Why are you searching for internal strength? Why is it missing from your Aikido training? Why are you going outside your art to find it? When I read the threads about this training it makes me wonder what you are lacking that you have to go find it.
Once when I was joined in the conversation I was told I don't have it because one of my students doesn't have it. If you asked me about that student I would agree. His heart was not in it. Aikido training lets your inner strength out. You don't have to go to an expert to find it.
Aikido training for me is a whole practice…the development of correct feeling is just part of the process in becoming what Aikido is molding me into. Are you missing the point in your impatience to be the strongest person alive?
From my personal perspective I suspect IS is what I've seen referred to as Kokyu Ryoku which is for me one of the bases of Aikido.
A long time ago one of my major influences expressed concerns that 'modern Aikido' was more concerned with the joy of movement and failed to recognise that the development of breath power was fundamental to the practice of Aikido.
Now on this forum there is a growing body saying the same thing.
So I'm very interested.
Some of the guys that post here are specialists with seemingly vast subject knowledge .
Naturally I'm interested ...sadly the person who initially evoked my interest is now in his 90's and no longer travels, ...so I do need another source since to date I've not been able to work it out for myself...If Aikido were more of a kicking art I'd kick myself for not paying enough attention when I had the chance...but perhaps I was not ready.
I'm ready now though - So this is why I'm looking ...i agree with you of course, Aikido is whole practice, it is all the good things in a dojo, it is also still a martial art so to me has to 'work' ...but if there are some shortcuts available or even just a few pearls of wisdom then why not look to those that have focussed on this Internal aspect and then bring that into the framework of your whole practice Aikido?