In this episode, Audrow Nash interviews Peter Adamczyk, Assistant Professor at the University of Wisconsin-Madison, on semi-active foot and ankle prostheses. 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.