This paper investigates the energetic effects of adding springs at the passive ankles of a planar five-link, four-actuator walking biped robot. The energetic cost of walking with springs was determined by using a walking motion designed for the biped without springs added. The walking motion was then optimized for the presence of springs of various arbitrarily chosen stiffnesses and offset angles. The stability properties of the motions that resulted were checked. The energetic costs of walking and standing were then computed. It was found that standing with springs was more efficient, while walking was more costly than the same action without springs. Finally, the spring characteristics (stiffness and offset angle) and the motion were optimized simultaneously. The costs for walking and standing were computed, revealing that walking with springs was more efficient and standing was more costly than doing either without springs. A methodical approach to choosing the size of the feet based on this analysis of the spring characteristics is also presented.