An ankle-foot orthosis comprising a leg member 12; a foot member 14; a frame 16 connecting the leg member to the foot member, wherein the frame comprises a first revolute joint 18 that rotates about a first axis 20 and a second revolute joint 22 that rotates about a second axis 24, wherein the first and second axes are non-parallel. The frame can further comprise a foot member segment 30 secured to the foot member 14 and extending to the second revolute joint 22. The orthosis can comprise a first force-torque sensor 36 located on the leg member, a second force-torque sensor 38 located on the foot member, and an encoder positioned on one of the revolute joints. The invention includes a method of measuring ankle-foot-related forces comprising positioning a subject's leg and foot in an ankle-foot orthosis; collecting data from the first and second force-torque sensors; and analyzing the collected data to determine the motion of the subject's ankle.