An ankle-foot orthosis for aiding or enhancing a user's foot and ankle movement, wherein the orthosis comprises at least one strut member, a calf shell, a foot shell, and a plurality of segments. Gaps formed between adjacent segments, an uppermost segment and the calf shell, and a lowermost segment and the foot shell have gap widths, wherein gaps at a higher location have larger gap widths than those at a lower location. Therefore, in dorsiflexion, the gaps close in series from bottom to top, gradually increasing the orthosis stiffness, creating a progressive dorsiflexion stop, and decreasing the magnitude of loads transferred into the user. In plantar flexion, the gaps similarly decrease in series from bottom to top, gradually increasing the orthosis stiffness, creating a progressive plantar flexion stop, and decreasing the magnitude of loads transferred into the user.