Re: Kodo Horikawa's aiki
When I first started training with the sword I was told that that when doing a lot of cutting practice I would first feel the fatigue/pain in my arms and shoulders. The story went that as I gained experience it would move in to the high upper back, then finally into the mid back and down. And yes, being the guy who was completely fixated on weapons who trained a *lot* daily that's precisely what happened over time.
But I also came to realize that maybe there's more than just "learning to do it right". In other words, it's not just choreography. The idea is that possibly it is the developing and conditioning of the body in a certain way through a lot of (proper) repetitions allowed me to be able to start using more core to "power" the movement. Maybe the long term training helps develop the connections allowing you to use more of the "core" muscles through those connections to support the movement, somewhat altering the role of the muscles originally more used in the movement.
So again we talk about those deshi with lots of hard, good training. Maybe they developed more of this body than they realized. And lacking an understanding of how and why things work start talking about things like "relax completely" and "let your ki flow" without realizing that they were talking about feelings they had inside their conditioned bodies that their students may not be capable of feeling in their unconditioned bodies. So the crux, again, becomes conditioning correctly and learning how to use that conditioning to do those things that the real greats could seemingly do without thought or effort.
Or... Just watch Kuroda and his sword. Amazingly fast. Amazingly fluid. Amazingly relaxed. Full integration of his entire body in every movement. Just like a smaller woman easily picking up a heavy bag of dried beans and tossing them on a shelf. Learn to do it right an develop the body that allows it to be done, and voila.
Which all reminds me of family who still live/work on farms and ranches. Some of those little guys can toss bales of hay around like they're nothing. Me, I struggle. The difference isn't "muscle strength" (I'm much bigger and "stronger"), but a lifetime of tossing the damned things.