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. As the thigh member reaches its maximum forward position, a return spring pulls the shank member forwardly until it is substantially parallel to the thigh member. At this point, the knee joint is again locked and the wearer's foot strikes the ground. This begins the rearward swinging of the thigh member which propells the wearer's torso forwardly to complete the step. The opposite knee brace is then operated in an analogous fashion to execute a second step.