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 dunno. They're really comfortable. I haven't sat on them for more than about 2 hours in one go, but my experience with them has been good so far.

One thing though, the one I've used isn't a rocking chair; it has wheels instead. The rocking may make a difference.