A respiratory muscle training device includes a chamber (1) containing a variable orifice valve assembly (3). An inlet (9) is provided at a first side of the valve assembly permitting air to be inhaled into the chamber, and an outlet (11) is provided at a second side of the valve assembly permitting air that has passed through the valve assembly to be inhaled by a user. A pressure sensor (7) determines a pressure differential across the valve assembly. Means is provided for determining the opening area of the valve assembly, and control means (15, 47, 49) is provided for varying the orifice of the valve assembly in dependence upon a pressure differential determined by the pressure sensor and upon an opening area of the valve assembly.