Forward progression of the Ground Reaction Force (GRF)
The GRF is presented by a vector. This vector has a size, a direction and a point of application and is visible with the aid of a yellow line (in this video). At initial contact the point of application is at the heel. During the roll off the point of application moves to the forefoot. At initial contact the GRF point a bit backward and at terminal stance (push off) the GRF points a bit forward.