Here's my thought about these under-desk types of things.
For any exercise to be beneficial, you need to really raise your heart rate. To do that with an under-desk type of machine, you're going to need to put out some serious leg movement which is going to ... well, make you look really silly behind your desk!
I personally would check out some books on isometric exercises and stretches, or a book on some yoga postures you can practice at your desk.
Here's a WebMD article about exercising at your desk:
http://www.webmd.com/fitness-exercis...e-at-your-desk
Some oft he things you won't be able to do since you can't leave, but some of them might help you feel more energetic and not quite so sedentary behind the desk. I do a lot of the strength building ones at my desk - the desk chair pushups have made a huge difference in my wrists and arms over the last month or so!
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