Optimal mechanical components were designed using biomechanical modeling and inverse dynamics analysis. The prosthetic knee includes a linear spring and friction dampers that engage and disengage at optimal times in the gait cycle to provide able-bodied kinematics with minimal patient energy expenditure. The spring and damping system provide early stance flexion-extension and late stance and swing control respectively. The design has an automatic early stance lock for stability. The spring stiffness and damper friction may be adjusted for patient and device weight. A small user trial showed a smooth transition from stance to swing and easy usability of the prosthetic. This invention is a passive prosthetic knee design that is low cost with improvements over current devices that could allow large scale manufacturing for widespread use in developing countries.