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:  Fixed compliance properties 
1. Abstract of Cited Paper (BIB001):  Play has physical, social, emotional and cognitive benefits for children.1 It has been suggested that opportunity for spontaneous play may be all that is needed to increase young children’s levels of physical activity,2 an appealing concept in view of our … 
2. Abstract of Cited Paper (BIB002):  A hermetic refrigeration compressor has a flat valve plate closing off a cylinder bore. The valve plate has an elongated recess on the outer side around the discharge port and a discharge valve assembly, comprising a flat reed valve and backing spring, fits within the recess beneath an overlying valve stop which engages the bottom of the recess at each end. The recess, the reed valve, the backing spring, and the valve stop are so configured that they can be assembled only in the correct configuration. The valve stop is held in place by an arcuate retaining spring having ends engaging notches in the valve plate and a projecting boss on a cylinder head defining a discharge valve plenum engages the retaining spring to press the spring and the valve plate into position within the recess to retain all of the parts in operating position. 
3. Abstract of Cited Paper (BIB003):  A safe robot arm can be achieved by either a passive or active compliance system. A passive compliance system composed of purely mechanical elements often provide faster and more reliable responses for dynamic collision than an active one involving sensors and actuators. Since both positioning accuracy and collision safety are important, a robot arm should exhibit very low stiffness when subjected to a collision force greater than the one causing human injury, but maintain very high stiffness otherwise. To implement these requirements, a novel safe link mechanism (SLM), which consists of linear springs, a double-slider mechanism and shock-absorbing modules, is proposed in this research. The SLM has the advantages of variable stiffness which can be achieved only by passive mechanical elements. Various experiments of static and dynamic collisions showed the high stiffness of the SLM against an external force of less than the critical impact force, but an abrupt drop in the stiffness when the external force exceeds the critical force, thus guaranteeing collision safety. Furthermore, the critical impact force can be set to any value depending on the application. 
4. Abstract of Cited Paper (BIB004):  In this paper a novel concept of embedded robotic actuator is presented which has been named the very versatile energy efficient (V2E2) actuator. This actuator stores energy during any force profile which generates negative work on the load and it does therefore have unprecedented potentials for robotics applications. 
5. Abstract of Cited Paper (BIB005):  A novel revolute nonlinear series elastic actuator called the HypoSEA is presented. The actuator uses a hypocycloid mechanism to stretch a linear spring in a nonlinear way. The actuator is optimized for highly dynamic tasks such as running and jumping, as it features a 120Nm torque capability and more than 30J of passive energy storage. When combined with a suitable controller, using the spring as an energy buffer can greatly reduce the work done by the rotor during periodic motions. The design has exceptionally low reflected mechanical impedance, making it robust against repeated impact loads. The nonlinear stiffening spring is optimized for the nonlinearities typically found in revolute-jointed hopping robots, and may be adjusted offline using a pretensioning mechanism. Finally, the low effective stiffness around the zero-torque equilibrium allows for extremely sensitive force control.

Literature Review:
The most famous inherently compliant actuator is the original Series Elastic Actuator (SEA)  , which is a spring in series with a stiff actuator. The actuator stiffness is fixed and determined by the spring selection, thus the physical stiffness cannot be changed during operation. Sugar BIB001 developed a spring-based actuator, using the concept of equilibrium controlled stiffness, in which, a linear spring is series to a stiff actuator and the spring equilibrium position is controlled to exert a desired force or stiffness. The stiffness is actively changed using a control law rather than by passively adding springs. The force control problem becomes a position control problem using electric motors. The motor position is adjusted based on the deflection of the spring to alter the tension or compression of the spring. Tsagarakis et al.  developed a ComPact © soft actuator for the iCub robot using 6 linear springs for the compliant element sandwiched between the three spoke structure of the motor side reduction drive and the three spokes of the output link. The velocity based controller generates velocity commands that are a function of the desired virtual stiffness using the spring deflection state. In the Distributed Elastically Coupled Macro Mini Actuation (DECMMA) approach BIB002 , a SEA is placed in parallel with a second smaller motor to recover control bandwidth in the high-frequencies, later called Distributed Macro-Mini Actuator-DM 2 BIB003 for an antagonistic setup. 3.1.1. Nonlinear SEA As nonlinear stiffness seems to be advantageous for tasks such as jumping and hopping [24] , the HypoSEA BIB006 was designed to use a hypocycloid mechanism to stretch a linear spring in a nonlinear way. For safety the safe link mechanism (SLM) by Park et al. BIB004 maintains very high stiffness up to the pre-determined critical impact force, but provides very low stiffness above this value, thus absorbing the impact acting on the robot arm. The critical impact force is set by adjusting the initial transmission angle of the double-slider mechanism, the spring constant and the initial spring length. In the V2E2 actuator BIB005 energy efficiency is achieved by combining an Infinite Variable Transmission (IVT) and an elastic element. The spring element is used to store mechanical energy where any force profile can be achieved by changing the transmission ratio of the IVT. Energy can be injected in the system by a motor-clutch mechanism making it very suitable for cyclic motions like walking or to compensate different constant loads at zero energetic cost. Since the actuator has only one control parameter (the transmission ratio), the torque and stiffness are coupled.