The ankle–foot complex in the human body is one of the major determinants in normal human walking. Most of the research study ankle and foot motion by observing people as they move, measuring desired kinematic and kinetic data indoor or outdoor, and numerical simulations in computer. However, very few studies are able to explore the fundamental mechanical principles underlying human musculoskeletal system. In this paper, we developed a three-dimension (3D) passive dynamic walker with flat feet, toes and ankle springs to investigate the impact of the ankle and toe stiffness in the walking motion. The results suggest that the ankle springs have a main impact on the walking motion, where the anterior spring, over any other position, plays a main role in providing sagittal stability. The springs from the sagittal plane control the pitch angle of the robot which impacts on its velocity and step length. The stability got worse along with the step length and velocity increasing especially when the step length overcame 8 cm. The fact that the best configuration of the ankle joint has stiffer stiffness in the sagittal plane than the coronal plane complies in nature with humans where Tibialis Anterior, Soleus and Gastrocnemius muscles are much stronger than other muscles around the ankle. Furthermore, it can be stated that the medial toe plays a more important role than the lateral one, as blocking the medial toe with the stiffest joint (rigid joint) has a negative effect on walking motion. In conclusion, we show that the ankle stiffness of the robot in anterior–posterior position should be higher than that in medial–lateral position and the stiffness in any position should exceed a minimum level to maintain walking stability. Also, adding toes (medial one should be softer than the lateral) in the foot of the robot may benefit biped locomotion especially when taking longer step length.