I wanted to ask the experts if the correct position that a pianist should assume while sitting on the bench is exactly in correspondence with the "middle" of the piano, or if it can be slightly off-center depending on the piece being performed.
In this specific case, I am studying Chopin (op 69 no. 2) and by positioning myself on the bench "slightly" to the right of the center of the instrument, I get a benefit in terms of left arm fatigue.
Shifting from the central position only occurs when playing four hands with another pianist. Furthermore, the waltz you mention does not seem to me to be a piece that is off-center towards the upper half of the instrument, but on the contrary, it seems very well balanced to me. I believe instead, since you are expressing fatigue, that at the root there are technical and/or postural issues that should be analyzed with great seriousness in order to prevent them from causing physical problems much more serious than simple fatigue over time.
I believe instead, seeing as you are manifesting fatigue, that at the base there are technical and/or postural issues that should be analyzed with great seriousness in order to prevent them from determining physical problems much more serious than simple fatigue over time.
I addressed the problem with my teacher but it seems everything is fine in terms of posture. Regarding "technical" issues, I didn't quite understand what you mean. Anyway, it is strange that the right limb is relaxed and the problem is only on the left. Maybe it depends on the fact that the movements are wider!
I share what Simone says, since it is often not easy to understand what causes us discomfort. I believe that in addition to the work done with the maestro, you should personally be able to understand what causes the fatigue. Furthermore, the accompaniment does not have passages that could in any way cause tension and so on. I would say to monitor closely if there are precise points where the fatigue begins to be felt. It is sometimes not easy to realize... I say this from personal experience.
Try playing the piece by making pauses, e.g., at the end of a phrase or a period
during the pause without changing your posture or position in any way, listen to see if there are any tensions anywhere including shoulders, upper back, as well as arms and hands; if there are, release them and then continue repeating the operation a bit further into the piece..