Why do you believe one has to "anaylse closely"? It should be obvious to anyone with a basic understanding of introductory physics, because our feet push into the ground and gravity exists.
Check out p. 48 pic. 22 in The Secrets of Judo, a judo book from 1959, that explains that feet pushing off the ground, plus gravity pulling down, creates a resultant vector that is diagrammed as the body (and hence any arm, leg, etc. attached to the body) advancing.
I was reading some of my older MA books this weekend and found that in addition, the karate book Dynamic Karate
, 1966, has a similar diagram (p. 109) of such vectors.