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 &# 39 ; 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 .