Abstract: There is provided a knee-joint orthosis for persons whose legs are weak or unstable. The orthosis includes a first elongate member adapted to be secured along the lower leg below the knee, and a second elongate member adapted to be secured along the upper leg above the knee. The two elongate members are hinged together at a pivot location which, when the elongate members are secured to the leg, can be substantially aligned with the hypothetical pivot axis between the upper and lower leg. A tension member is secured between a first location which moves with the first member and a section location which moves with the second member, the force of the tension member acting along a line which is spaced from the pivot location and which is such that tension in the tension member increases as the leg is flexed. In this manner, the moment acting to extend the leg increases as the leg is flexed.