This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 2 • IEEE ROBOTICS & AUTOMATION MAGAZINE • MONTH 2020 1070-9932/20©2020IEEE. TRANSLATIONS AND CONTENT MINING ARE PERMITTED FOR ACADEMIC RESEARCH ONLY. PERSONAL USE IS ALSO PERMITTED, BUT REPUBLICATION/REDISTRIBUTION REQUIRES IEEE PERMISSION. SEE HTTP://WWW.IEEE.ORG/ PUBLICATIONS_STANDARDS/PUBLICATIONS/RIGHTS/INDEX.HTML FOR MORE INFORMATION. R obotic ankle-foot prostheses aim to improve the mobility of individuals with below-knee amputations by closely imitating the biomechanical function of the missing biological limb. To accomplish this goal, they must provide biomechanically accurate torque during ambulation. In addition, they must satisfy further requirements such as build height, range of motion (ROM), and weight. These requirements are critical for determining the potential number of users, range of activities that can be performed, and clinical outcomes. Previous studies have proposed addressing this challenge through the use of advanced actuation systems with series and parallel elastic actuators, clutchable leverages, and pneumatic artificial muscles. These advanced actuation systems have shown improved mechanical and electrical efficiency compared to conventional servo motors, making powered ankle prostheses possible. However, the improved efficiency comes at the expense of a tall build height, reduced ROM, and significant increase in weight, thus limiting the clinical viability of currently available powered prostheses. In this article, we show how a polycentric design can enable a lightweight powered ankle prosthesis to fit within the anatomical foot profile while providing physiological torque, energy, and ROM. Our simulations demonstrate that the moving instantaneous center of rotation (ICR) of the proposed polycentric mechanism has a twofold effect. It improves electrical efficiency by affecting the torque and speed required at the motor output and reduces the load on the main transmission system. Using the proposed powered polycentric design, we developed the first powered ankle- foot prosthesis that fits within the biological foot profile of the 50th percentile male adult (12-cm build height) and matches the weight of the 50th percentile female ankle/foot (1.3 kg in total weight, including battery and covers). Experi- ments with two below-knee amputee subjects show how the proposed powered polycentric prosthesis can provide physi- ological torque and speed as necessary to perform common ambulation tasks that require net-positive energy, such as walking and climbing stairs. XXXXXX By Lukas Gabert, Sarah Hood, Minh Tran, Marco Cempini, and Tommaso Lenzi Featuring a Powered Polycentric Design Digital Object Identifier 10.1109/MRA.2019.2955740 Date of current version: 20 January 2020