Christophe Lecomte

Prosthetic foot quasi-stiffness (QS) and ankle joint QS (AJQS) are factors impacting user biomechanics and comfort. While QS is typically determined by machine-based test methods, AJQS is usually derived from user testing. Despite the use of various methods to assess QS, the relationships and comparability remain unclear. In this study, measurements were performed using five commonly described test methods on three different prosthetic feet (SACH, K2, energy storage and return). A uniaxial test machine was used to record heel and forefoot QS, while a 2D motion tracking system allowed calculation of AJQS. The results revealed significant (p < 0.001) inter-method QS differences of up to 24% (equivalent to approximately two foot categories), as well as differences in AJQS of up to 19% for the heel and 55% for the forefoot. Anterior-posterior forces (APF) varied by as much as 161%, likely due to ambiguities in certain test methods. Overall, each method demonstrated a foot sample-specific non-linear increase in QS, AJQS, and APF. Notably, two of the five methods were unable to differentiate between foot stiffnesses of the prosthetic foot samples.

Finding a balance between adaptability for unexpected or non-standard movements and repeatability for cyclic standard movements is a known control challenge for active lower-limb prostheses. This study integrates a volitional electromyography (EMG)-driven musculoskeletal model controller with a finite-state machine impedance controller. During the stance phase, the hybrid controller is activated, and during the swing phase, it functions as a position controller. To provide a seamless transition to the swing phase of the gait, the controller first runs in hybrid mode until it validates that the foot is entirely off the ground. The Gastrocnemius and Tibialis Anterior muscles were modeled using a Hill-type muscle model to function around an ankle joint. The system uses input from ankle sensors and EMG data from antagonist muscle pairs to activate the muscle models. In addition, muscle parameters within the model are optimized using a surrogate-based optimization technique to improve the neuromuscular model controller’s performance and responsiveness. A transtibial amputation participant assessed the hybrid controller online for various activities, including ambulation on level ground, stairs, and ramps, using EMG signals from the residual limb as input. Combined with the impedance controller, the EMG control showed exceptional efficiency and repeatability, particularly for stair ascent, stair descent, and ramp descent. Moreover, according to user feedback, the user felt more involved and capable of using their muscles to control the motion, which improved their overall sensation of stability and control during these activities. The hybrid controller functioned more like an impedance controller during ramp ascents and walking on level terrain.

Objective: Running-specific prosthetic feet are curved cantilever springs, distinguished from ordinary prosthetic feet by their size, shape, and greater ability to store and return elastic potential energy. Finite element analyses show how modifications to prosthesis shape alter the multidimensional and nonlinear mechanics, but designers seeking to prescribe custom mechanics are limited by the complex constraints and high dimensionality of possible geometries. Methods: We introduce two simple formulations for describing foot mechanics, and use them with a custom spline-based shape optimization to generate new prosthetic foot shapes given desired endpoint mechanics. We then tackle a relevant example problem using this approach, designing and characterizing three running-specific prosthetic feet with similar vertical and angular deflections but varied horizontal deflections, with the expectation that knee extension moments would increase with anterior deflection of the toe. Finally, we compare the prostheses' endpoint mechanics and resulting biomechanics in an athlete with unilateral transfemoral amputation. Results: The shape optimization was able to derive shapes that substantially alter prosthesis mechanics along all dimensions of endpoint behavior, and benchtop testing validated the behavior of two new feet constructed from the optimization. The subject's knee moments increased with horizontal endpoint deflection as expected. Conclusion: We developed and validated a shape optimization tool using a simplified formulation of foot behavior to achieve desired running prosthesis mechanics. Significance: With this framework, researchers can begin to elucidate the link between prosthesis mechanics and athlete biomechanics and performance.

Energy-storing-and-returning prosthetic feet are frequently recommended for lower limb amputees. Functional performance and stiffness characteristics are evaluated by state-of-the-art biomechanical testing, while it is common practice for design engineers and researchers to use test machines to measure stiffness. The correlation between user-specific biomechanical measures and machine evaluation has not been thoroughly investigated, and mechanical testing for ramps is limited. In this paper, we propose a novel test method to assess prosthetic foot stiffness properties in the sagittal plane. First, biomechanical data were collected on five trans-tibial users using a variable stiffness prosthetic foot on a split-belt treadmill. Gait trials were performed on level ground and on an incline and a decline of 7.5°. The same prosthetic foot was tested on a roll-over test machine for the three terrains. The sagittal ankle moment and angle were compared for the two test methods. The dorsiflexion moment and angle were similar, while more variability was observed in the plantarflexion results. A good correlation was found for level-ground walking, while decline walking showed the largest differences in the results of the maximum angles. The roll-over test machine is a useful tool to speed up design iterations with a set design goal prior to user testing.

This paper presents a novel design of a prosthetic foot that features adaptable stiffness that changes according to the speed of ankle motion. The motivation is the natural graduation in stiffness of a biological ankle over a range of ambulation tasks. The device stiffness depends on rate of movement, ranging from a dissipating support at very slow walking speed, to efficient energy storage and return at normal walking speed. The objective here is to design a prosthetic foot that provides a compliant support for slow ambulation, without sacrificing the spring-like energy return beneficial in normal walking. The design is a modification of a commercially available foot and employs material properties to provide a change in stiffness. The velocity dependent properties of a non-Newtonian working fluid provide the rate adaptability. Material properties of components allow for a geometry shift that results in a coupling action, affecting the stiffness of the overall system. The function of an adaptive coupling was tested in linear motion. A prototype prosthetic foot was built, and the speed dependent stiffness measured mechanically. Furthermore, the prototype was tested by a user and body kinematics measured in gait analysis for varying walking speed, comparing the prototype to the original foot model (non-modified). Mechanical evaluation of stiffness shows increase in stiffness of about 60% over the test range and 10% increase between slow and normal walking speed in user testing.

The unique rheological properties of discontinuously shear thickening fluids (STF) have been employed in various engineering applications in recent years. In a commercial aspect, this has most notably been body armor and protective equipment, but also specialized smart structures in damping and force-coupling applications. The topic of this work is the application of STF in an articulating prosthetic foot for adaptable force response. Connected in series and parallel to a spring system, a STF based element can be used to affect the force transfer within the system, and thereby, influence the stiffness of the prosthetic foot dynamically over the gait cycle. The device described, prototyped and tested in this work, is a STF filled piston/cylinder design. The objective is a velocity dependent force response over the translational motion. Ranging from dampened, compliant deflection at low velocity, to a more efficient force transfer (coupling) for energy storage and return in the spring system, at higher speed. The rapid viscosity increase in the STF at a critical shear rate is used to approach a stepwise force response, thereby enabling adaptive response of the foot for different load rates. The adaptive response results in a greater range of motion with easier rollover for slow movement, for instance standing up from a seated position and adaptation to inclined surfaces, without sacrificing the energy return favorable for normal walking.

Energy storage and returning prosthetic feet do not provide a well-defined articulation point compared to the human ankle. Calculation of user relevant parameters, such as ankle power, requires such a joint center point when using traditional mechanical models. However, shortcomings of current calculation methods result in some errors. The aim of this case study was to compare conventional ankle joint calculations to a functional joint center (FJC) using data collected on a roll-over test machine and in a motion lab during dissimilar walking tasks. Three prosthetic feet were evaluated on a roll-over test machine. Then, two trans-tibial amputees were each fitted with the same three prosthetic feet matching their weight and activity category. Kinematic data were collected during walking on level ground, as well as up and down a slope. The FJC during the stance phase of gait was calculated for each test method and compared with outcomes using conventional methods. The location of the FJC was generally anterior and inferior to the estimated anatomical joint position. Importantly, the FJC location varied for the different prosthetic feet and was task dependent as per the three gait conditions. This was reflected in different ankle angles and moments of FJC calculations compared to conventional methods for level ground walking. Differences in the calculated FJC between conditions represented the variations in prosthetic foot deformation, and explained how this parameter is influenced by the prosthetic's stiffness. For level ground walking, calculated FJC location between human subject testing and machine evaluation were strongly correlated. Both stiffness and task dependent demands of the prosthetic foot should be considered during testing. The FJC of elastic ankles can serve as a parameter for characterization and differentiation between various prosthetic foot designs and be an important parameter for prosthetic foot designers to consider. As the position of the FJC is dependent on the design and task, it is a more informative measure of the prosthetic foot's response to the user's needs. Furthermore, prosthetists could use this metric in clinical practice to better appreciate amputee feedback and perception. FJC provides an alternative center during calculation of ankle power using standard methods.

Different tasks and conditions in gait call for different stiffness of prosthetic foot devices. The following work presents a case study on design modifications of a prosthetic foot, aimed at variable stiffness of the device. The objective is a proof-of-concept, achieved by simulating the modifications using finite element modeling. Design changes include the addition of a controlled damping element, connected both in parallel and series to a system of springs. The aim is to change the stiffness of the device under dynamic loading, by applying a high damping constant, approaching force coupling for the given boundary conditions. The dynamic modelling simulates mechanical test methods used to measure load response in full roll-over of prosthetic feet. Activation of the element during loading of the foot justifies the damped effect. As damping is in contrast to the main design objectives of energy return in prosthetic feet, it is considered important to quantify the dissipated energy in such an element. Our design case shows that the introduction of a damping element, with a high damping constant, can increase the overall rotational stiffness of the device by 50%. Given a large enough damping coefficient, the energy dissipation in the active element is about 20% of maximum strain energy.