Abstract: A generally butterfly-shaped inflatable/deflatable bag which is freely insertable in the upper front portion of a prosthesis designed to be worn by a below-the-knee amputee. The bag, when in place, and in use, extends around the front and lateral sides of the wearer's knee. Through a quick-disconnect hand-operable pump, which is coupled to the bag via a flexible tube, the user can adjust the degree of bag inflation to suit the particular circumstance.