Sure, there are plenty of studies on exercise in general (quality is lacking in plenty, also). Exercise is a hormetic activity...so does hanging upside down cause a type of stress? I don't know, I see no reason to argue for or against it. For use in recovery it's pretty much a no-brainer if you enjoy it...since basically any activity that you find relaxing or enjoyable will help with recovery.
Per the Mayo Clinic, there is no scientific evidence that inversion therapy has any significant value in providing long-term pain relief. Studies of traction therapy such as inversion have found *NO* significant long-term benefits. Besides that, persons with high blood pressure and certain diseases place themselves at risk by performing inversion therapy. And the results of accidentally taking a nose dive while struggling to extricate oneself from this position wouldn't be pretty.
http://www.mayoclini...therapy/AN01614
I did a lot of research on these things when I was looking to buy one. I found a lot of info out there on traction, etc. not being that effective, but I read something that made sense to me. Apparently some of the information out there that disputes the effectiveness of traction is based on evidence from standard mechanical traction devices that use pullies and harnesses to distract the joints. Inversion is a natural way to decompress that uses a persons own body weight... and it's progressive - which means that each part of the back receives a traction load equal to the load that compresses is while upright. With standard traction devises, the loads must be determined by the adminstrator, and they are not progressive... plus it's difficult for the person to fully relax.
I've had my own machine for several years now and have found that nothing else can compare to the type of stretch you get when inverted. I sit at a computer all day and it's actually pretty amazing how I can just get on this thing and stretch out the strain of the day. I only need to do it for about 5 minutes. Sometimes I forget to do it more regularly, and I start to notice that tension creeping back. It has honestly been a lifesaver because I can just get on it when I need it.
I'm not sure that there are a lot of full studies on inversion in the sense that they are double-blind (how can you do that?) but the Teeter site does list a few articles and studies that sure suggest there's something to it, even if the results are temporary (what isn't??)
ANything is risky if you don't use the equipment right (i would sure hate to fall upside down) but my equipment is pretty rock steady - i'm not going anywhere. it's actually pretty easy to use - no struggles involved!
Re: the person who mentioned hanging my your hands. I used to try that and got a little relief in the lower back. The problem with that is you don't get the full stretch - only to the lower back. The upper back, shoulder and neck area don't have any chance to decompress. Plus, who can hang by your hands for the amount of time necessary? I find that I need to hang at least 5 minutes for my back muscles to relax enough and *release*. The inversion table is the best thing you can buy for your back, hands down.