So if both Mitch and Jerry are right, then its not the point of attack , but the over use by an FM vest on your shoulders (many injuries reported to date concerning shoulders and fm vests, and back, I need not report on that part of the body to any of us here). The hips bearing the brunt of the weight allow us as operators to use our brains for more operating and less survival. In other words, a back mounted vest is more energy efficient...........
The only question I have concerning point of attack is if your back begins to tilt forward as is the case after a long day in an FM vest, aren't you effectivly bringing your center of gravity foward and placing more and more stress on your lower back. Additionally, in a non rigid vest, if the vest twists as you work, say in low mode, doesn't that also place different stress on the body at different points?
Finally, the design of the BM vest, provides for a "hinge effect in the back, pushing the vest into your back as you walk, rather than pushing the vest away from you as is the case with an FM vest. If the vest pushes into you rather than away from you, doesn't that affect the where the weight affects you? So maybe its not about where the Center of Gravity exists on a vest, if all equal, but how that CG affects you in a rigid vs non rigid vest.
For my body, and career the back mounted system has made a tremendous difference, all of which has been explained.
The bottom line, regardless of the science, is that there is a notable difference between the two systems. I personally prefer the Back Mounted to the front.