An estimated 623,000 individuals are living with a major lower leg amputation in the United States . Of these amputations, 78% were due to peripheral vascular disease (PVD) and 45% were due to PVD in individuals with type I or II diabetes . With diabetes and PVD incidence levels on the rise  and those in a depressed socio-economic situation more susceptible to develop type II diabetes , the demand for affordable, high quality ankle prostheses has never been higher. Prostheses currently available on the market include both passive and active devices, neither of which fully satisfies user requirements.
Passive prostheses, the more commonly prescribed style, are economically priced but lack the powered push-off observed in a natural ankle  due to the absence of an actuator. As a result, passive prostheses cause a multitude of quality of life detriments to the end user including asymmetrical gait (for unilateral amputees), slower self-selected walking speeds, higher metabolic cost per distance traveled and increased pain in the residual limb [5–6].
Conversely, active devices can nearly match the functionality and powered push-off of a natural ankle  but are cost prohibitive. Among active devices, one of the most successful models is the BiOM. Initially developed at MIT, the BiOM uses an actuator in series with a spring to achieve near natural ankle behavior. In 2013, two years after the product’s official launch, the device cost approximately $50,000 and had only sold about 1,000 units .
The limitations of currently available ankle prostheses motivates work on a new solution, the EaSY-Walk (Early Stance Y-deflection), a passive ankle device that mimics several key aspects of a natural ankle joint, especially nonlinear rotational stiffness and rotational work output (powered push-off) that increases with walking velocity while remaining relatively inexpensive.