# Mechanical Structural Design and Actuation Technologies of Powered Knee Exoskeletons: A Review
## 2 Mechanical Structural Design of Knee Exoskeletons
### Simplified Structure Design  
Although the knee is a complex joint, most degree of freedom (DOF) of the knee joint can be ignored and only the flexion/extension movement in the sagittal plane is considered in biomechanical studies [26; 27]. Hence, to simplify the mechanical structure and promote the system control by simplifying the dynamic model, a single DOF motion has been designed for the knee exoskeleton based on the assumption that simplify bio-joints to engineering joints. In general, a simple single DOF hinge structure is mainly used to assist the flexion/extension motion of the knee joint in the sagittal plane of the human body.  
For example, as a portable mechanism for knee exercises, the ERF device utilized a simple revolute model driven by an electro-rheological fluid-based rotary actuator at the knee joint [28], as shown in Figure 2a. The energetically autonomous powered RoboKnee exoskeleton [29], a wearable device specific designed to facilitate running, was constructed with a motor-driven screw bars-based rotary actuator, which also adopted a simple hinge model for the knee joint, as shown in Figure 2b. On the basis of the pure rotary DOF modeling of human knee, EICOSI orthosis employed two cables with antagonistic actuation to provide sufficient torque for the knee joint [30], as described in Figure 2c. In other work, a compliant support exoskeleton with elastic energy storage called KAFO was developed in [31], and the feasibility of compliance assistance of the designed 1-DOF knee joint was experimentally verified based on quasi-passive compliant stance control, as shown in Figure 2d. A powered exoskeleton using Inertial Measurement Unit (IMU) sensors was developed to provide effective assistance during walking for impaired users with knee dysfunction in [32], as shown in Figure 2e, the active knee joint of the mechanical system was designed as a perfect revolute joint actuated by a high torque motor and a low-ratio gear transmission. Similarly, in [33] a new type of knee exoskeleton consisting of a thigh segment and a shank segment was designed and its unique DOF of rotation was adjusted by a hydraulic damper. In order to provide walking assistance and reducing the metabolic demand of locomotion, a parallel-elastic knee exoskeleton operated by a custom interference clutch with integrated planetary gear transmission was proposed in [34].  
Figure 1: Human knee joint and its movement: (**a**) anatomical diagram of the knee joint; (**b**) knee polycentric motion in the sagittal plane.  
Additionally, in some commercial exoskeletons, one purely rotary motion was also arranged at the knee joint, such as the Reo Ambulator used for enhancing strength and endurance during walking [35], the most well-known automated gait trainers Lokomat [36], and another popular gait rehabilitation robot LOPES [37]. It is worth noting that although the exoskeleton or orthosis mentioned above can help or adjust the human musculoskeletal system, it may cause discomfort and injury if it is not adaptive to the wearer.