Enabling technologies for prosthetic legs

Enabling technologies for user-friendly, energy efficient, active lower limb prostheses

 

Motivation

People with above-knee amputation require an adequate replacement for their lost limb that allows for a natural gait and is accepted by them as a part of their body. A promising approach is provided by prostheses, that support gait actively using drives. For the investigation and assessment of enabling technologies for active lower limb prosthetics various gait scenarios are analyzed gradually in this project. Those describe the limitations of contemporary prosthetic solutions in everyday life, and thus mark the key challenges for the development of new concepts:

  • Walking on inclined planes
  • Walking backwards
  • Climbing and descending stair
  • Walking on level ground with frequent changes of velocity and direction (as in a lively pedestrian zone)

Based on these, appropriate existing concepts should be identified and improved and new concepts that are are both energy efficient and user friendly, will be developed. For increased usability, the integration of the prosthesis as part of the body is very important. Thus, this is a primary goal in developing concepts. Therefore, the development methodology elaborated in the project focuses on the human as the future user. To involve users during the whole development process, questionnaire and interview studies as well as experiments with healthy individuals, amputees and orthopedic specialists are performed.

Concept

For the development of enabling technologies for user-friendly, energy efficient and active prosthetic concepts, this project focuses on three key fields of research. For each of these, one of the three cooperating groups is in charge supported by the other ones:

Sketch of a simulation model of the lower extremeties
Sketch of a simulation model of the lower extremeties

Modeling and simulation of human locomotion provides an important basis for the understanding of functional processes and allows the specification of technical design parameters of prosthetic systems. Modeling the time-varying, dynamic behavior of the human musculoskeletal system leads to a high-dimensional mechanical multibody system consisting of sub-models for skeletal and soft tissue, muscles and tendons as redundant drives as well as optimization models to simulate activated muscle groups.

A promising, but numerically challenging approach to the study of biomechanical systems with redundant drives, is forward dynamics simulation. Here, the control of the drives is determined by the evaluation of an optimization problem with a suitable quality function. By combining a biomechanical simulation of the user with a technical simulation of the prosthetic system. Preliminary approaches can be investigated and optimized before manufacturing prototypes. In computer science, the research is focused on the following topics:

  • Setup of an effective and modular modeling and simulation environment for biomechanical multibody systems
  • Identification of user-specific biomechanical model parameters
  • Dynamics simulations of human locomotion
  • Tracking of three-dimensional human locomotion
Test rig for the evaluation of drive trains with variable torsion stiffness
Test rig for the evaluation of drive trains with variable torsion stiffness

Besides technical features of enabling technologies such as prosthetic mechanics, drive trains and controls soft factors as usability and ergonomic design are important in the development of user-friendly prostheses. Thus, an advanced development methodology incorporating users throughout the entire development cycle is elaborated in parallel to the investigations of prosthetic concepts and their control. With this, the psychological integration of the prosthesis and thus its usability and functionality can be improved.

The tools of this methodology, both questionnaires and simulations are used with the patent-pending concept prosthetic simulator “Prosthesis-user-in-the-loop”. Based on the ,ethodology and these tools, existing technical prostehtic concepts are examined and new approaches that focus on the requirements of users are prepared. Among the enabling technologies, the research emphasis lies on drive trains and their control. Currently, the following points are considered:

  • Development of drive train concepts with variable torsional stiffness (VTS)
  • Development of a design methodology for integrating users and transfering user requirements to technical ones
  • Setup of a robot for the assessment of the psychological factor body schema integration
  • Prosthesic design and initial implementation of the user-in-the-loop simulator conceptProsthesic design and initial implementation of the user-in-the-loop simulator concept
Experiements for the assessment of body scheme integration
Experiements for the assessment of body scheme integration

An amputation usually initiates a completly new stage of life, showing holistic impact (cognitive, motor, emotional, etc.) to the concerned person. Hence, the rehabilitation of amputees aims at dealing with new sensitive psycho-physiological states of patients and preparing them for their “new” everyday life as good as possible.

Since the nervous system of humans can be described to be plastic, early intervention or control of these processes is important in order to achieve the best possible integration of the new artificial limb into the body image and thus prevent symptoms such as phantom sensations and pain. Technology can here make a valuable contribution. Psychology aims at providing the following ones:

  • What psycho-physiological information can be used to develop engineering-specific instructions?
  • How can you integrate a prosthesis better into the body schema?

A major connection between the three participating groups is the joint development of the prosthetic concept simulator “Prosthesis-user-in-the-loop”. This is intended to provide the simulation of the biomechanical interaction between the prosthesis and the stump of the amputee and a visual simulation of the environment to make the subjects feel to really walk with a prosthesis that only exists as a simulation model.

The simulator concept thus enables the integration of users in all phases of the development process, since new technologies can be examined and evaluated without a prototype. In combination with the user-centered design methodology this shuld close the current gap between the collection of user requirements through questionnaires and interviews, and clinical testing of the final prosthetic components.

Funding

This project was funded by the Forum of Interdisciplinary Research FiF of Technische Universität Darmstadt.