## 1 Introduction
It improves gait symmetry and reduces musculoskeletal degeneration of transfemoral amputees to a degree [18]. The power prosthetic knee enables amputees to perform activities such as walking up stairs and standing from a sitting position. However, the active prosthetic knee is energy-intensive and requires a large actuator, which increases the weight of the prosthetic knee and reduces the battery life. The biological knee provides active and passive torque during walking [19]. Active or passive microprocessor-controlled prosthetic knees cannot simulate the hybrid work mode. The prosthetic knee with active-passive control is a solution to solve the problem. Thus, the prosthetic knee combines the advantages of powered and variable damping prosthetic knees--namely semi-active prosthetic knee, which has shown good potential for improving the walking performance and ensuring wearability. [PERSON] et al. proposed the design of a semi-active knee prosthesis. The power was added to a hydraulic knee by a pump driven by an electric motor. The active torque and passive damping were adjusted by a hydraulic pump and variable position valve. A compact, low power system while providing the benefits of an active device was achieved by the hybrid design [20]. [PERSON] et al. presented a design for a knee-ankle prosthesis which ultimately controlled two linear hydraulic actuators using a single pump. The hydraulic nature of the prosthetic allows it to function in a semi-active mode [21]. [PERSON] et al. characterized the dynamic coupling interaction between the thigh and an electrical semi-active above knee prosthesis. The prosthetic knee worked as a passive knee in part of the gait cycle phases and as an active knee prosthesis during other parts [22]. [PERSON] et al. presented a prosthetic knee operated in the semi-active and active modes. The semi-active mode is achieved from a flow mode magnetorheological (MR) damper, while the active mode is obtained from an electronically commutated (EC) motor [23]. These studies provided a helpful method to the design of semi-active prosthetic knee. Our team proposed several designs of hydraulic damping prosthetic knees and tested their performance in different walking conditions [24; 25; 26]. In this work, we proposed a prosthetic knee for transfemoral amputees operated in semi-active and variable damping modes. A four-bar structure was used to simulate the instantaneous center motion of the human knee joint. The active torque was provided by the ball-screw driven by the motor and the variable damping was provided by a hydraulic cylinder. In the passive mode, the adjustable hydraulic damping is used for the control of swing flexion and extension. The active torque is provided to realize the stance extension and ascending stairs in the active mode. By the hybrid drive mode, the knee joint can better simulate the torque changes of the knee joint and improve gait symmetry. The major contributions of this work are as follows:  
1. A novel prosthetic knee structure operated in semi-active mode was presented. 2. The semi-active prosthetic knee could provide similar torque and angle of the biological knee in the simulation. 3. The gait symmetry of the prosthetic knee was realized by simulation.