A knee movement assistive device, comprising: - a waist garment attachable to a user waist and a calf garment attachable to a user calf, said calf garment being arranged along a longitudinal direction (X-X); - an actuator attached to the waist garment and comprising a driver, said driver being configured to switch between an active state, wherein the driver generates a rotational force, and an idle state, wherein the driver generates a null force, the actuator being configured to transform the rotational force of the driver in a tension force; - a transmission cable connecting the actuator to the calf garment and being configured to transmit the tension force, generated by transforming the rotational force of the driver, to the calf garment; said assistive device being configured in that the actuator comprises: - a first gear connected to the driver and being configured to rotate around a first rotation center, the rotational force generated by the driver making the first gear rotate around the first rotation center; - a second gear operatively engaging the first gear and being configured to rotate around a second rotation center, the transmission cable being connected to said second gear; - locking means operatively engaging the first and the second gears; said locking means being configured to switch between a locking configuration, wherein the second gear is prevented to rotate around the second rotation center, and a releasing configuration, wherein the rotation of the first gear makes the second gear rotate around the second rotation center so that the rotational force generated by the driver is transformed in the tension force and can be transmitted to the calf garment through the transmission cable, the locking means being configured to switch from the locking configuration to the releasing configuration due to the rotation of the first gear when the driver is in the active state, said locking means being configured to automatically switch from the releasing configuration to the locking configuration when the driver is switched to the idle state. - a waist garment attachable to a user waist and a calf garment attachable to a user calf, said calf garment being arranged along a longitudinal direction (X-X); - an actuator attached to the waist garment and comprising a driver, said driver being configured to switch between an active state, wherein the driver generates a rotational force, and an idle state, wherein the driver generates a null force, the actuator being configured to transform the rotational force of the driver in a tension force; - a transmission cable connecting the actuator to the calf garment and being configured to transmit the tension force, generated by transforming the rotational force of the driver, to the calf garment; - a first gear connected to the driver and being configured to rotate around a first rotation center, the rotational force generated by the driver making the first gear rotate around the first rotation center; - a second gear operatively engaging the first gear and being configured to rotate around a second rotation center, the transmission cable being connected to said second gear; - locking means operatively engaging the first and the second gears; said locking means being configured to switch between a locking configuration, wherein the second gear is prevented to rotate around the second rotation center, and a releasing configuration, wherein the rotation of the first gear makes the second gear rotate around the second rotation center so that the rotational force generated by the driver is transformed in the tension force and can be transmitted to the calf garment through the transmission cable, the locking means being configured to switch from the locking configuration to the releasing configuration due to the rotation of the first gear when the driver is in the active state, said locking means being configured to automatically switch from the releasing configuration to the locking configuration when the driver is switched to the idle state.