Northwestern University Feinberg School of Medicine

Northwestern University Prosthetics-Orthotics Center

Further Development of an Adaptable Prosthetic Ankle Unit

Co-Project Directors: Steven Gard, PhD, and Matthew Major, PhD 

Co-Investigators: Andrew Hansen, PhD; Stefania Fatone, PhD; Dudley Childress, PhD; Edward Grahn, BSME; Eric Nickel, MS; Brian Ruhe, MS; Rebecca Stine, MS; Dilip Thaker; Sam Kwak; José Luis Zavaleta, BS

Funded by: National Institute on Disability and Rehabilitation Research (NIDRR)

Status: Completed


The ability to ambulate on different terrains is a distinct advantage that legged locomotion has over wheeled transportation. However, some of the lower-limb adaptations that able-bodied persons automatically and subconsciously utilize on uneven terrains are not possible with most current lower-limb prostheses, making it difficult for prosthesis users to walk on irregular surfaces. It is not sufficient for the prosthetic foot and ankle to merely bend in order to accommodate the surface inclination, because doing so induces moments in the prosthesis that reduces stance phase stability for the user. Instead, the prosthetic alignment must be modified instantaneously and automatically to provide the user with optimal function during stance phase while walking on slopes. This project will focus on the development and evaluation of a prosthetic foot-ankle mechanism that can adapt (without manual intervention) to different levels of walking surface inclination. This project is a continuation of the project "Development of an Equilibrium-Point Prosthetic Ankle Joint" from the last RERC cycle, which developed 1st generation prototypes of the concept shown in the figure below.

Adaptable ankle on level walking surfaceAdaptable ankle on uphill walking surfaceAdaptable ankle on downhill walking surface
Figure 1: Animations of the design concept on level, uphill, and downhill slopes. The locking mechanism engages at foot flat and disengages at toe-off, setting the equilibrium-point of the triceps-surae spring on every step and at different ankle angles for different slopes.

Specific Objectives

Our objectives in this project include continued development and refinement of a 2nd generation prototype mechanism and evaluation of the device on level and non-level terrains. The prototype will be tested against commercially available prosthetic foot and ankle components. Performance goals include similar or superior walking of persons with transtibial amputations when using the prototype as compared to commercially-available prosthetic foot-ankle components on level surfaces, and superior function when using the prototype on inclined and declined surfaces. Performance will be assessed using walking speed, roll-over shapes, gait kinematics and kinetics, and subjective evaluation by the users. Gait measurements will be compared with walking data from able-bodied persons.


Mr. Eric Nickel worked on this project to satisfy requirements for his Master’s Degree at Northwestern University. The purpose of his project was to develop a prototypical prosthetic ankle unit that adapted to sloped surfaces and was sufficiently durable for short-term take-home trials. The prototype that Mr. Nickel developed switched between low and high rotational impedances by means of a wrap spring clutch mechanism. The clutch is held in a disengaged position when unloaded and deflection of a compressible pylon under axial load rotates a control collar and engages the clutch. The prototype was subjected to 100,000 cycles of mechanical endurance testing based on ISO 10328 standards to determine the suitability for two-week take-home testing. Three persons with unilateral transtibial amputations were recruited to test the prototype in the laboratory, providing subjective feedback through a survey and participating in a motion analysis study to confirm the performance of the slope adaptation function. Translation of the ankle moment-angle curves for all subjects along the ankle angle axis demonstrated a change of the ankle alignment when subjects walked with the adaptable ankle on surfaces of different slopes. The ankle moment-angle curves had a lower slope than the subjects’ usual prostheses, and some subjects had distinct flat regions in the moment-angle curves when using the adaptable ankle. The arbors of the clutch demonstrated significant wear when tested to 100,000 cycles based on ISO 10328 standards, yet the adaptable ankle continued to hold testing loads. The alignment change observed for sloped surfaces suggests the prototype was providing slope adaptation. The flat regions on the ankle moment-angle curves suggest the clutch may have been slipping. Refinement of the clutch engagement mechanism and continued development to reduce the weight and size of the prototype is needed prior to take-home testing.

Mr. Samuel Kwak, a student with the Illinois Math and Science Academy (Aurora, IL), performed a case-study analysis of a bilateral transtibial prosthesis user walking with the prototypical mechanism that Mr. Eric Nickel developed for this project. The subject was asked to walk at a self-selected pace across a level surface, and up and down a ramp with a 5° slope using his regular prostheses with non-adapting ankles while gait data were recorded. These data were then compared to data collected while walking with the NUPOC prosthetic ankle. The NUPOC prosthetic ankle provided greater range of motion than the non-adapting ankle in all walking conditions. Motion of the NUPOC prosthetic ankle more closely resembled that of able-bodied individuals, although neither prosthetic ankle provided entirely “normal” motion. Ankle angle-moment data demonstrated the adaptation of the NUPOC prosthetic ankle to inclines and declines by changing the ankle angle during stance while maintaining the ankle moment. This case study suggests that it is possible for persons with bilateral transtibial amputations to benefit from prosthetic ankles that mechanically adapt to changes in ground slope on each step, consistent with results previously reported for persons with unilateral transtibial amputation. More research and development is needed to confirm these possible benefits and to address the performance problems encountered with the current prototype.

Mr. José Luis Zavaleta, an engineering student from Iberoamericana University (Mexico City), began working with Steven Gard and Matthew Major and explored several different mechanical solutions to the problem before developing a physical prototype. The prototype, however, is for illustration purposes only and is not substantial enough for testing on human subjects. José Luis Zavaleta and the research he conducted while at NUPOC were featured in an issue of Domingo. The journal spotlights “The 42 Mexicans of the Future: Science and Education”. (Or copy and paste this address into your browser: José's work was also featured in multiple Mexican media outlets, including a Yahoo (Mexico) News article. (Or copy and paste this address into your browser:

Clinical Implications

The successful development of this product should allow lower limb prosthesis users the ability to more easily and safely traverse sloped surfaces. The adaptability of the ankle-foot system should result in more consistent loading of the residual limb and increased comfort of the prosthesis.

Patent Granted 

Hansen, A., Gard, S.A., Childress, D., Ruhe, B., Williams, R. (2013). Equilibrium-Point Prosthetic and Orthotic Ankle-Foot Devices. United States Patent 8,597,369.

Related Presentations and Publications

Nickel E. Development and Fatigue Testing of a Durable Prosthetic Ankle-Foot Prototype Capable of Adaptation to Sagittal Plane Surface Inclination. Masters Thesis, Evanston IL: Northwestern University; 2010.

Nickel E, Hansen A, Gard S, editors. Prosthetic ankle-foot system that adapts to sloped surfaces. 10th Design of Medical Devices Conference; 2011 April 12-14; Minneapolis, MN.

Kwak S, Fatone S. A Case Study Evaluation of a Bilateral Transtibial Prosthesis User Walking with the NUPOC Mechanically Adapting Prosthetic Ankle. Capabilities, 2011, 19(3)6-7.

Kwak S, Fatone S. Development and evaluation of a prosthetic ankle that mechanically adapts to sloped surfaces. IMSAlloquium Student Investigation Showcase, Illinois Mathematics and Science Academy; April 21, 2011; Aurora, IL.

Nickel E, Hansen A, Gard S. Prosthetic ankle-foot system that adapts to sloped surfaces. Journal of Medical Devices - Transactions of the ASME. 2012;6:011006-1-6.