A wind turbine includes a blade. The blade includes a blade body; an active member mounted to the blade body and configured to move between a retracted position and an extended position to change an aerodynamic property of the blade; and a bladder which is configured to be connected to a pneumatic or hydraulic system of the wind turbine to move the active member when the bladder is filled by a fluid supplied by the pneumatic or hydraulic system, or when the fluid is removed from the bladder by the pneumatic or hydraulic system. The wind turbine includes a retaining means configured to prevent the active member from moving towards the extended position.