For the past 2 months, I've been doing an exercise that I personally called "Plank Runs." What it is, is like the Handstand Wall Runs:
https://www.gymnasticbodies.com/forum/topic/1030-handstand-wall-runs/
Though instead of tapping on the shoulders in a handstand position, I've been doing it in a plank position (PPT/full protraction/locked elbows still applies.) - my hands taps the opposite shoulder. Last week I've built up mastery for 5x60r. Doing this exercise, the place I feel it the most is probably my biceps tendon area, I get a huge pump in the front of my shoulders.
I started doing this as a kind of "sPL/PE3.5," in-between the Planks(PE3) and the Single Arm Plank (PE4). There was 2 reasons I decided to do the Plank Runs instead of continuing onto Single Arm Plank . One was out of curiosity the benefits I would gain from it, if doing the "Runs" in a Handstand position was so beneficial, theoretically wouldn't I gain a lot from it doing it in a Plank position too? Second reason I did it was because when I started sPL/PE4, I felt A LOT of pressure on my elbow in that exercise, it didn't hurt, but I felt a lot that I was scared whether I shouldn't advance to it so quickly. Doing this exercise at least allowed my elbows to experience at least half a second per rep of putting my weight on 1 elbow anyways.
Results of doing this? Quite a few "WTH?" effects. My planks gotten better, I'm able to hold sPL/PE4 with full protraction (though I haven't tested myself on this yet, it's deload week for me.) Also surprisingly, I got quite a bit of transfer from doing this "plank runs" to planche lean. Played around with the planche lean and was pleasantly surprised, at how much new "ROM" I gained without even starting planche leans yet. The other day, I decided to walk up into a handstand on the wall for the first time in a while, never have I held a wall handstand so effortlessly before. I'm pretty sure the reason for this isn't due to my HS sessions because I've been re-cycling the same PE exercises due to iM, and because of a few reasons, I've actually been using 10 less pounds on the strengthening element, compared to my last cycle.
So overall, I'm pretty glad that I did decide to spend 2 months working on this