It has been shown that compass-like bipeds can walk passively by gravity-induced motion. Similarly, it has been stated that the unipodal phase of human gait may be regarded as a passive motion. This paper is aimed at determining precisely to what extent the swing phase of human-like walking can be considered as a non-actuated motion. More specifically, a method is developed that makes it possible to adjust initial conditions, step length and walk speed, as well as mass and inertia distribution of a bipedal robot, in order to generate a passive swing motion. The approach is demonstrated by using a 7-link planar biped. Numerical simulations show that purely passive unipodal transferring cannot be fully satisfactory. This approach is further considered in Part 2 of the paper which consists in computing optimal actuating joint torques to create perfectly feasible motions.