Chengxiang Liu

Lower-limb prostheses: optimal human-in-the-loop co-adaptation

After finishing my Master’s degree in Mechatronic engineering, I worked in institutes developing medical robotics for more than two years. I started my HTRIC PhD project in March 2023. This project aims at developing a human-in-the-loop approach in which the user and the (control settings and mechanical parameters of the) prosthesis co-adapt to each other in an optimization process. The research I do for this project can contribute to  the life quality improvement of amputee patients.

This project is applied for by Prof. Dr. Raffaella Carloni (FSE) and Prof. Dr. Han Houdijk (UMCG).

The optimal transfemoral amputee-prosthesis co-adaptation (Co-adapt)


Let ‘s begin with an introduction of my PhD project: The optimal transfemoral amputee-prosthesis co-adaptation (Co-adapt). The project proposes to enhance the user-prothesis interaction from the perspective of co-adaptation, which takes into account the reciprocal nature of this process. The proposed approach combines the complexity and inherent unpredictability of the behavior of the user’s musculoskeletal and motor control systems.

This way, the prosthesis can incorporate the input of the amputee to find optimal control and mechanical settings for the correct human movement. Furthermore, this project aims to provide solutions to optimize the function of the powered prosthesis. With the realization of this objective, people with lower limb amputation will better accept the prosthesis, their quality of life will improve, their social contacts will be enlarged, and their (re-)integration in society and the labor market will be facilitated.

The project is based on a previous project, MyLeg (awarded by the European Union in Horizon 2020 in 2018). Hence, we have the complete mechanical structure of the prosthesis, and most of the electronic systems are ready. The rest of the work mainly focuses on control scheme development, clinical experiments, and validation.

In the past six months, I mainly worked on developing a leg prosthesis to track a predefined trajectory and tune critical parameters between the interface and the user. Further work will explore the stiffness and equilibrium effect on walking performance measured by metabolic energy and asymmetric and step length variability. After that, the user’s response to the prosthesis’ settings/parameters will be continuously assessed during gait, and iterative adaptations to these settings/parameters will be made to optimize defined cost functions related to gait performance.

Developing a prosthesis leg can sometimes be tedious and cause many setbacks. Sometimes, to fix a bug, you must spend several days going through thousands of lines of code and checking dozens of cable connections. More frustratingly, more bugs may pop out after one bug is solved. Nevertheless, when bugs are solved, the feelings of joy and reward are also with you, and the prosthesis leg works as expected. Below you can find a photo of me wearing the prosthesis leg with an adaptor.

In conclusion, I am excited about what I am doing and how the potential contribution may be made to improve the daily life quality of amputees. Please feel free to reach me if you are interested in more details about my journey in developing the prosthesis leg!

MyKnee powered variable stiffness prosthetic knee joint


Prepare to embark on a journey into the heart of innovation as we unveil the latest breakthroughs in prosthetic technology. Today, we shine a spotlight on the MyKnee powered variable stiffness prosthetic knee joint. our current endeavor lies in developing a control architecture for energy absorption and restoration during the stance period, a symphony of state machines and impedance controllers. The state machine guarantees to reliably discriminate between the stance phase, and the flexion/extension of the knee joint during the wing phase. The impedance controller tunes the variable stiffness joint and, namely, its equilibrium angular position and mechanical stiffness

To assess the impact of the impedance control and of the variation of the walking speed on the energy absorption and restoration of the prosthetic joint, we conducted a series experiments with a healthy participant wearing the MyKnee prosthetic knee joint, as shown in the figure below. We are preparing more tests with both the able-body subjects and people with a transfemoral amputation. These trials were not just tests of functionality; they were journeys of discovery, aiming to illuminate the intricate interplay between impedance control parameters and their effects on gait patterns and metabolic cost. Through the careful analysis of the experimental data, we uncovered a wealth of insights into the dynamic nature of energy storage and restoration during the stance period. We found that variations in the stiffness and equilibrium of the prosthetic knee joint, coupled with changes in walking speed, exerted a profound influence on energy absorption and restoration within the springs, as well as on knee angle variation during the stance period and metabolic cost during walking. Specifically, a greater equilibrium angle, lower stiffness, and higher walking speed correlated with enhanced energy exchange and enhanced knee flexion during the stance period.

We still don’t know what is the optimal setting of these parameters. However, these findings provide a solid foundation for further optimization and refinement, paving the way for the development of even more advanced prosthetic systems that empower individuals with a transfemoral amputation to move with confidence and grace.