An actuating device for orthosis includes a transmission that is operatively connected to a motor such that the motor provides power to the transmission. The transmission includes a first stage, a second stage, and a third stage. Each of these stages includes at least two sprockets and a drive belt tensioned between the two sprockets. The transmission also includes a first shaft and a second shaft. A sprocket of each of the first, second, and third stages of the transmission is attached to the first shaft. An actuating arm is operatively connected to the third stage of the transmission such that the power provided to the transmission by the motor causes the actuating arm to provide an output torque.