The first method in which inversion tables relieve back pain is by separating the vertebrae of the spine into wider gaps. This allows nerve endings to be unclamped, which is a great way to keep pains in the back a nonissue. If pain relief isnt noticed, try increasing the degree of the tilt or try using the device for a longer period of time. Also keep in mind some types of back pain may not be aided by this method.
Having a bad posture will inevitably result in back pain. Poor posture will allow the muscles to shrink in size, making them less limber. This allows for much less mobility and movement, and easily results in back pain. The trick comes to stretch out the back muscles in a safe way; something inversion tables are great at. Inversion tables may not teach good posture, but they help fix the situation bad posture has caused. Fixing bad posture should be a secondary goal, however.
When back muscles aren't able to get the oxygen they need, there is going to be a certain amount of pain involved. This is sometimes caused by blood vessels that are constricted, so the real solution comes to be widening them. This can be done with inversion tables, since the upside down angle allows blood to flow through back muscles at an increased pressure.
To help the process along, some like to use warming packs or small massagers during the inversion table usage. Doing so allows the back to relax even further, and promotes expansion of blood vessels to again give a benefit to increased blood flow. Having someone else give a massage immediately before or after using an inversion table will also help the user to reap further relief from back pain.
Those with chronic back pain, any health conditions, or other worries should consult a doctor before planning to use an inversion table for back pain. It's never a bad time to get a second opinion, since some types of complications such as high blood pressure may render some effects of the inversion table useless. A local general doctor will be able to tell users if they are fit to use such devices or not.
In Conclusion
Obtaining the right inversion table usually requires asking around. Check with friends or employees at health stores to see if they have ever used such devices. If not, going online and reading user reviews is always the next best option. But, as always, consul a general physician before purchasing one, as health risks may arise otherewise.

