According to structural form, VSAs are classified into two categories, namely, antagonistic VSA and serial VSA. Antagonistic VSA [
20,
23–
26] is inspired by the skeletal system of living creatures with the antagonistic arrangement of muscles, usually possessing two motors. Each motor is connected to a nonlinear elastic element to drive the actuator output antagonistically. The position and stiffness of antagonistic VSA are simultaneously controlled by two same motors: Motors move in the opposite direction for stiffness adjustment and work in the same direction for changing actuator position. Despite the simple structure for easy engineering implementation, the main limitation of antagonistic VSA is the synchronous control of position and stiffness that leads to control complexity and high energy consumption. Compared with antagonistic VSA, the position and stiffness of serial VSA are controlled by two different motors: One principal motor changes position, and an auxiliary motor adjusts stiffness independently. The representative serial VSAs, such as MACCEPA 2.0 [
27], DLR VS-joint [
28], FSJ [
29], and S
3VSA [
30] adjust stiffness by changing the pretension or preload on the elastic element with the stiffness motor. The main limitation of these serial VSAs is rooted in the higher requirements on the driving capacity of the stiffness motor. Other serial VSAs adjust stiffness by changing the transmission ratio between the actuator output and the elastic element. Among these serial VSAs, the ones based on the lever principle with variable pivot [
31–
37] have a theoretical stiffness adjustment range from zero to infinity. Furthermore, as described in Refs. [
21,
38–
40], the output stiffness of these actuators can be regulated without energy injection or extraction from the elastic element, which means that all the energy from motors work on the load. Therefore, compared with the other counterparts, VSAs based on the lever principle with variable pivot dominate in terms of stiffness adjustment range and energy efficiency. Nevertheless, the existing VSAs based on the lever principle with variable pivot [
31,
32,
36,
37] mostly have the asymmetric structure of a single pivot and lever that will cause stress concentration, bring additional bending moment, and decrease mechanical reliability. Though the SVSA-II [
41] based on a symmetrical adjustable pivot lever mechanism is proposed, the stiffness adjustment mechanism is designed together with the VSA as a whole rather than being designed as a separate module, which makes the VSA difficult to maintain. A leaf-spring-based stiffness adjustment mechanism following the modular design is presented in Ref. [
42], but the stiffness can only be adjusted manually. In Ref. [
43], a new configuration synthesis method to design modular stiffness adjustment mechanism based on guide-bar mechanisms through the addition of linear springs and the use of length-adjustable links is proposed. However, the stiffness adjustment mechanism designed is still in the modeling stage, rather than the prototype.