A passive knee locking mechanism using the position of the shank and the weight of the user to lock. The mechanism has translational motion, which allows the shank to move linearly along the sagittal/vertical plane, which locks and unlocks the knee, and rotational motion for the shank to swing when unlocked. The knee block, which is disposed at the top of the shank, has a spur gear that locks with a matching gear rack on the top plate of the knee housing. The side plates for the knee housing have a linear path that makes for the translational motion. During heel strike, the stopper in the front plate of the knee housing positions the shank in a straight position. When the user applies their weight for initiating stance phase, the knee block traverses up the path and the spur gear meshes with the matching rack, locking the knee.