In human walking, it is often assumed that the arms have a passive movement which reduces the energy consumption of walking. The issue addressed in this work is the influence of the arms on the walking of a humanoid robot. The study has two objectives: to verify the effect of arms on a sthenic criterion during walking, and to determine whether the optimal movement of the arms is passive or not. Firstly, we defined optimal cyclic gaits for a biped robot moving in 2D. These gaits are composed of single support phases with a supporting flat foot, double support phases with rotation of the feet and an impact. Different evolutions of arms are studied: bound arms, arms having an active motion and passive arms. The comparison of our results for different walking speeds show the importance of an active movement of the arms. The part of the sthenic criterion supplied in the joints of arms allows reducing the global sthenic criterion especially for high walking speeds. A passive movement of the arms will have large amplitude when the natural frequency of the arms coincides with the frequency of the walking gait. Adding springs at the shoulders allows to adjust the natural frequency of the arms to that of walking gait. However, the sthenic criterion with the active arms with or without spring remains less than with the passive arms.