Ueshiba spoke of "a limited sphere of strength" outside of which no opposition is possible. There is reason to believe he may have been speaking at a micro-level as well as a macro-level.
The questions of "what adjustment" and "what sensation triggers adjustment" are key.

For an alternative model of adaptive static equilibrium try "Colulomb's Memoir on Statics." Jacques Heyman, tr.

The principles are graphically illustrated by an inverted chain of spheres on the cover of the 1998 edition. See here:

http://www.amazon.com/gp/reader/1860...15#reader-link

The adaptive signal for that model is any force differential outside the center of the articulating joint.