Following last week's question regarding Dr. Marshall's wrist weights exercises, I exchanged several emails with Dr. Marshall and one of his students.
The student addressed the question, but claimed that the two arm actions are the same despite the obvious visual differences. He then took the opportunity to tell me that I don't understand how Dr. Marshall's pitchers throw a baseball. The student also claimed that the vertical elbow extension was the result of centripetal force. Apparently, this student wasn't paying much attention to Dr. Marshall when he explained forearm flyout.
Dr. Marshall had more useful things to say. He explained that his pitchers are taught to drive their upper arms in a position described to be "as vertical as possible." From this position, a pitcher clearly can only extend his elbow vertically. This matches exactly what I have seen from his pitchers when they throw baseballs.
When performing the weighted exercises, Dr. Marshall's pitchers appear to me to be powerfully extending their upper arms toward the target. This is because their upper arms seem to move from nearly vertical to nearly horizontal in the direction of the throw prior to release. As a result, their elbow joints extend their forearms toward the target instead of the sky.
Take another look at some of Dr. Marshall's students performing a weighted training exercise. This time, they are throwing iron balls which are obviously more similar to baseballs than the wrist weights are.
Again, his pitchers appear to be driving the heavy weights in a nearly straight 3-dimensional line. Some of them do this better than others. In this video, it appears that the pitchers who raise the ball higher are better able to keep the ball on a 3-dimensionally straight path through release.
Compared to the wrist weight exercises from last week's entry, the iron ball exercises appear to result in more skyward elbow extension. This could be an illusion caused by the arm's reaction to the release of a heavy object. Without high-speed video, it's virtually impossible to tell which happens first.
Because Dr. Marshall wants his pitchers to accelerate their upper arms in as vertical a position as possible, elbow extension is necessarily skyward. This is really the answer to last week's question. In Dr. Marshall's view, when performing his motion, skyward elbow extension is expected and unavoidable.
It seems that the difference in arm actions is the result of the weights being too heavy for Dr. Marshall's pitchers to duplicate the intended arm action.