High Score of 1069! Nice!
I played on keyboard btw., it's totally fine, also you made if correct by setting the physical location of "Z" to the gamepad button "A", because like this it was "Y" for me on the german keyboard. If you now were to display the Keyboard buttons in the game, there would be zero problem with keyboard play.
One little bug I found (at least I think you didn't want that) is that with time, your muscle don't cool down to white, but at first they became like pinkish for me, then later they stayed orange after fully cooling down, so maybe you're modifying the color strangely. I guess you are doing like "intensity += 1" and later "intensity -=1", but then the internal calculations drift off. I you instead would track your muscle fatigue in a designated variable, then calculate the color from that fatigue, something like this shouldn't be able to happen.
All in all, a very nice idea, reminding me of the simple sports game I played with my father a long time ago, where you also had to button mash ;-)
PS: would have been easy to put a chair in here somewhere, right?