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:  Active spring length 
1. Abstract of Cited Paper (BIB001):  Interaction with humans is inevitable for service robots, which results in safety being one of the most important factors in designing the robots. Compliant component is an answer to the safety issue at the cost of performance degradation. In order to reduce the performance degradation, manipulators equipped with variable stiffness have been studied by many researchers. This paper presents a variable stiffness joint (VSJ) designed for a robot manipulator, as well as a control scheme to control the stiffness and position of the VSJ. Compliance is generated by leaf springs and two actuators are used to control the position and stiffness of the joint using four-bar linkages. Two actuators in parallel configuration are connected to the spring. Changing the effective length of the spring results in a change in stiffness. The position of the joint is controlled via two actuators rotating at the same speed in the same direction. A nonlinear controller is used to control the VSJ, and a singular perturbation model is adopted to prove the stability of the closed-loop system. Experiments are conducted to show that the position and stiffness are controlled independent of each other, and having less stiffness at the joint helps in making an unexpected collision with an object safer. 
2. Abstract of Cited Paper (BIB002):  This paper proposes a novel variable torsion stiffness (VTS) aiming on biomechanical applications like prosthetic knee joints. By varying the effective length of a torsional elastic element via a relocatable counter bearing, the stiffness of a rotational joint is adjusted. This functional concept is described in detail by the authors as well as the design of such VTS joints. Additionally, analytical models for the transfer behaviour of drivetrain and stiffness control are derived. These are used for a simulative evaluation of a pendulum driven by a VTS unit. Based on the results of this simulation, the power requirements of VTS are analysed. Furthermore, an analysis of its structural strength is presented. For practical comprehensibility, the example of the design of a prosthetic knee joint is taken up for several times in this paper. Finally, the concept, modeling and design of VTS as well as the simulation results are concluded and discussed in a final assessment and in comparison to other contemporary concepts.

Literature Review:
Stiffness may also be adjusted by varying the effective length of a compliant element. An active knee brace varies the beam length to adjust the stiffness  . The Mechanical Impedance Adjuster  contains a leaf spring, connected to the joint by a wire and a pulley. The effective length of the spring can be changed by a slider, with a roller on the slider holding the leaf spring close to the structure. The motor rotates the feed screw, which moves the slider, and thus changes the stiffness. A rotational version was implemented in a robotic joint. The two spindles, which are actuated by a motor, can move the slider with four wheels, rolling over the leaf spring to vary its effective length. An advantage of both of these mechanisms is that they are easy to construct and easy to control since the stiffness setting and equilibrium position are completely independent. This mechanism allows all possible states between compliant and very stiff. An advantage of both of these mechanisms is that they are easy to construct and easy to control since the stiffness setting and equilibrium position are completely independent. This mechanism allows all possible states between compliant and very stiff. Choi et al. BIB002 presented a variation of this method. The effective length of four springs split at 90°around an axis were changed by moving the pivot, which had rollers sliding on the spring. The pivots were moved with a fourbar linkage. Controlling the two adjacent bars together in opposite direction, the pivot moved along the spring causing a stiffness change. When the bars rotated at the same speed in the same direction, the axis rotates without stiffness change but with change of equilibrium position. Sugano et al.  designed a finger that incorporates a leaf spring that can be adjusted to vary the stiffness. The joint was not mounted rigidly but could move according to the stiffness of the leaf spring. De and Tasch designed a two dof degree-of-freedom finger, which again uses leaf springs, but can also control the coupling stiffness BIB001 . In BIB003 a torsional elastic element with a relocatable counter bearing changes the active spring length. Stiffness adjustment in the Jack Spring is achieved by in/decreasing the number of coils used in a spring through a rotation of either the spring or the shaft/nut  .