One way of avoiding repetitive stress on the fingers
I believe that the ability of the fingers to move in one plane of action (up and down) is abetted by its being free and able to move in other planes as. Once in a while I’m practicing, when I momentarily run out of ideas what to do, I will, turn my hand upside down and I will extend rather than curl my fingers while trying to make the finger tips cause sounds on the piano. It is but a moment’s diversion.
Another time, I might spend a few moments focusing on the lateral motion of my fingers. Here is the series of steps: 1) lay a flat, closed hand on piano. 2) separate two adjacent fingers, say fingers two and three. 3) have the third finger move laterally in the third knuckle until it can play a note that is located where the 2nd finger is. and vice versa.
Here’s something to try that is half way between a right side up hand and a hand turned upside down. I turn the hand sideways, either pinkie down or thumb down. We’ll take thumb down first. The thumb, which is now the ‘lowest’ part of the hand, is extended downwards toward the keyboard and away from the other fingers, and tries to make a sound on the keyboard. Then I attempt to do something similar with the other fingers. These seconds’ long exercises can be done with either hand as well as with the hand turned pinkie down, so that it is the pinkie that first tries to extend down and away from the other fingers to try to make a sound.
Another way of going about freeing up the muscles in the fingers is to use one hand to hold onto a finger in the other hand, so that the latter finger can move only from the first knuckle; later the second knuckle; or the third knuckle. Usually all three knuckles take part in the flexion of the finger when making a sound (although the first knuckle a lot less than the others); so it is interesting to separate apart these “partial” motions and then putting them back together.