Every year, thousands of above-knee amputations are carried out around the world due to circulatory problems, complications of diabetes, or trauma. The loss in mobility results in a degradation of the quality of life of the amputees. Therefore, there is a real need to develop an efficient lower limb prosthesis to restore mobility functions. The current trends in knee prostheses research are limited to either purely passive (mechanical), actively controlled using adaptive damping, or actively driven (powered) knee prostheses. Attempts to develop actively powered knee prostheses have sought to completely replace the muscle activity with an actuator. However, they do not take into consideration the fact that the human muscle works either passively or actively according to the phase or task. Hence, there is a lack of research in developing a prosthetic knee based on a hybrid approach (semi-active), which behaves closely to the natural functioning of a limb. This research involves the design and development of a novel electromechanical semi-active prosthetic knee that is back-driveable when operating passively under the influence of gravity. It also takes advantage of the dynamic coupling interaction between the amputee's stump and the prosthetic knee. The mechanism is driven in active mode via a de permanent magnet motor and a ball screw. In addition, this mechanism is back-driven by either the gravity forces or the dynamic coupling energy in the passive mode. This research, based on extensive investigations, provides the key design factors and parameters that affect the prosthetic knee performance. The assembly modes, the governing kinematic and dynamic relationships of the proposed knee mechanism are presented. The research also covers the study of the energy reflected from the hip to the prosthetic knee due to the dynamic coupling effect. This dynamic coupling is generated due to the change in the kinetic, and the potential energies of the segments and it is controlled by the kinematic parameters of the hip. A finite element model (FEA) was developed for the CAD model, and normative data for the ground reaction forces and the knee trajectory for level ground walking were applied and imported to the model. The FEA was used to VI check the von Mises stress in addition to the factor of safety for the main parts and components under the required loads and trajectories. A prototype of the proposed system was then manufactured. Experiments were carried out on the prosthetic knee for three prosthesis weights to characterise its performance in both open and closed loops for the active mode. Furthermore, the passive mode performance was tested for three prosthesis weights and braking (damping) scenarios due to braking torques. Furthermore, the dynamic coupling effect was tested, and the results showed that the energy generated by dynamic coupling helps to reduce the power consumption by the motor in the active mode if it works in the same direction. This leads to more efficient lower limb prostheses, and paves the way to significant improvements in prosthetic knees.