# Mechanical Structural Design and Actuation Technologies of Powered Knee Exoskeletons: A Review
## 3 Actuation Design
### Variable Stiffness Actuators  
In the applications of exoskeletons, variable stiffness actuators (VSAs) as another optional driving model allows the adaptation to environmental changes and task requirements through changing the output stiffness of the active joint [76]. Compared with other types of actuators, variable stiffness actuators can more efficiently imitate the nonlinear characteristics of the human joints to improve the performance of shock absorption and reduce energy consumption. Therefore, numerous knee exoskeletons with compliant joints exhibiting variable stiffness performances have recently been proposed.  
[PERSON] et al. [77] designed a knee exoskeleton integrating flexible drive with wearable mechanism to help users recover their walking ability. As shown in Figure 5(a), the variable-stiffness module equipped with two springs mounted on an adjustable knee brace allowed the wearer's knee joint to move freely in the terminal stance and the swing phase. As shown in Figure 5(b), [PERSON] et al. [78] proposed a novel compliant actuator that powers the knee joint by employing springs working both in series and in parallel to the joint. Note that the parallel elastic element was a quasi-passive mechanism composed of an EC motor, a spindle, and a spring, whereas the series elastic element was a variable stiffness actuator which remained engaged throughout the gait cycle. To effectively assist people in walking or perform rehabilitation training, [PERSON] et al. [79] designed a wearable exoskeleton actuated by tendon-sheath artificial muscles, as shown in Figure 5(c). The actuator completed the contraction and relaxation of the artificial muscle with the controllable clutch to adjust series and parallel springs, which was utilized to imitate the output characteristics of the muscle. For the purpose of helping patients with knee motor dysfunction recover their walking ability, [PERSON] et al. [80] designed a robotic knee exoskeleton with a variable stiffness actuator, as shown in Figure 5(d). As the elastic element of the actuator, a disc-type torsion spring was designed in the compliance module, thus enabling the mechanical device to reduce the output impedance of the system. Furthermore, [PERSON] et al. [76] proposed a novel intrinsic energy-efficient actuator to achieve the stiffness regulation by using the variable lever arm principle. To minimize the large contact force and safely interact with the user, [PERSON] et al. [81] conceived a new force-controlled compliant actuator with an embedded sensor and a locking mechanism. Regarding the driving compliance of the knee joint exoskeleton, [PERSON] et al. [82] analyzed the influence of the mechanical flexibility on the performance of the actuator under different ideal conditions in the experimental setup.  
In terms of safety and compliance of human-robot interaction, the proposed variable stiffness actuators can effectively regulate the stiffness of the knee exoskeleton according to the requirements under most conditions. This is possible due to their novel mechanical configurations make full use of natural dynamics to achieve the stiffness regulation and deliver biologically-relevant knee joint torque-angle output. However, to the best of our knowledge, the ability of VSA-based actuation units to realize driving characteristics found in biological systems usually comes at the cost of higher weight, complexity, and lack of robustness.