A wind turbine comprising a tower, a nacelle mounted to a top of the tower, a hub being rotatable mounted to the nacelle, and at least one blade mounted to the hub, the blade comprising: 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; 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; and a retaining means configured to prevent the active member from moving towards the extended position, wherein the retaining means comprises at least one of the following: a) a suction means configured to control a predetermined fluid pressure within the bladder which prevents the bladder from inflating due to a pressure build up due to centrifugal forces on the fluid inside the bladder, which is created when the hub rotates, wherein the bladder is mechanically fixed to the active member and directly or indirectly to the blade body, or wherein a flexible portion is provided which is configured to move or maintain the active member to or in the retracted position; b) the bladder is fixed both to a baseplate and/or to the blade body and the active member, wherein the active member is pressed and/or retained in the retracted position by a suction means configured to apply a negative pressure in the bladder; and c) a suction pressure reduction element being provided at the blade body adjacent a trailing edge of the active member or at the trailing edge itself, the suction pressure reduction element being configured to reduce a suction pressure induced by an air flow on the active member when the active member is in the retracted position. 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; 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; and a retaining means configured to prevent the active member from moving towards the extended position, wherein the retaining means comprises at least one of the following: a) a suction means configured to control a predetermined fluid pressure within the bladder which prevents the bladder from inflating due to a pressure build up due to centrifugal forces on the fluid inside the bladder, which is created when the hub rotates, wherein the bladder is mechanically fixed to the active member and directly or indirectly to the blade body, or wherein a flexible portion is provided which is configured to move or maintain the active member to or in the retracted position; b) the bladder is fixed both to a baseplate and/or to the blade body and the active member, wherein the active member is pressed and/or retained in the retracted position by a suction means configured to apply a negative pressure in the bladder; and c) a suction pressure reduction element being provided at the blade body adjacent a trailing edge of the active member or at the trailing edge itself, the suction pressure reduction element being configured to reduce a suction pressure induced by an air flow on the active member when the active member is in the retracted position. a) a suction means configured to control a predetermined fluid pressure within the bladder which prevents the bladder from inflating due to a pressure build up due to centrifugal forces on the fluid inside the bladder, which is created when the hub rotates, wherein the bladder is mechanically fixed to the active member and directly or indirectly to the blade body, or wherein a flexible portion is provided which is configured to move or maintain the active member to or in the retracted position; b) the bladder is fixed both to a baseplate and/or to the blade body and the active member, wherein the active member is pressed and/or retained in the retracted position by a suction means configured to apply a negative pressure in the bladder; and c) a suction pressure reduction element being provided at the blade body adjacent a trailing edge of the active member or at the trailing edge itself, the suction pressure reduction element being configured to reduce a suction pressure induced by an air flow on the active member when the active member is in the retracted position.