It seems gravity, kicking the ground to push us up, the sum of their (trivial) vectors moving us forward, and ultimately any thing attached to us moves forward, supplemented by as much or as little muscle as needed, is universal to movement, certainly across 'internal' and 'external' martial arts.
For example, 'peng exercises' (http://denner.org/reinhard/neijia/pe...Exercises.html
Push open doors by dropping down slightly, arm sort of straight (plenty of angle under the shoulder, too) and straighten slightly up into the door. Use the absolute least muscular effort. This will begin to train peng to the arm and pretty quick your setup motions will be almost unnoticeable.
Where does the basic physics model as presented in the Judo book and the Karate book (both not considered 'internal') not apply?