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:  Preload Adjustment of Single Spring 
1. Abstract of Cited Paper (BIB001):  A quasi-passive dynamic walking robot is built to study natural and energy-efficient biped walking. The robot is actuated by MACCEPA actuators. A reinforcement learning based control method is proposed to enhance the robustness and stability of the robot's walking. The proposed method first learns the desired gait for the robot's walking on a flat floor. Then a fuzzy advantage learning method is used to control it to walk on uneven floor. The effectiveness of the method is verified by simulation results. 
2. Abstract of Cited Paper (BIB002):  Facing new tasks, the conventional rigid design of robotic joints has come to its limits. Operating in unknown environments current robots are prone to failure when hitting unforeseen rigid obstacles. Moreover, safety constraints are a major aspect for robots interacting with humans. In order to operate safely, existing robotic systems in this field are slow and have a lack of performance. To circumvent these limitations, a new robot joint with a variable stiffness approach (VS-Joint) is presented. It combines a compact and highly integrated design with high performance actuation. The VS- Joint features a highly dynamic stiffness adjustment along with a mechanically programmable system behavior. This allows an easy adaption to a big variety of tasks. A benefit of the joint is its intrinsic robustness against impacts and hard contacts, which permits faster trajectories and handling. Thus, it provides excellent attributes for the use in shoulder and elbow joints of an anthropomorphic robot arm. 
3. Abstract of Cited Paper (BIB003):  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. 
4. Abstract of Cited Paper (BIB004):  It is widely recognised that compliant actuation is advantageous to robot control once high-performance, explosive tasks, such as throwing, hitting or jumping are considered. However, the benefit of intrinsic compliance comes with high control complexity. Specifically, coordinating the motion of the system through a compliant actuator and finding a task-specific impedance profile that leads to better performance is non-trivial. Here, we utilise optimal control to devise time-varying torque and stiffness profiles for highly dynamic movements in compliantly actuated robots. The proposed methodology is applied to a ballthrowing task where we demonstrate that: (i) the method is able to tailor impedance strategies to specific task objectives and system dynamics, (ii) the ability to vary stiffness can be exploited to achieve better performance in this class of movements, (iii) in systems with variable physical compliance, our methodology is able to exploit the energy storage capabilities of the actuators. We illustrate these in several numerical simulations, and in hardware experiments on a device with variable physical stiffness.

Literature Review:
The main feature of this subclass is the use of a nonlinear connector between the output link and the spring element hence only one linear spring is required. The stiffness adjustment is however still performed by changing the preload on this single spring. In the Maccepa  (see Fig. 6a ), the position of a lever arm is controlled to set the equilibrium position. The lever arm is connected to a spring loaded pivot point on the output link via a wire. When the link is moved out of the equilibrium position, the spring extends forcing the joint back to the equilibrium position. By pretensioning the spring different stiffness setting can be achieved. The MARIONET (Moment arm Adjustment for Remote Induction Of Net Effective Torque) BIB001 , replaces the pretensioned spring with a tensioning motor. In the Maccepa 2.0  (Fig. 6b ) the lever arm is replaced by a cam so the torque angle and stiffness-angle relation can be chosen depending on the application. The Maccepa can be built with offthe-shelf components and has a linear angle-torque characteristic, making it readily suitable for different applications BIB002 BIB005 . The VS-joint BIB003 (Fig. 6c) can be seen as the complement of the Maccepa. In this case a preload is responsible for the change of stiffness rather than a pretension. A roller is pushed by a spring to the lowest position in the cam disk which is the equilibrium position. When a torque is applied to the joint, there will be a joint deflection of the roller and pushing the roller upward causing a translational deflection of the springs. The spring pushes the roller downwards, which will generate a force in the direction of the lowest point of the cam disk. The advantage of this design is that it can easily be integrated into a robotic arm. The shape of the cam disk can be adjusted to obtain a progressive, regressive, or linear system behavior. Although one spring is enough, the VS joint uses three springs for symmetry. Both designs allow that two motors of different sizes can be used: a small one for the stiffness preset and a more powerful motor for the link position. In contrast to the mechanics of the VS-joint the new mechanics of the FSJ (see Fig. 6d ) is not equipped with a single cam system but with two opposing cam profiles. The two cam disks are coupled with each other by a single floating spring (from there also the name Floating Spring Joint (FSJ) BIB004 ), which means that the spring has no connection to the joint base or output shaft. It is designed to use the spring energy of a single mechanical spring as good as possible to generate the desired torque and reduce losses due to pretension in order to alter the joint stiffness. One cam disk is fixed to the link side and the second to the stiffness actuator. When it is rotated axially the stiffness is increased.