The muscles that move the fingers are mostly in the forearm, but I don't hear of many students plowing through the schmitt exercises as I did when I was young. They are on pianostreet,
https://www.pianostreet.com/schmitt-sheet-music/preparatory-exercises-op-16.htmMy teacher bought mine in a book from G. Schirmer
4 5 strength is a typical weakness of the new student, or the old person that has lapsed (me).
At my age and skill level, I play 3 Scott Joplin rags every night when I have a piano available. Since I don't have a piano at my summer trailer, I'm suffering fatigue this month (September) on the 4th and 5th finger muscles. You lose 10% of your muscle mass after age 60 if you don't exercise, and it only takes a few weeks off to start the process. The rags I play are Paragon, Magnetic, and Maple Leaf. Lots of octave jumps in left hand, lots of octave chords in the right.
Signing off to get my rags exercised before
Roosevelts airs on PBS at 2000.