Abstract: A mechanical prosthesis of the knee to be locked by a brake system, and formed by the thigh and a leg joined by a brake means pierced by a spindle. In the thigh a drum is integral with an inverted U-shaped support, the base of which is fixed to a rigid member on the thigh. The drum is surrounded by a band open at a single point. Above and below this opening it has two parallel and perforated flat radial lugs pierced by a tie-rod, which is bevelled on its upper portion. This upper portion is threadably engaged with a nut pinned to lock it. The tie-rod has an inverted U-shaped lower portion connected with a lever system having unequal arms. The pivoting point or fulcrum of the lever system is seated on a projection of the band.