People aren't stickpins stuck in the ground. Their bodies can pivot and rotate; go back-and-forth and side-to-side. Probably the person is not much aware of it.
This is an irrelevant observation. This has nothing to do with anyone's body pivoting or rotating. Brehm's legs never change their horizontal position, and the son moves from being behind him to being beside him at an impossible speed. You are simply ignoring the problem of how anyone could carry out the son's movements in 0.56 seconds, or barely over half a second. Try it. I've done several experiments with one of my sons, and he was never able to even come close to doing those movements in 0.56 seconds.
Another physically impossible movement in the current Zapruder film is the head turn of the driver, William Greer, from Z315-321. Greer turns his head about 165 degrees in six frames, or in only 1/3rd of a second. Mike Pincher and Roy Schaeffer observe that Greer's head turn should create blurring in the film because the human eye can't remain focused when following such a rapid movement, but no blurring is seen:
"If the reader flashes his hand in front of his face in approximation of one-third of a second, it appears as a blur. The eyes are incapable of staying in full focus in following this action. If Greer's 165-degree movement in one-third of a second truly depicted real time, it would likewise appear as a blur. But blurring of this nature is not seen in the Zapruder film." (Assassination Science, p. 223)
Or, look at the astonishing speed at which Malcolm Summers moves his leg in Z353-356. In this sequence, we see Malcolm Summers diving to the ground. Summers is to the right of James Altgens. In Z353 Summers' left leg is extended most of the way out. But, in the very next frame, Z354, amazingly, the foreleg is bent markedly backward. Can anyone flex their foreleg to that degree so quickly, in 1/18th of a second? Then, in Z355 Summers' left leg is bent even farther backward. Can anyone move their foreleg that much in 1/9th of a second (from its position in Z353 to its position in Z355)? Then, in Z356, the left foot seems to be on the ground. Can anyone whip their left foreleg backward and then put their foot on the ground in the space of three frames, 1/6th of a second?