## 1 Introduction  
Thousands of people lose their lower limbs due to circulatory and vascular problems, diabetes complications, cancer, or trauma every year. Among them, some people lost part or all of their lower limbs and the ability to walk need to be restored by lower limb prosthesis [1]. Transfemoral amputees often face more difficulty in the walking ability reconstruction since both the knee and ankle joints are lost. The prosthetic knee is the most important joint components of the lower limb prosthesis for transfemoral amputee [2]. The gait parameters are important for the safe design of prosthetics and orthotics [3, 4]. The transfemoral amputees often more relied on intact side due to the distrust of prosthetic limb. The daily ambulation requires the prosthetic knee to accommodate for force, speed, or terrain alterations. The limited and abnormal gait, decreased safety, and higher energy expenditure were observed of transfemoral amputees. The performance of the prosthetic knee decides the life quality of the transfemoral amputee. The prosthetic knee can be divided into three categories according to structures and knee torque realization methods. They are mechanical passive, variable damping, and dynamic active prosthesis [5]. The performance of conventional mechanical passive knee joints is limited due to the lack of sensing and driving. Transfemoral amputees often have asymmetrical gait and have to walk more slowly than able-bodied people. This leads to more metabolic energy being consumed [6]. The major commercial microprocessor-controlled prosthetic knees are variable damping. The typical damping types are pneumatic, hydraulic, and magnetorheological [7]. [PERSON] et al. conducted a comparative study of energy expenditure and walking speeds between able-bodied people and young users of pneumatic Intelligent Prosthesis (IP). This study showed that the pneumatic IP could improve the walking performance of the users. However, the pneumatic IP just had electronic control of the swing phase and the energy expenditure of users increased by 24% compared to able-bodied people [8]. [PERSON] et al. determined the differences in knee flexion between the Genium and C-Leg microprocessor-controlled hydraulic knees while walking on level ground and ramps [9]. The Genium and C-Leg are able to electronically control the hydraulic damping in both swing and stance phase. However, they still could not provide active torque during upstairs or stance extension. [PERSON] et al. conducted comparative biomechanical evaluation of Rheo Knee with magnetorheological damper and C-Leg [10]. Although the passive variable damping prosthetic knees have advantages in weight and clinical application, the common defect of variable damping prosthetic knees is difficult to extend the leg during active knee torque phase due to its lack of any active actuator. Their performance is difficult to improve during stair ascent or upslope because of the lack of active knee torque [11]. Many researchers have focused on the research of active prosthesis which can inject power to the knee joint. A powered knee-ankle prosthesis was developed by the team of Vanderbilt University. The motor and ball-screw were adapted to transmit the active torque [12; 13]. [PERSON] et al. presented the design of a powered knee-ankle prosthetic leg, which implements high-torque actuators with low-reduction transmissions [14]. [PERSON] et al. presented a powered knee and ankle prosthesis with indirect volitional swing control enables level-ground walking and crossing over obstacles [15]. However, the powered prosthesis is often heavy and consumes significant energy [16]. The weight of active knee-ankle prostheses is still over the clinically acceptable level due to the large motor and heavy transmission mechanism. Compared with the mechanical passive knee joint, the variable damping prosthetic knee has the advantages of providing different damping torque, enhancing the stability, adapting to environmental changes, and the walking speed adaption [17].