You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Variable impedance actuators: A review 
Chapter Title:  Antagonistic Springs with Independent Motors. 
1. Abstract of Cited Paper (BIB001):  In this paper we introduce a classification of intrinsically compliant joint mechanisms. Furthermore, we outline design considerations for realizing such devices in order to match the requirements for robust and performant actuation. Based on this elaboration, a new design concept is presented, the DLR QA-Joint. Its performance is investigated by various experiments, covering velocity increase using the elastic energy, joint protection capabilities, and control performance.

Literature Review:
The main disadvantage of using antagonistic motors is that both motors need to work synchronously to either change the equilibrium position or the stiffness. This means the motors cannot be dimensioned for a specific task. For Antagonistic Springs with Independent Motors, the motors are arranged to (partially) decouple the control of equilibrium position and stiffness 4. For the Quasi-Antagonistic Joint (QA-Joint) BIB002 one motor (the link drive) adjusts the link side position, while the second motor (the stiffness drive) operates the stiffness adjustment. This is a partially decoupled system since when the stiffness is changed, the equilibrium position must be adjusted by the link side motor. Complete decoupling of the equilibrium position setting and the stiffness occurs when the endpoints of the two springs are mechanically coupled either by a lever arm or a pulley like e.g. the AMASC actuator  . Here the motor to set the equilibrium position is not on the joint but on the other side of the nonlinear springs. It is also possible to move this motor to the joint, making the design less complex. In this case the equilibrium position of both lever arms is horizontal, and the motor for the equilibrium position of the actual joint sets the relative position of the arm of the joint with respect to the lever arms BIB001 . With this design the angular joint range is extended, since it is not limited to the maximum extension of the springs.