Based on Cortot exercies like these...
...I've been practicing patterns like this for seventh chords...
...and left hand...
The spacing of the fingers with these major seventh chords feels different than other seventh chord types like minor sevenths, etc. The fingers get crowded at the minor seconds, but then stretch to reach the major thirds. The finger spacing feels more even for the other chord types.
When I reach with finger 4 to get to a black key I notice that my left hand has a more difficult time than my right, but my two hands are the same size.
When if finish playing like this my palms feel a little warm and sort of 'itchy.' I'm not sure exactly how to describe it. It does not hurt or ache or spasm, etc. I definitely feel like my hands need a rest by the end. I don't push through any discomfort.
It feels similar to the fatigue of playing scales in octaves or full octave chords, but it's more centered in my palms.
Let's just say I've had worse feeling from using the computer mouse wheel, and I have enough sense to stop before I've really irritated by hand.
Of course I don't want to do any damaging repetitive motions. But I can't really tell if this is just a phase of coordinating and training my hand.
Just looking for some guidance.
- does my pattern show some misunderstanding of the Cortot exercises?
- are there some time limits I should keep in mind?
- should I avoid certain things when doing such tremolo patterns?
EDIT for a bounty.
The one answer I received from @MalcolmKogut is appreciated but is more of a general overview of anatomy than a response about these particular exercises. I'm looking for help with how to approach these exercises safely. Or, if the recommendation is to avoid them entirely, some explanation of why such exercises were recommended by Cortot - whose teaching I understand is still used today - but avoided by some teachers.
Also, I removed the word 'strengthen' from my question. I can tell now that word is a big piano instruction buzz word. This issue isn't about strength, but extension (meaning spreading my fingers apart - which apparently uses the interossei muscles) and finger independence, which I understand is mostly an issue or coordinating the nervous system. To be clear: I do not think these exercises are about working out finger muscles. I can hold my fingers fully spread apart without discomfort or fatigue. I can do motions like this for an extended time - 20 minutes for example - so the point is already understood these exercises are not about strength. I want to know how to approach such exercises safely.
I found some videos like this one...
...from what I have seen in this particular video I gather that I don't need to play them fast and the ability to rock your hand in subtle circular motion while the keys are depressed sort of tests or demonstrates a firm touch without undesirable tension and stiffness. He calls it 'loose' around 5:47 in the video.