foweyman wrote:The reason for controlling this variable is that it would allow the determination of whether it is the shoes or changes in the running form that causes the observed differences. It would be important to know if the shoes caused the changes or if they only allowed or encouraged them.
I get what you are saying... Basically you just want to understand what effect shoes have, given that the running form stays the same. But honestly I don't think that's the best way to think about it, human running form consists of the interaction of the human musculoskeletal system and shoes & ground. You don't get to just change one without changing the other.
The body moves in a certain manner dependent on the nature of the surface beneath it. Dynamics of the center of mass and segments of the body have to match in accordance with the reaction force underneath the feet.
Now people can model the properties of the shoes, that's been done plenty. Then, in a dynamic model simulation the running motion, you could try to keep the same running motion and change the property of the foot - surface interaction and see what happens. But that isn't necessarily realistic.