Powered transfemoral prostheses can give above-knee amputees more flexibility than passive devices, for example allowing them to ascend and descend stairs more easily, or allowing a more natural and symmetrical gait pattern. To achieve the high torques required, most devices employ an electric motor with a ball-screw transmission, as is done in this work. The geometry of such a design determines how the peak torque is modulated as a function of joint angle. Therefore, it is important that this geometry is optimized to fulfill the requirements of the application. In this paper, we optimize this geometry to approximate a physiological peak torque versus joint angle versus joint velocity profile. Other powered knee prostheses commonly employ a single-axis joint. We investigate four different joint types: a single-axis joint, a biomimetic polycentric joint, a polycentric joint used in conventional passive prostheses, and an optimized polycentric joint. Our simulations suggest that employing an optimized polycentric joint can generate a uniform torque profile over the whole range of motion. An optimized geometry using a single-axis joint, however, can be used to obtain a peak torque versus angle profile that is similar to a physiological profile, and should, hence, be suitable for our application.