A prosthetic knee having a passive knee locking mechanism that uses the same four-bar mechanics found in a natural knee. The flexible four-bar mechanism guides the motion of the knee, aids in the return of the knee from full flexion to extension, and connects a femoral gear to a tibial gear. The gears have a circular radius and are connected using parallel links to keep the femur and tibia together when the knee is active. The knee stays locked throughout the stance phase. At toe off, no weight is applied on the prosthetic knee allowing the knee to flex. The flexible links are stretched, thereby increasing the stiffness in the springs, and at terminal swing phase, moments before heel strike, the flexible links in the four-bar mechanism snap back to the extended/locking position and lockout once the user applies his/her weight on the knee.
