The difference is that active below-knee prostheses work to move the person's weight, emulating the calf muscle, while semi-active devices use small amounts of power to improve the performance of the prosthesis. Adamczyk discusses the motivation for semi-active devices and gives three examples: shiftable shapes, controllable keels, and alignable ankles. Peter Adamczyk directs the UW Biomechatronics, Assistive Devices, Gait Engineering and Rehabilitation Laboratory (UW BADGER Lab) which aims to enhance physical and functional recovery from orthopedic and neurological injury through advanced robotic devices. We study the mechanisms by which these injuries impair normal motion and coordination, and target interventions to encourage recovery and/or provide biomechanical assistance. Our work primarily addresses impairments affecting walking, running, and standing.