This is probably the six trillionth time I've brought up this topic. But one of my favorite physical problems that I've been learning how to conquer is the 4-5 trill. Even when not playing the piano I practice trills with my fingers, on whatever surface is near. For the longest time it really seemed like the 4-5 trill (particularly in my right hand, I suspect playing the guitar eventually made it easy for my left hand) was unattainable by me.
However in recent months I've noticed it becoming remarkably easier, without tension, to play a 4-5 trill in my right hand. I think I've figured out why------when you're a beginner at piano, you only think about the "down" muscles in your fingers. Later, you think, "oh, there are UP muscles too!" but then that doesn't work because it makes your fingers tense, and you finally realise the motion you want is not just moving up but moving up AND RELAXING AT THE SAME TIME. I think this is the secret to really fluent physical movement at the piano. I don't care whether you call it finger strength or arm weight or whatever else, but that is what is really going on "behind the scenes" in our fingers, I think.. That's just my perception anyway.