"This concept of training separately in internal power is not really known in Japan."
--Really? When I lived there I trained at two separate and unrelated aikidojo, and both consisted of a majority of students who were chiefly interested in waza training and a small but dedicated minority who were aware of and actively seeking the "stuff" (kokyu ryoku, IP, inner muscle, whatever you want to call it) through extracurricular training methods. Kind of like.....how it seems to be here in the states.
True - Minoru Akuzawa is based out of Japan and has whole set of exercises for training the body independent of the waza.
Going back further, Tohei has a whole system for it in the Ki Society, complete with a separate ranking structure.
Of course there are a number of Daito-ryu and Aikido instructors who have had solo training regimens for building the Budo body seperate from waza, and that goes back quite a long time, although it wasn't always public.
Sagawa talks a little about it in this article
"If you train sincerely in any martial art eventually you will develop inner power."
--I disagree with this, too. If it were true, why do I keep hearing so many high level practicioners from aikido (and other arts, to be sure) claim that it can't be developed just by doing the typical training regimen? It's not largely new folks who are seeking out these training opportunities and methods; it's typically people with decades of experience in their chosen art. They can't all be deluded fools, can they?
Yes, that's right, we're deluded fools. Nothing more to see here, please move along...