US 8870967 B2 abstract artificial limbs and joints that behave like biological limbs and joints employ a synthetic actuator which consumes negligible power when exerting zero force consumes negligible power when outputting force at constant length and while performing dissipative nonconservative work is capable of independently engaging flexion and extension tendon like series springs is capable of independently varying joint position and stiffness and exploits series elasticity for mechanical power amplification application_number US 60862709 A description in the construction of a biologically realistic limb system that is high performance light weight quiet and power efficient a agonist antagonist actuator design is proposed herein comprising a plurality of actuators and series elastic structures since it is desirable to minimize the overall weight of the limb design the efficiency of the agonist antagonist actuator design is critical especially given the poor energy density of current power supplies e g lithium ion battery technology by understanding human biomechanics the lightest most energy efficient agonist antagonist actuator design can be achieved in the next section the key features of biomechanical systems are highlighted a more complete description of biomechanical systems is found in the patent applications cited in the foregoing u201c cross reference to related.