Abstract: An orthosis for powering the walking movement of a paralyzed wearer, comprises torso members to be firmly connected to the opposite sides of the wearer's torso. A pair of leg braces is connected to the torso members. Each leg brace comprises a thigh member pivotally mounted at a hip joint to one torso member on one side of the wearer. A shank member connected to the shank and foot of the wearer, is pivotally mounted at a knee joint to each thigh member. A four link drive mechanism which is powered by a rechargeable power unit mounted on the torso member, swings the thigh member forwardly and rearwardly about the hip joint. A knee lock keeps the knee joint locked during the rearward movement of the thigh member and during a portion of the forward movement of the thigh member. At an appropriate point during the forward movement of the thigh member, a cam operated crank unlocks the knee joint and swings the shank member rearwardly to simulate a natural forward step.