This paper addresses the way the rolling mechanism of the human foot influences the Generalized Inverted Pendulum (GIP) model during normal walking. A test on 6 subjects was performed , and two ways of modelling were used: the first one uses the filtered data of the Center of pressure (CoP) trajectory to rebuild the human inverted pendulum; the second one linearizes the CoP trajectory with time, which is equivalent to ignoring the effects of the rolling mechanism of the foot. The results show that the linearized model leads to an observable clear area of convergence of the dynamics support lines showing a neat inverted pendulum, while the non-linearized forces to make the distinction between the three sub-phases of single support: damping, stabilizing, propelling.