Mortensen uses the four legs of a piano stool as an analogy for his four pillars but, like others, he does not describe where to put it. The result is JV and my bio-mechanic advise sitting farther back. But how much farther back?
One rule of thumb could be to describe how outstreched the arms should be when reaching out to the lower base and the upper treble, subject to comfortably reaching the pedals.
Would anyone like to say something about this?
Robert Estrin has answered my question with poster image for this video showing hands hanging down against the fallboard. Nice to know I have got something right for once.
It's a subtle point really. Estrin is right that finding optimum position at the piano takes years and, more to say, that position changes with time in accordance with pianist's musical and technical preferences.
When you sit closer to the piano and higher it promotes "pushing", or as it is commonly said in Russia, "playing into the grand". When you sit farther and lower it promotes "pulling", or as it is said in Russia, "playing out of the grand". Finding the right balance is a subtle task. You may try the "hanging bridge" test for that, it's done like that: play a note, or better a triad, and let your hand rest on the pressed down keys and relax your upper arm completely. If your hand can rest on the keys with just a little tension in the fingers, and your elbow can sway freely, than the balance is right. If much tension is required to maintain that position, or no tension at all, or if the elbow becomes stiff, than some posture adjustments may be necessary.