Hi Beto-Music,
Player health is a fantastic topic by the way!
When I took conservatory lessons as a kid, everyone was taught to sit very stiff & upright (as in that "wrong" PC sitting picture) in another post here. Fingers did all the work (eg. in Bach inventions), and fatigue was just waiting to happen. Being a flexible kid at the time, I got away with it I suppose.
Now I'm in my early '50's so, I have to really watch all kinds of health & ergonomic problems in my life.
A few years ago, I had the pleasure of getting a few jazz improv. (and general life & meditation) lessons from Canada's Paul Tobey. The first thing he did, was have me sit at the piano bench, relax, breath etc, lean forward very slightly, and put my legs in a "diagonal stride" (sustain pedal leg forward, other leg well back under the bench). It was reminiscent of the main diagonal stance in Tai Chi. It immediately relieved my mid-back of a lot of pain. He then got after my hands and told me to use the weight of my entire forearm (relaxed again) for much of the power stroke in pressing keys. (Instead of arms stiff & dead while & hands & fingers alone madly try to supply all the energy).
He had, at the time, a nice newer Ibach 7 foot grand, and while I sat there, that way, all relaxed (with the pre-meditation he inisted on*) & no worldly nor "harsh teacher" judgements in his "sanctuary" - well it was was MAGICAL & happy. No fatigue! He made my journey seem so safe & doable.
(Most teachers preface everything with "You'll never be like......". -How many of us have been heart-broken by that line?
And now, with all this research coming out in brain-plasticity, it turns out you can get good at things at an advanced age! (And you're supposed to, to prevent dementia).
*Paul Tobey also insisted on me playing for 5 minutes right after meditation - but while still halfway "under" in a meditative state! (No sheet music, no agenda). Allow pure stream of consciousness - all "wrong" notes if you will - sort of child-like improvisation, and no negative judgements. Much later, you approach "serious" sections of pieces in mangageable chunks, slow, and totally relaxed, getting each section perfect, but without any tension, nor judgement, nor "ivory tower anality". -Just pure joyous detachment - oddly - with accuracy in the end. 'Very strange process** - but it works. You sort of shut off all the pride & competition of the world, and get good in this soft, humble, ergonomic, & meditative state.
It's a slow process at first, but eventually - from what I can understand - it becomes a very mercurial way of mastering music.
I have - regardless of piano playing - mid-back (thoraic) pain issues, and do a lot of core work now (huge amounts of situps, and also many straight-arm reps with small hand weights while standing bent over at the waist. These arm swings are called "flys" I am told. They work wonders for my upper back & reduce my tendancy to hunch & get "computer round shoulders".
Many years ago, my first electronic controller was a Yamaha PF-80. It used the KX-88 era keyboard, and was a very stiff & heavy action. I used the stiff, fingers-only, playing technique of my childhood, and it used to kill my hands. I did have the foresight to put the PF-80 on a huge slab of dense 16 cm thick bed-foam on my table. It absorbed the shock at the bottom of keystrokes. I didn't notice the unit bouncing a bit.
**Paul was big on Kenny Werner's "Effortless Mastery". This is more geared to the mental-health, consciousness, & psychology aspects of piano playing. (Hence the "meditate at the piano for a few moments" to clear away bad habits & bad judgements). A lot of it involves stopping yourself the moment you either tense up, or, when you get self-critical, or, when you race ahead in a piece without knowing what you are doing (repeating bad habits). It's a way of gently & joyously unlearning all the stiff things & "fear things", and perhaps throwing out the un-developed consciousness things we all seem to get stuck with. Artists use his system to kill stage fright too.
I just wish I had more time & energy to play. My job takes a lot out of me, so, I have a lot of things to do in terms of conserving energy, making time for music etc...
Hope this 2 cents worth helps...:)
-Brian.