I. Introduction
The dynamics of human walking are well-characterized, and the roles of individual lower-extremity joints during the gait cycle are understood in relation to their functional and clinical significance. Of particular importance is the role of the ankle joint in the sagittal plane of movement. The ankle performs net work against the environment with a typical peak rate of during preferred level-ground walking speeds [1]. This effort has been alternately hypothesized to redirect the body's center of mass [2], reduce energetic losses at the leading leg during touch down [3], and efficiently propel the trailing leg into its swing phase [4]. Correspondingly, loss of the joint from amputation commonly results in detrimental health outcomes such as increased metabolic cost of transport [5] [6] and patellofemoral osteoarthritis in the contralateral leg [7].