The energetic effects of passive motion of the arms and addition of torsion springs to the arms joints of a nine-link planar bipedal robot are studied for walking gaits. Only the locomotor system of the biped is actuated. Starting from a cyclic optimal motion of the biped with the arms stuck to the trunk, we explored the existence of a passive motion of the arms produced by the dynamics of the locomotor system of the biped. Multiples solutions including large amplitude of arms motion exist if the duration of walking step corresponds to the natural period of arms oscillation. The effect of torsion springs on the natural period and the cost functional is explored. Finally, costs functional of reference trajectories and trajectories with passive motions of the arms are compared.