Hi, I have had my wonderful yamaha with a practice mute, for over a year now. I ONLY practice with a practice mute.
This is what my advice is:
Replace the mute felt (the stuff that hangs between the strings and hammers when you activate the mute) with thinner felt!
I practiced my said yamaha for a long time, hours and hours, and felt so much more stress in my forearms, yes. BUT I have also noticed great things over time.
Take lots of ibuprofen, don't quit practicing on the mute.
What you are feeling is resistance. If you feel it in your wrist, then you are playing wrong.
You can burn through the resistance as you get stronger. In fact, I have a very old Chickering baby grand, and the action is extra heavy. It feels like nothing now, and I am able to play evenly and very forte, no problem. I don't even have to use much pedal. I just play legato with a ton of strength. IF ANYONE WANTS TO FLAME ME DOWN FOR PLAYING WITH STRENGTH VS ARM WEIGHT, YOU CAN EAT IT!! >:u)
Also, how is the bench? I recommend an upgrade to an artist chair, make sure you sit high enough. There is nothing worse than sitting too low and too close.
Get used to the resistance. Work your way up to it, starting with very short, incremental practice. Multiply the sessions, then do a few shorter, longer sessions. Then try one longer session. Then Try two longer sessions. Then try one long session. ETC.
Also, work out.
Do an upper body work-out, multiple times a week.