Editorial Feature

The Mechanics of Robotic Prosthetic Legs

Discover how robotic prosthetic legs are revolutionizing mobility for amputees worldwide, overcoming challenges through advanced technology and ongoing research. Read on to explore the latest innovations and future developments in this transformative field.

In Robotics Development Laboratory: Engineers and Scientists Work on a Bionics Exoskeleton Prototype with Person Raising Bent Leg.

Image Credit: Gorodenkoff/Shutterstock.com

Introduction to Robotic Prosthetic Legs

In the US, there are over 2 million people living with limb loss,1,2 a number predicted to potentially double by 2050.2 Amputations can be devastating and can have a significant impact on a person’s psychological well-being, can affect their ability to work, and lead to disintegration within a community.1 

Robotic prosthetic legs are advanced artificial limbs that take the place of amputated legs. These devices allow people who have had leg amputations to regain motor capabilities, and in some cases, they can regain sensory feedback.3 Modern robotic prosthetic legs incorporate sensors, actuators, microprocessors, and power sources. Some devices are under development that aim to control the prosthetic legs via mind control.

Robotic prosthetic legs offer those who have experienced leg amputations access to improved mobility and a better quality of life. Currently available devices, however, face several limitations which limit their viability and utility in daily life.4 Advances in robotic prosthetic limb technology that are currently underway will hopefully make these devices more usable in the daily lives of those who have had amputations.

Components and Design Principles of Robotic Prosthetic Legs

There are several components that tend to be common across robotic prosthetic limb designs. The socket is the interface between the prosthetic leg and the terminal point of the natural limb. Sockets are often customized so that they fit comfortably to the person. A well-designed socket is vital for ensuring comfort and a secure fit.

Actuators are the components of the robotic prosthetic leg that generate movement. A range of actuators have been used in various robotic prosthetic leg designs, including electric, pneumatic, and hydraulic.

Sensors are an important component of robotic prosthetic legs; they detect parameters that are important to managing movement, such as pressure and angle. Sensors provide feedback to enable the leg to adjust its movement in response to the wearer’s actions.

Microprocessors are a vital component of the prosthetic leg, acting as a central processor, or “brain”, where the information collected by the sensors is processed and used to coordinate movement. Microprocessors are crucial to optimizing gait patterns.

Batteries or rechargeable power sources are used to supply energy to the robotic prosthetic leg. Finally, there is a user interface that allows the user to customize the settings of the prosthetic leg.

Biomechanics of Human Gait and Prosthetic Leg Functionality

Historically, scientists have struggled to design robotics that can mimic human walking. Standing up, walking, climbing up and down stairs, and navigating inclines and uneven terrain have been challenging to replicate in robots. Particularly, the transitions between these movements, sitting to standing, standing to walking.5 Robotic prosthetic legs have yet another challenge when it comes to completing these movements - they are not wired into the user’s central nervous system and, therefore, are not in tune with the rest of their body.

For over a decade, researchers at the University of Michigan, led by Robert Gregg, have been working on this problem.5 Their research has achieved several successes along the way in developing a robotic system that continuously represents all stages of the gait cycle and is capable of controlling knee and ankle positions. Previous systems had used separate controllers to navigate each stage of the gait cycle.

With a continuous model, the researchers found they had developed a method of replicating a natural gait. In 2018, the team began to extend the walking control model, developing it to navigate inclines, stairs, and transitions such as sit-to-stand, and stand-to-walk. While the model was successful in producing a more natural movement and gait, they identified the robotic prosthetic limb’s struggle with maneuvering a change in activity. When doing so, the movement became rigid and jarring.5

Advanced Technologies and Innovations in Robotic Prosthetic Legs

To overcome this issue, the team at the University of Michigan is now attempting to control joint position indirectly by copying biomechanical impedance instead. This method resets the joint back into position if it is disturbed. This allows the leg to accommodate a range of movement while offsetting any disturbance.

To understand the mechanical properties of the human gait so that the robotic prosthetic leg can be designed to mimic it, the team used an exoskeleton to collect data. By collecting measurements from the exoskeleton, the team can determine mechanical properties such as stiffness, inertia, and viscosity, thus helping to design a more advanced prosthetic limb that more accurately mimics the healthy human gait.

Challenges and Future Developments in Robotic Prosthetic Leg Mechanics

Before the optimal robotic prosthetic leg can be designed and widely available, there are a number of challenges that must first be addressed. The team at Michigan is currently working on one key challenge, that is, mimicking the natural human gait. Other challenges include optimizing energy efficiency and battery life, customizing the prosthetic for maximum comfort, increasing the affordability of the device, adapting the device to work effectively across uneven terrain, and providing sensory feedback such as tactile sensation.

An Introduction to the Biomechanics of Prosthetics

References and Further Reading

  1. Bumbaširević, M. et al. (2020) The current state of bionic limbs from the surgeons viewpoint, EFORT Open Reviews, 5(2), pp. 65–72. doi.org/10.1302/2058-5241.5.180038.
  2. (2021). The future of bionic limbs. Research Features. Available at: https://researchfeatures.com/future-bionic-limbs/
  3. Millar, A. (2022) Robotic-powered prostheses – state of play. Medical Device Network. Available at: https://www.medicaldevice-network.com/features/robotic-powered-prostheses-state-of-play/
  4. Tran, M. et al. (2022) A lightweight robotic leg prosthesis replicating the biomechanics of the knee, ankle, and toe joint. Science Robotics, 7(72). doi.org/10.1126/scirobotics.abo3996.
  5. (2023) University of Michigan project seeks to improve control of robotic prosthetic legs. News Medical. Available at: https://www.news-medical.net/news/20231221/University-of-Michigan-project-seeks-to-improve-control-of-robotic-prosthetic-legs.aspx

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Sarah Moore

Written by

Sarah Moore

After studying Psychology and then Neuroscience, Sarah quickly found her enjoyment for researching and writing research papers; turning to a passion to connect ideas with people through writing.

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