Aw lawrd!!!!! Now you guys are starting a conversation in my area of expertise. It is soooooooo hard to resist not regurgitating a bunch of stuff, considering that for months I've been reading threads after threads from detailing experts, and although I can hold my own, I really have no real credentials to speak from, and therefore contribute little.
So what kind of alphabet soup do I have in front and behind my name to answer to this thread?? Well, I hold a post-professional doctorate in the field of physical therapy with a sub-specialty in orthopedic and manual physical therapy. I've been a practicing clinician (working for myself) for about 18 years. <---- sheesh, this always looks pompous when I read that kind of intro.
The answers provided so far are in line with common sense thinking, but unfortunately lacking a few important elements.
So here we go..... The basic answer is none of the DAs I have tried so far prevented me from having progressive back discomfort/pain. It is true that some vibrate more than others; some are heavier than others; and some have more torque than others etc... Using the GG6 required less effort than the Flex but also corrected slower. I also noticed the same with my Rupes. The Rupes was faster and more ergonomic than the GG6 and did correct faster but was a little more weight to handle.
The simplest answer is not which is best in general, but which is best
for you! It all depends on your capacity and tissue tolerance, and how long you are planning on using the machine in one session. The most important question though is: "are you are planning to work with your body in mind (take break when it says) or fight it all the way (finish the job regardless of the consequences)?"
I agree with the above that exercising on a regular basis will help, same with using proper ergonomics, but no amount of exercise can prevent the consequences of chronic repetitive strains.
Here we could talk about the induced biomechanical forces of one polisher over another (as it pertains to horizontal vs. vertical surface polishing; using spinal force closure or form closure...... blah, blah, blah....). Let me spare you the scientific babbling and tell you that there are no safe polishers regardless of vibration, weight etc..
You can definitely improve your "life expectancy" through proper technique and regular exercises, but you cannot run away from the consequences of the wear and tear you are putting on your body. Think about the regular maintenance and polishing you are doing to your car. The more you touch it the more your induce scratches that you will then gently polish to restore the shine and maintain the value of your investment...... but you are not restoring the clear are you?
If you have a Rupes and physically feel that you can work longer hours compared to a Flex which, because of the forced rotation, and walking, and vibration etc..., only allows you to work 2/3 the amount of time, then you have to appreciate that the limitations you are experiencing are based on your physical capacity. Your bodies only have so much capacity/tolerance to handle and process the physical strain and associated byproducts of cellular work. Regardless of the way you want to justify it your capacity will be challenged by:
low work/long duration vs. heavy work/short duration.
In the end though, your body only has so much to give (flexion, extension, rotations, compression, distraction etc...). This relative maximum is based on your personal genetic maximum. Exercising regularly helps to maximize your capacities/tolerance but does not immunize you from the long term effects of repeated abuse.
So what should be done? The answer was provided before in the form of proper technique and ergonomics, as well as exercising on a regular basis to achieve the most out of your own body.
Yoga only answers the need for mobility/flexibility and conditioning, but does not fullfill the requirements for strength and endurance (which you need). Similarly, strength training only fulfill the strength challenge but no cross over benefits are found in endurance or conditioning.
There are no AIO exercises.... you need diversification and experimentation, always challenging yourself to achieve the best results....sounds familiar??