Wow, the article you cited was great.
Could you share more?
Unless I am mistaken reference to the seika tanden[tantien ] is made in a book on Judo by E.J.Harrison.The tanden according to Eastern viewpoints is the source of/ centre of Ki /power.If one acquires the ability to activate /use the tanden a person can become light as a feather /or be come a heavy as a mountain.Of course this is not saying a man loses or gains weight.
My own judo teacher [who was 90 % disabled]in over thirteen years no one who ever came to the dojo ever threw him.When attempting to throw him it was like moving a building.
In aikido Tamura Sensei[ a great man ] was tiny yet very few guys could budge him.He would just stand a smile while you tried in vain to move him.He is a great loss to the aikido community.