# Mechanical Structural Design and Actuation Technologies of Powered Knee Exoskeletons: A Review
## 1 Introduction  
Nowadays, the development of medical technologies and the improvements of living conditions bring convenience and improves quality of life, but the situation of population aging is not optimistic. In the aging process, the gradual decline of lower limb motor function and the mobility inconvenience caused by other diseases have largely affected the life of the elderly [1, 2]. In addition, the contradiction between the increasing number of patients with limb injuries caused by stroke and traffic accidents and the current shortage of medical resources (the gap of professional medical nursing personnel has reached millions) is increasingly prominent. Exoskeleton robots, which combine human intelligence with robot power, are an important technical path to eliminate these challenges and alleviate the current medical dilemma. The lower limb exoskeleton robot forms an intelligent device for man-machine coordination by parallel connection with the wearer and utilizes the human action guidance function and specific programs to assist the wearer in walking or rehabilitation training [3, 4, 5]. Therefore, as an intelligent man-machine integration system, the lower limb exoskeleton robot is significant in the fields of helping the disable and the elderly, medical rehabilitation, and industrial production. Various exoskeleton robots have been developed according to functional requirements in different fields, including developing different types of joint orthotics and exoskeletons to aid the movement of ankle, knee, and hip joint [6; 7; 8]. In particular, as the most vulnerable joint of the human body, the knee joint not only supports walking, running, and squatting to standing, but also absorbs the impact of human walking [9]. It is noteworthy that the dysfunction of a knee joint caused by diseases and injuries is the most common factor for gait abnormalities and seriously affects patients' living ability and mental health [10]. Therefore, many methods have been proposed to reduce the internal force of the knee joint, carry out rehabilitation exercises after the knee joint injury, and develop the corresponding exoskeleton for improving the exercise ability. In the past few decades, many universities and scientific institutions have developed active and semi-active knee joint exoskeletons to help users recover their walking ability and strengthen their muscles in a more natural way [11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. According to the intended use, the powered knee joint exoskeleton robots can be classified into exoskeletons for walking aids for the wear and therapeutic exoskeletons for rehabilitation. The first type of knee exoskeleton is mainly designed for patients with stroke, spinal cord injury, muscle weakness or other diseases that may lead to walking difficulties because they allow patients to complete some actions that they cannot implement independently [11; 12; 13; 15]. The second type of knee exoskeleton can help, resist or interfere with the user's movement to achieve therapeutic exercise and train the individual's muscle and nervous system to overcome the limitations of disability [14; 16; 17; 18]. From another point of view, some knee exoskeleton devices have the functions of both assisted walking and therapeutic rehabilitation, and can help medical treatment and improve the current physical ability of the wearer [19; 20]. However, the application scope of these devices is limited by several factors. In particular, wearing comfort, energy supply, actuator design, and adaptive compliance control of all exoskeletons need to be improved [21; 22; 23]. The review summarizes the design and development of mechanical structures and drive actuators related to the knee joint exoskeletons for rehabilitation and enhancement of human performance, and points out possible research directions to improve existing devices.