This is actually a bigger problem in the world of pianists, especially ones who haven't been taught about the concept of Relaxation.[All of this has been taught by my teacher and has strongly influenced my thoughts and approach to technique and playing the piano]What you're experiencing is tension, which you are quite aware of. This is due to loss of blood flow in your arms, which is from straining your arms to stay still. Many pianists think that if they hold their arms very still that they will be able to play faster, but in all reality, it will the do exact opposite. You're going to feel pain, and will eventually cause damage to your arms and hands (tennis elbow, carpal tunnel and other similar problems).Another problem with this tension is that your tone and control is severely decreased and is very "sharp" and "loud." It's difficult to play quiet, and your focus is decreased due to you using up your physical stamina.So, what do you do? How do you fix these demons?You gotta relax!Follow these points and stick to them anytime you're playing the piano:* Drop your weight--Make sure you "drop" your elbows/forearms and especially your shoulders..let 'em hang!* Breathe--This allows blood to flow through your body, making it able to move and adjust the best it can* Keep your wrists relaxed--Sure, this could be apart of dropping your weight, but often times people forget that one of the main problems with tension come from your wrists...drop them, and keep them limber* Think of the fingers as "needles"--And that's all they should be! Simply think about positioning your fingers on (not over!) the keys and press down with your "weight." As you can tell by now, your weight is the gravity pushing down on your shoulders, arms, elbows and wristsWith these 4 tips, you'll be able to gain control and play 10 times better than you do now. Your tone will increase, as well as your speed. Don't get me wrong, this is an extremely hard technique to master, as it requires you to just lay back and play; I had the same problem as you did, as well as every professional artist in history...however I do not compare myself with somebody like Rubinstein.I hope you've learned something from this! Feel free to ask any questions!