I've come to the conclusion that playing a classical guitar is not an ergonomic activity. Basically, it's trying to adapt your body to a box with a neck sticking out of it. As one gets older, the problems are likely to increase.
If I sit with in the typical classical position, playing in the first position moves the left arm way out, aggravating any shoulder problems. Then there is also the back pain associated with having your spine twisted by using a foot rest.
I've resorted to placing the guitar on the right leg and using a small Dynarette cushion. That way, both of my feet are solidly planted on the floor and my back is straight. Even then, it makes sense to get up regularly and move around to avoid maintaining a static position for too long.
Lately, I've been wondering whether guitarists outside the classical realm who move around a lot aren't at less risk of developing the back problems associated with maintaining a static position for a long time. I also think contoured electric guitars, such as Fender Stratocasters, held on a strap, are much more ergonomic, provided that they are not heavy.