I'm still exploring the idea that "the Correct Way" happens in 2 stages, the all in one direction model evolving into the eventual model that we see in the videos. As I've written, I still wonder if this morphing happens because the most beginning students do not have the sensitivity to use the intrinsic muscles in the hand, but must rely upon the larger muscles in the forearm (the digitorum profundis, and the digitorum superficalis).
What I see is what guit-box sees, that the professional brings the finger to the string by simultaneously extending the PIP and DIP, while flexing the MCP. After the "work" of the force exerted on the string (the "pluck"), we observe the exact opposite of this motion; the MCP extends while the PIP and DIP flex (the "relax"). This is complex flexion/extension motion exactly describes the function of the lumbricals,which, not so incidentally, arise from the digitorum profundis tendon.
So you see, if you are still reading all this tripe, the "all in one direction model" for beginners engages the same tendons as one of the most import and and observable intrinsic muscles in the hands, but at an unrefined stage. Perhaps it just takes a while for the body to discover that it doesn't need all that extra effort to displace the string and to redirect the work from the forearm to the hand. I also suspect that there are assists to the advanced players motion, likely from both the interosseous muscles in the hand and a perhaps a continued but lighter assist from the digitorum superficialis in the forearm, at least for some folks.
But what do I know. I would love to hear an opinion from a physiologist.
[Cross-posted to guit-box's thread]
Dr. Lawrence A. McDonald, D.M.A., Art Kaplan Fellow
Author of The Conservatory Tutor for Guitar
2018 Michael Thames "Ancient Dragon" Cd/Ir
2008 Michael Thames Cd/Br
Royal Conservatory Advanced Guitar and Theory Instructor