Piano Forum

In the last days I am facing a big problem, I'm having sore forearm and I can't play relaxed, I have a competition next week and I have to fix this as soon as I can. The piano there is very heavy and my repertoire is almost all in forte and requires great stamina and technique.  I am playing Chopin op. 25 no. 12 and from the third page my hands are getting very stiff, Sonate Beethoven op. 22 which is ok, and the Hungarian Rhapsody no. 6 with the finale octaves that gets on my nerves. I don't think that I'm using the wrong technique...
So, how to fix this muscle pain? Does one day with no practice would be useful?
And another question, how to play on a heavier piano? I found out that I get tired more easily and because the sound it's not really as powerful as I want I am doing a lot more effort.
Thank you!