This paper deals with the decision mechanism analysis and the design of bipedal trajectories, for the stepping motion. For that we have used biomechanical model of the human body and dynamic control scheme previously developed by Gorce. We based our study on an experimental protocol, in order to determine behavioural laws for the task execution. We have developed a biped trajectory generation process, taking into account the biped height and the obstacle dimensions. Furthermore, we characterize the stepping motion feasibility by introducing a security notion, and we define an “Admissible control Domain”, which relies on the relative position of the biped to the obstacle and the obstacle dimensions. This domain definition has led us to define the biped behavioural strategies facing an obstacle: the biped executes the task at an accurate “chosen distance”, or stops or takes another way. Experimentations have allowed to validate simulations results.