Rethinking symmetry: a new framework for powered knee prostheses

The National Institute of Child Health and Human Development awarded Rehabilitation Institute of Chicago, doing business as Shirley Ryan AbilityLab, $711,175 under the Child Health and Human Development Extramural Research program (CFDA 93.HDR) on June 1, 2026, to develop and test a low-profile powered knee prosthesis designed to reduce device inertia and improve gait function in individuals with transfemoral lower limb loss.

The project addresses limitations in current powered prosthetic knees by relocating the powertrain above the knee joint rather than below it, concentrating device mass more proximally to the trunk and reducing perceived inertia during ambulation. The research tests whether this mass redistribution improves step time symmetry and gait outcomes while minimizing the knee height asymmetry that would result from above-knee powertrain placement in conventional designs. Performance extends through May 31, 2030, at the recipient's Chicago, Illinois location.

Abstract

Millions of individuals around the world are affected by transfemoral lower limb loss, reducing their mobility and profoundly impacting their quality of life. Reduced mobility is a major contributor to adverse physical and mental health outcomes, including limited social participation, inability to return to work, and increased depression. Restoring the maximum possible mobility is, thus, a critical concern. Prosthetic devices are accepted treatment options for lower limb amputation, but even advanced prostheses like microprocessor-controlled knees (MPKS) fall short of restoring pre-amputation gait function. Powered prosthetic knees offer the potential to restore joint torque and replicate human limb functions. However, their increased mass and inertia often diminish their benefits and limit user adoption. While reducing the mass of a powered prosthesis has a practical limit, a more straightforward and tractable approach to overcome these limitations is to reduce the inertia of the device by placing the powertrain above the knee instead of below IT, concentrating the mass more proximal to the trunk. This redistributes, rather than reduces, the mass to improve gait. While studies of have shown that above-knee mass placement is superior to below-knee mass placement in passive prosthetic knees, the build height of current powered devices has prevented direct testing of this approach. Specifically, moving the powertrain above the knee would necessitate moving the prosthetic knee center substantially below the sound knee, introducing a large degree of asymmetry. Although a moderate amount of knee height asymmetry has been shown to actually improve step time symmetry when using a passive prosthesis, the disparity in knee height required for powertrain repositioning in current devices would likely have adverse impacts on gait. However, our new low-profile powered knee prototype has a small build height that makes powertrain repositioning possible. The proposed study seeks to gain fundamental biomechanical insights on how mass placement, knee height, and powered assistance interact to affect gait. In aim 1, we will utilize predictive simulation and biomechanical modeling technology to perform in silico testing of various knee height and mass placement configurations of a prosthesis on virtual subjects with differing physical characteristics, such as residual limb length. This allows our team to test a wide array of configurations in a rapid and efficient manner. In aim 2, we will perform human subjects testing to assess how knee height asymmetry and mass placement affect gait when using a passive prosthetic knee. Finally, in aim 3, we will assess how the placement of the powertrain, which accounts for most of the mass and influences knee joint height in our low-profile powered knee prototype, affects walking performance in lower limb prosthesis users. This work leverages our new device to systematically investigate how mass placement, knee height asymmetry, and powered assistance interact to affect mobility. Unlike prior studies, which examine these factors in isolation and only in passive knees, our work will generate foundational data needed to inform evidence-based design of next-generation powered prostheses to optimize outcomes.