Sitting in one of these (goofy, overpriced) chairs does not involve tensing any muscles other than those used for balance, which don't really get tired, so that's probably the worst analogy on the internet today. These chairs are meant to rest your legs, not your back. Your back doesn't need to be supported all day. It is the support. There are people who stand for 8 or more hours a day, you know. And they're healthier for it.
I forgot to mention that I've used a cheap knockoff (and even goofier) model of these chairs for a couple of years. And it was quite comfortable indeed.
