For the more than one million Americans living with lower limb amputations—a number expected to double in the next 50 years—prosthetic technology is essential for mobility and independence.
However, most existing lower-limb prostheses feature a fixed-stiffness design, limiting adaptability to different activities. Walking on flat ground may feel stable for users, but navigating inclines, stairs, or uneven terrain can become a challenge. Without an adjustable response, users often experience discomfort, fatigue, and an unnatural gait.
Despite significant advancements in prosthetics, no commercial prosthetic ankle has successfully delivered real-time stiffness adjustments without sacrificing efficiency or performance. That’s why Dr. Max Shepherd, Assistant Professor of Physical Therapy, Human Movement, and Rehabilitation Sciences at Northeastern University, is leading the charge to change that.
His team is developing a Variable-Stiffness Ankle (VSA), an innovative prosthetic solution that dynamically adapts to users’ movements—enhancing comfort, stability, and overall quality of life.
A Breakthrough in Prosthetic Design
The VSA is a next-generation prosthetic device designed to improve mobility and comfort. Unlike traditional prosthetic feet, which rely on a fixed mechanical response, the VSA adapts to different movements and terrains.
Key features of the VSA include:
- Fiberglass Springs: The design retains the energy-efficient, spring-like behavior of conventional prosthetic feet
- Energy-Efficient Stiffness Adjustment: A small, battery-powered motor adjusts stiffness only during specific transitions, such as shifting from standing to walking, reducing unnecessary energy consumption
- Backlash-Free Mechanism: A novel linkage system eliminates mechanical backlash, a common challenge in adjustable prosthetic designs
Dr. Shepherd and his team, including mechanical engineering PhD student Kathryn Lee, have spent the last couple of years refining this design to ensure it meets the needs of active prosthetic users. The VSA represents a significant step forward in prosthetics that adapt to patients’ needs.
“An open question in the field of prosthetics is, ‘How do we best adapt a prosthesis to a patient depending on what they do?’” says Dr. Shepherd. “Our work tries to answer that.”
By fine-tuning the mechanical response of the prosthetic foot in real-time, the VSA offers a more natural and responsive experience, helping amputees move more freely across different environments.
Real-World Impact: Restoring Natural Movement for Lower-Limb Amputees
The ability to adjust stiffness dynamically has significant real-world benefits:
- Seamless Adaptation: Users can comfortably walk on flat surfaces, inclines, and stairs without needing to change their gait or exert additional effort
- Lightweight and Quiet: Unlike bulky, power-intensive robotic ankles, the VSA is designed to be lightweight, quiet, and energy-efficient—making it a practical solution for everyday use
- Immediate Quality-of-Life Boost: With improved comfort and mobility, thousands of active prosthetic users could experience a near-instant enhancement in their day-to-day lives
By providing a more natural walking experience, the VSA has the potential to significantly reduce joint strain, improve posture, and allow amputees to engage in a wider range of activities with greater ease.
Looking Ahead: Bridging Research and Industry
With the funding support from the Spark Fund, Dr. Shepherd’s team will be able to build and refine the first fully functional prototype, conduct fatigue testing, and prepare for take-home studies—allowing real users to test the device in daily life and provide valuable feedback.
“I’d love for this to get into the hands of patients,” emphasizes Dr. Shepherd. “With the Spark funding, we’ll be doing clinical testing to refine the design and do cycle testing.”
Dr. Shepherd’s team is focused on ensuring the VSA reaches the people who need it most. With a provisional patent in place on the core technology, the project is well-positioned to attract additional funding and licensing opportunities. The ultimate goal, when they are ready, is to partner with a major prosthetics manufacturer to scale production and bring the VSA to market.
By combining cutting-edge research with commercialization support from the CRI, Dr. Shepherd’s team is ensuring that this breakthrough innovation doesn’t remain in the lab—it reaches the real world, where it can change lives.
Key Takeaways: A Step Closer to Enhanced Mobility with The Spark Fund
For individuals with lower limb amputations, mobility challenges extend beyond simply walking. Adapting to different environments, minimizing strain on the body, and achieving a natural gait are essential for long-term health and comfort. The VSA is poised to address these needs, offering an adaptable, user-friendly solution that enhances movement across a variety of real-world conditions.
With the support of the Spark Fund, Dr. Shepherd’s team is now advancing toward clinical testing and, ultimately, commercialization.
“This project will help a specific population of people—people with lower limb amputations—to walk more comfortably and be more mobile across a wide range of activities.”
Written by Elizabeth Creason