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:  Nonlinear mechanical interlink 
1. Abstract of Cited Paper (BIB001):  There is a growing interest in utilizing pneumatic artificial muscles (PAMs) as actuators for human-friendly robots. However, several performance drawbacks prevent the widespread use of PAMs. Although many approaches have been proposed to overcome the low control bandwidth of PAMs, some limitations of PAMs such as restricted workspace and torque capacity remain to be addressed. This paper analyzes the limitations of conventional circular pulley joints and subsequently proposes a design methodology to synthesize a pair of variable radius pulleys to improve joint torque capacity over a large workspace. Experimental results show that newly synthesized variable radius pulleys significantly improve position tracking performance in the enlarged workspace. 
2. Abstract of Cited Paper (BIB002):  Bringing mechanically compliant joints to robots is in the focus of interest world wide, especially in the humanoid robotics community. Variable Stiffness Joints (VSJ) promise to gain a high performing and robust robotic system. The presented DLR Floating Spring Joint (FSJ) is a VSJ module designed for the first 4 axes of the anthropomorphic DLR Hand Arm System. The DLR Hand Arm System aims to match the skills of its natural archetype. For this purpose, the joints have to be extremely compact to fit into the arm. At the same time they require a high power density in order to approximate the human arm skills. The new DLR FSJ is designed completely from an energy based point of view. This addresses not only energy efficient components and low friction design, but also that the potential energy of the spring is used as good as possible. A demonstration of robustness is given by the investigation of a blunt impact to the tip of the arm.

Literature Review:
By changing the properties of the mechanical interlink one can obtain different transmission ratios. In this category no devices have been reported in literature that use this as the primary principle to adapt the stiffness. However different connections principles exist to yield torque-deflection characteristics in function of the application (see Fig. 8 ). Mostly the compliant elements are connected to the joint using a pulley (see Fig. 8a ). PAMs for example have a limited joint motion due to the limited contraction ratio of the muscles and by choosing different connection principles the torque characteristic can be adjusted. In the biped Lucy  and the manipulator arm BIB001 a pull rod and leverage mechanism are implemented (see Fig. 8b ). By changing the angle and length of the lever arms the strong nonlinearity of the force-angle characteristic of the muscles is compensated in order to flatten the torque-angle characteristics of the joint. The knee rehabilitation robot Knexo uses a four-bar mechanism (see Fig. 8d ) in order to be able to encapsulate the desired torque and joint range requirements with a relative simple and compact mechanism. Another advantage is that the coupler of each four-bar linkage is easily equipped with two pairs of strain gauges for joint torque measurements. An exhaustive search approach is proposed by Beyl et al.  to solve this multi-objective optimization problem. Shin et al. BIB002 developed a design methodology to synthesize a pair of variable radius pulleys (see Fig. 8c ) that maintains high torque capacity while satisfying passive stiffness and workspace requirements. In the VS-joint BIB003 , Maccepa 2.0  and FSJ BIB003 a cam profile is used to have a desired torque-deflection characteristic.