A cushion (16) for use with a mask (10) in delivering a flow of breathing gas to a user includes a first portion adapted to contact a surface of a user, a second portion adapted to be coupled to a mask shell (18), and a wall portion extending between the first portion and the second portion. The wall portion includes a chamber formed therein, the chamber having one of an electro-rheological fluid or a magneto-rheological fluid disposed therein. The cushion further includes a means (44, 50) for selectively varying the apparent viscosity of the fluid disposed within the chamber.