enough to send me to a doctor and a physical therapist. I'm learning more about ergonomics than I ever wanted to know, but I wish that I had learned it several years ago. I really like the portability and power of laptop computers, but I now realize that they are not well-designed for most of us who must use them many hours per day. As far as I know there is no ergonomically advisable way a healthy adult human being can use a laptop computer daily for more than an hour per use. Of course, I'm not claiming any real expertise beyond what anyone might learn when highly motivated by frequent pain. Here are my conclusions:

1. If you work in anything resembling an office environment, either you die young or you are VERY likely to have lower back trouble.
2. It is a VERY good idea to separate the keyboard from the monitor/display, so that each can be positioned best for your body.
3. BUT, there is no combination of furniture and posture that can prevent cumulative damage from frequent periods of intense concentration while being physically inactive for more than an hour.

So, move around. Learn some stretches or other exercises that work well for your body in between periods of intense computer use. But most important, get up and move at least a little at least twice every hour!

Oops. That's all I have time for now. My alarm clock is buzzing and I have to get up to turn it off ... and then walk around a little.