Similar to some people who could smoke and yet live to 100 years, or work with asbestos and do not develop cancer etc.
But majority of guitarists who spent thousands and thousands of hours playing in sitting position develop different problems and back pain as they aging...
I am also a doublebass player. If you think guitar is bad, try bass: most players stand, some sit. Sitting is considered rather wimpy, by many. Yet those players who stand often end up with back injuries. Gary Karr has a system (leaning forward, into the bass) which works for him, but not for me. Yet I digress.
Much of this discussion sounds too theoretical.
Pain and discomfort is not theoretical: it is to be avoided.
We can only reach our best performance when there is ergonomic balance - relaxation provides not only prevention of repetitive stress, but prepares the body for the widest range of musical expression.
I have tried many, many different sitting positions and have spent years experimenting with commercial products and around a dozen of my own inventions, from arm-rests, to knee pads, to chair mounted, clamp-on support pedestals, to strap configurations, to footstools, etc.
Each time I think I have found the perfect solution, after a few weeks, or a few months of playing, I begin to notice some problems.
For what it is worth, my current playing setup involves three appliances, plus a lot of body posture awareness:
1. Ergoplay Professional, set at minimum elevation:
+ Ergoplay rests over my RIGHT knee, with the guitar tucked in tightly to the base of my thigh, with my torso leaning forward, pushing the top bout into my right forearm.
+ I Position the guitar so that my line of downward sight (perpendicular to the neck) is at the 4th or 5th fret and my line of horizontal (eye-level, leftward facing) sight is a little above the 4th string tuning mechanism.
+ From a birdseye view, I sit with my crotch at the left corner of a chair, so that my body can face directly forward toward the front of the stage, while the neck of my guitar makes about a 45 degree diagonal angle to the stage, yet my left arm is usually very relaxed, with elbow hanging downward, with gravity.
2. Footstool, set at minimum (1st notch) elevation
+ The only reason for the footstool is to provide just a bit of extra height to the thigh, to prevent any feeling of wandering, or slippage of the Ergoplay, on the right thigh.
3. Classical guitar hook/strap - the kind that a plastic hook wraps around the lower bout of the guitar and hooks onto the edge of the soundhole.
+ This is my trade secret.
+ With the strap in place, I am now able to lean my chest forward, against the upper bout of the guitar, which, being the center of lateral balance, provides great stability, while the strap tethers the guitar, in opposition to the resting weight of my right forearm, at the lower bout.
This is crucial: my right arm can always relax completely, since its resting weight is counter-balanced by the strap / tether, on the left side.
+ This is a very important subject which, I think, is well-understood probably here, and certainly in any forum that deals with repetitive stress.
My current configuration provides very strong stability around a fluid axis: i.e., the slightly forward leaning vertical axis from where the Ergoplay rests at my thigh and where my upper right chest area rests against the upper bout of the guitar - this is the axis of lateral rotation, which is easily moveable.
No matter how perfect a playing setup may be, music involves movement, thus, unless there is counterbalancing movement of the body, in response to movement of the hands, stress will develop, and also, related compromise of relaxation and dynamic, expressive musical capabilities.
So, my goal is to become more and more aware of how my entire body needs to move, to counter-balance each and every hand or finger motion.
But, these micro-movements revolve around a consistent axis of rotation, which is a constant, and mechanically stable, point of reference.
That is my two cents for today.
Who knows? Maybe tomorrow I will ditch the current setup for something even better.