Hmmmm...
This sounds a little like that weird feeling one gets after rollerskating (or ice skating) for several hours.
The only thing I can offer is that perhaps different muscle groups are being bombarded than your body is used to, and as a result your muscle needs to build itself up.
When I was using the other Vest (HillRom) I did feel very achy and "rattled" even after using it. That was several years ago, and well before I had much muscle tone at all. As some of you have seen from my blog, I've been working out quite a bit. When I was first starting to lift weights, my muscles trembled a bit. Now that they're more toned, they don't tremble. I guess I'm basically saying that anything as new and aggressive as the good pounding that the vest offers, is bound to impact your muscles.
Does anyone else work out or do yoga or anything that has really improved their back muscles? If so, do you still experience the shaky feeling that's been described?
On a similar note, we should all be careful to use good posture. I know that's very difficult for those with worse lungs, since it's our body's natural response to gulp air as easily as possible, even if that means not breathing a sturdy breath using the diaphragm. Keep those shoulders straight, and your head straight ahead don't slouch! Maybe that'll help.
If not, I tried <img src="i/expressions/face-icon-small-smile.gif" border="0">