A system for performing a variable function such as a variable spring resistance comprises a plurality of selectively operable spring devices and movable actuator means for selecting/deselecting devices over a range of movement of the actuator means. The actuator means includes stepped drives for selecting/deselecting at least one of the spring devices at a predetermined ratio with respect to another of the devices whereby spring devices can be selected to operate individually or in combination by movement of the actuator means to provide multiple levels of spring resistance.