This paper focuses on the study and design of an anthropomorphical light biped robot. The robot presents a total of twelve degree of freedom that permit it to perform a walk in a three dimensional space. Each joint resemble the functionalities of the human articulation and is moved by tendons connected with actuators located in the robot's pelvis. We implemented and tested an innovative actuator that permits to set the joint stiffness in real time maintaining a simple position control paradigm. The controller is able to estimate the external load measuring the spring deflection and demonstrated to be particularly robust respect to system uncertainties, such as inertia value changes. Comparing the resulting control law with existing models we found several similarities with the Equilibrium Point Theory.