Neurobionics Lab

Variable Stiffness Prosthetic Ankle


During various tasks, the human ankle is able to alter its stiffness in response to changing conditions. This ability is a key part of stable locomotion and efficient gait, but a majority of state-of-the-art commercial prosthetic ankle-feet can’t adjust their stiffness in real time. This lack of adjustment can lead to poorer walking performance in people with amputations. To address this need, we designed a novel ankle prosthesis called the Variable Stiffness Ankle Prosthesis, or VSPA. The VSPA can change its stiffness in real time thus generating a more biomimetic stiffness profile which is relevant for the specific task.


The VSPA foot is able to adjust its stiffness by using a small DC motor and lead screw to change the location of the support under the spring. In addition to altering the spring’s stiffness, the VSPA utilizes a cam profile at the ankle joint which compresses the spring during stance. By altering the shape of the cam profile, we can create a highly customizable torque-angle relationship (below). These variety of functions are contained in a small, anthropomorphic form factor which is vital for widespread, commercial viability of any prosthetic foot. We characterized stiffness profiles of the ankle-foot prosthesis at various support conditions using a custom dynamometer. These results showed that we can render a wide range of stiffnesses appropriate for multiple mobility tasks (below).

(A) Annotated rendering of the VSPA foot. (B) Moving the slider alters the relationship between the torque produced by the ankle at a specific angle. (C) As the slider moves towards the cam, the stiffness of the foot increases as indicated by the orange arrows. Figure from Shepherd, M. K., et al. (2018). Amputee perception of prosthetic ankle stiffness during locomotion. Journal of NeuroEngineering and Rehabilitation.

The VSPA foot is also a valuable research tool that allows us to explore the effects of foot stiffness without having to continuously change and re-align prosthetic feet of different stiffnesses. Current efforts for this project include studying the energetic and biomechanical effects of stiffness modulation during mobility tasks like walking, stair ascent/descent, and incline/decline walking. Beyond biomechanical factors we are also exploring how users learn and perceive the changing stiffness profiles. We are conducting research into user-preferred stiffness and just noticeable difference in stiffness changes for various tasks and how it may differ from prosthetist-preferred stiffness. Our work in these areas in ongoing, so check out our recent publications to learn more.

Contributors: Hannah Frame, Max Shepherd, Tyler Clites, Elliott Rouse


Clites, T. R., Shepherd, M. K., Ingraham, K. A., Wontorcik, L., & Rouse, E. J. (2021). Understanding patient preference in prosthetic ankle stiffness.  Journal of NeuroEngineering and Rehabilitation18(1), 1-16.

Shepherd, M. K., and Rouse, E. J. (2020). Comparing preference of ankle–foot stiffness in below-knee amputees and prosthetists. Scientific Reports 10(1), 1-8.

Shepherd, M. K., Azocar, A. F., Major M. J., & Rouse E. J. (2018). Amputee perception of prosthetic ankle stiffness during locomotion. Journal of NeuroEngineering and Rehabilitation15(1), 99.

Shepherd, M. K., Azocar, A. F., Major, M. J., & Rouse, E. J. (2018, August). The Difference Threshold of Ankle-Foot Prosthesis Stiffness for Persons with Transtibial Amputation. In 2018 7th IEEE International Conference on Biomedical Robotics and Biomechatronics (BioRob) (pp. 100-104). IEEE.

Shepherd, M.K., & Rouse, E.J. (2017). The VSPA Foot: A Quasi-Passive Ankle-Foot Prosthesis With Continuously Variable Stiffness. IEEE Transactions on Neural Systems and Rehabilitation Engineering, 25(12), 2375-2386,

Shepherd, M. K., & Rouse, E. J. (2017, May). Design of a quasi-passive ankle-foot prosthesis with biomimetic, variable stiffness. In 2017 IEEE International Conference on Robotics and Automation (ICRA) (pp. 6672-6678). IEEE.