The present invention discloses an ankle foot orthosis device that is adjustable to a patient's drop foot. The device may include, inter alia, a foot-support that is sized to fit into a corresponding patient's footwear; an elongate member that is mechanically coupled to the foot-support; and a lower leg holder that is coupled to the elongate member. The lower portion of the elongate member may extend from the foot-support's heel portion upwards and rearwards towards a corresponding lower leg's calf and may further extend forwardly towards the corresponding lower leg's shin. Forward tilting of the lower leg in the gait cycle causes the elongate member to bend forward, which in turn causes the development of potential energy in the device that may be released during the gait cycle's toe-off and swing phase, thereby compensating for the energy lost in the drop foot.