Ability of a Robotic Ankle Prosthesis to Augment Effective Foot-Ankle Stiffness Relative to Standalone Prosthetic Feet

Well-prescribed prosthetic feet are critical to restore gait for persons with limb loss. Criteria for prescription can be difficult to define due to differences in individuals and few measurements defining the mechanical properties of prostheses. The use of a robotic ankle in series with a prosthesis can provide biological levels of mechanical power during gait. In addition, active ankle prostheses can adapt to different use cases. To assess how this paradigm influences user assistance, we quantified the effective stiffness of standalone feet of varying clinical stiffness categories in comparison to a robotic ankle in series with a fixed category level prosthetic foot. We hypothesized that control of a powered ankle across its range of stiffness and damping parameters can expand the effective stiffness range offered by commercially available passive feet, and better explain the effective stiffness rendered during loading. Benchtop compression loading was completed on energy storage and return feet of manufacturer-defined stiffness category levels (4–9), as well as an integrated prosthetic foot (category 9) and robotic ankle system. Force-displacement data were used to characterize stiffness in toe- and heel-only loading, at low (--0-50% body weight) and high (--50-100
^%
) end levels. Control of the ankle captured well most of the profiles of standalone feet, as well as responses outside of these behaviors at low stiffness. Generally, there were stronger linear relationships between effective stiffness and category level of standalone feet (r=~0.9), and less so between the stiffness gain of the robotic ankle and effective stiffness (r=~0.8). The exception was for high-end toe-only loading of the standalone and robotic conditions (r=0.76 and 0.92, respectively).