INTRODUCTION:With an appropriate prescription, the use of a lower limb prosthesis can help mitigate mobility limitations and increased risk of falling for people with lower limb amputation. Prosthetic feet cannot replicate all the functions of a biological foot-ankle. Different feet have different designs and properties, and therefore there are functional trade-offs. There is insufficient evidence as to the effects these different prosthetic foot properties have on users' stability and balance, which would be helpful to guide clinical prosthesis prescription. Prosthesis users also rarely have opportunities to try walking with different prosthetic feet to give experiential input during the prescription process. Therefore, this study aims to determine 1) the effects of prosthetic foot stiffness on stability in lower limb prosthesis users while walking on varying terrains, and 2) whether a brief 'test-drive' strategy for selecting prosthetic feet can be used to predict longer term stability, balance confidence, and foot preference outcomes in lower limb prosthesis users. METHODS AND MATERIALS:In this multisite, participant-masked, randomized cross-over study, participants with unilateral, transtibial amputation will walk on different treadmill conditions (flat, incline, cross-slopes, uneven ground) with a variety of commercially-available prosthetic feet ('actual') and corresponding 'emulated' prosthetic feet in the laboratory. Participants will also wear the actual prosthetic feet at home and in the community for one week at a time. After each community trial, participants will return to the laboratory to complete walking trials on different terrains and at a range of speeds while we collect kinematic data. We will assess the effect of prosthetic foot stiffness on biomechanical and self-reported measures of stability. We will also assess how well brief 'test-drive' trials of walking with different prosthetic feet can predict longer-term self-reported measures of stability, balance confidence and preference. TRIAL REGISTRATION:This study was prospectively registered at www.clinicaltrials.gov (Clinical Trials Study ID: NCT05473065). Study start date: March 1, 2024.

BACKGROUND:Selection of a foot is an important aspect of prosthetic prescription and vital to maximizing mobility and functional goals after lower limb amputation. Development of a standardized approach to soliciting user experiential preferences is needed to improve evaluation and comparison of prosthetic feet.OBJECTIVE:To develop rating scales to assess prosthetic foot preference and to evaluate use of these scales in people with transtibial amputation after trialing different prosthetic feet.DESIGN:Participant-blinded, repeated measures crossover trial.SETTING:Veterans Affairs and Department of Defense Medical Centers, laboratory setting.PARTICIPANTS:Seventy-two male prosthesis users with unilateral transtibial amputation started, and 68 participants completed this study.INTERVENTIONS:Participants trialed three mobility-level appropriate commercial prosthetic feet briefly in the laboratory.MAIN OUTCOME MEASURES:"Activity-specific" rating scales were developed to assess participants' ability with a given prosthetic foot to perform typical mobility activities (eg, walking at different speeds, on inclines, and stairs) and "global" scales to rate overall perceived energy required to walk, satisfaction, and willingness to regularly use the prosthetic foot. Foot preference was determined by comparing the rating scale scores, after laboratory testing.RESULTS:The greatest within-participant differences in scores among feet were observed in the "incline" activity, where 57% ± 6% of participants reported 2+ point differences. There was a significant association (p < .05) between all "activity-specific" rating scores (except standing) and each "global" rating score.CONCLUSIONS:The standardized rating scales developed in this study could be used to assess prosthetic foot preference in both the research and clinical settings to guide prosthetic foot prescription for people with lower limb amputation capable of a range of mobility levels.

Prosthetic foot stiffness plays a key role in the functional mobility of lower limb prosthesis users. However, limited objective data exists to guide selection of the optimal prosthetic foot stiffness category for a given individual. Clinicians often must rely solely on manufacturer recommendations, which are typically based on the intended user’s weight and general activity level. Availability of comparable forefoot and heel stiffness data would allow for a better understanding of differences between different commercial prosthetic feet, and also between feet of different stiffness categories and foot sizes. Therefore, this study compared forefoot and heel linear stiffness properties across manufacturer-designated stiffness categories and foot sizes. Mechanical testing was completed for five types of commercial prosthetic feet across a range of stiffness categories and three foot-sizes. Data were collected for 56 prosthetic feet, in total. Testing at two discrete angles was conducted to isolate loading of the heel and forefoot components, respectively. Each prosthetic foot was loaded for six cycles while force and displacement data were collected. Forefoot and heel measured stiffness were both significantly associated with stiffness category ( p = .001). There was no evidence that the relationships between stiffness category and measured stiffness differed by foot size (stiffness category by size interaction p = .80). However, there were inconsistencies between the expected and measured stiffness changes across stiffness categories (i.e., magnitude of stiffness changes varied substantially between consecutive stiffness categories of the same feet). While statistical results support that, on average, measured stiffness is positively correlated with stiffness category, force-displacement data suggest substantial variation in measured stiffness across consecutive categories. Published objective mechanical property data for commercial prosthetic feet would likely therefore be helpful to clinicians during prescription.

Prosthetic foot selection for individuals with lower limb amputation relies primarily on clinician judgment. The prosthesis user rarely has an opportunity to provide experiential input into the decision by trying different feet. A prosthetic foot emulator (PFE) is a robotic prosthetic foot that could facilitate prosthesis users' ability to trial feet with different mechanical characteristics. Here, we introduce a procedure by which a robotic PFE is configured to emulate the sagittal plane effective ankle stiffness of a range of commercial prosthetic forefeet. Mechanical testing was used to collect data on five types of commercial prosthetic feet across a range of foot sizes and intended user body weights. Emulated forefoot profiles were parameterized using Bezier curve fitting on ankle torque-angle data. Mechanical testing was repeated with the PFE, across a subset of emulated foot conditions, to assess the accuracy of the emulation. Linear mixed-effects regression and Bland-Altman Limits of Agreement analyses were used to compare emulated and commercial ankle torque-angle data. Effective ankle stiffness of the emulated feet was significantly associated with the corresponding commercial prosthetic feet (p <0.001). On average, the emulated forefeet reproduced the effective ankle stiffness of corresponding commercial feet within 1%. Furthermore, differences were independent of prosthetic foot type, foot size, or user body weight. These findings suggest that commercial prosthetic foot properties can be effectively mimicked by a PFE, which is the important first step toward enabling prosthesis users to quickly trial different feet using a PFE as part of prosthetic foot prescription.

Background: Mechanical testing is the principal method used to quantify properties of commercial prosthetic feet in a controlled and standardized manner. To test feet in a mechanical testing machine without overconstraining the system, tangential shear forces must be minimized. However, there is scant published information comparing techniques for reducing shear forces during mechanical testing. Furthermore, there are no data on variability in linear stiffness across testing sessions. Objectives: To compare techniques for reducing shear forces during mechanical testing of prosthetic feet and to evaluate variation in linear stiffness across testing sessions. Study design: Repeated measures. Technique: Force–displacement data were collected at two pylon progression angles, one for the forefoot and one for the heel, and compared across three conditions: roller plate (RoPl), low-friction interface on the shoe (SB), and no method for reducing shear forces (NoSB). Data were collected for a range of commercial prosthetic foot models and sizes. Select data were collected over multiple days to assess variation over test sessions. Results: Differences in stiffness between RoPl and SB test conditions ranged from −0.9% to +2.6% across foot models. By contrast, differences between RoPl and no method for reducing shear conditions ranged from −2.9% to +14.6%. Differences in linear stiffness between test sessions ranged from −2.2% to +3.6%. Conclusions: Methods for reducing shear force in this study demonstrated roughly equivalent effects. Thus, a low-friction interface may be used as a less expensive and less complex method for reducing shear force in prosthetic foot testing. In addition, mechanical testing results were relatively consistent across multiple test sessions, lending confidence to test consistency.

Background: Despite the effects of prosthetic foot mechanical properties on gait of people with lower limb amputation, scant forefoot and heel stiffness data exist to help guide prosthetic foot prescription. Objective: To measure forefoot and heel linear stiffness properties across commonly prescribed commercial prosthetic foot models and to describe variations in stiffness across feet targeted for users with different body weights and foot sizes. Study design: Mechanical testing of five types of commercial prosthetic feet across nine user body weight and foot size combinations. Methods: Linear forefoot and heel stiffness (force vs. displacement) data were collected for 41 prosthetic feet. Quasistatic testing was conducted at −15 and +20 degrees to isolate loading of the heel and forefoot, respectively. Results: Overall, there was a significant relationship between user body weight and both forefoot and heel stiffness, when adjusted for foot size and type (P < 0.001). However, there were a substantial number of inconsistencies across foot type within example user body weight and foot sizes combination. Furthermore, the relative order of forefoot stiffness across foot type differed from the relative order of heel stiffness across foot type. Conclusions: The inconsistencies and differences in relative order of forefoot and heel stiffness across commercial foot type suggest the importance of publishing objective stiffness and other mechanical properties of prosthetic feet. These data can aid clinicians in better matching mechanical properties of prosthetic feet with the functional goals and abilities of prosthesis users.

In individuals with transtibial limb loss, a contributing factor to mobility-related challenges is the disruption of biological calf muscle function due to transection of the soleus and gastrocnemius. Powered prosthetic ankles can restore primary function of the mono-articular soleus muscle, which contributes to ankle plantarflexion. In effect, a powered ankle acts like an artificial soleus (AS). However, the biarticular gastrocnemius connection that simultaneously contributes to ankle plantarflexion and knee flexion torques remains missing, and there are currently no commercially available prosthetic ankles that incorporate an artificial gastrocnemius (AG). The goal of this work is to describe the design of a novel emulator capable of independently controlling artificial soleus and gastrocnemius behaviors for transtibial prosthesis users during walking. To evaluate the emulator's efficacy in controlling the artificial gastrocnemius behaviors, a case series walking study was conducted with four transtibial prosthesis users. Data from this case series showed that the emulator exhibits low resistance to the user's leg swing, low hysteresis during passive spring emulation, and accurate force tracking for a range of artificial soleus and gastrocnemius behaviors. The emulator presented in this paper is versatile and can facilitate experiments studying the effects of various artificial soleus and gastrocnemius dynamics on gait or other movement tasks. Using this system, it is possible to address existing knowledge gaps and explore a wide range of artificial soleus and gastrocnemius behaviors during gait and potentially other activities of daily living.

ABSTRACT Introduction The design and selection of lower-limb prosthetic devices is currently hampered by a shortage of evidence to drive the choice of prosthetic foot parameters. We propose a new approach wherein prostheses could be designed, specified, and provided based on individualized measurements of the benefits provided by candidate feet. In this manuscript, we present a pilot test of this evidence-based and personalized process. Methods We previously developed a “prosthetic foot emulator,” a wearable robotic system that provides users with the physical sensation of trying on different prosthetic feet before definitive fitting. Here we detail preliminary demonstrations of two possible approaches to personalizing foot design: 1) an emulation and test-drive strategy of representative commercial foot models, and 2) a prosthetist-driven tuning procedure to optimize foot parameters. Results The first experiment demonstrated large and sometimes surprising differences in optimal prosthetic foot parameters across a variety of subjects, walking conditions, and outcome measures. The second experiment demonstrated a quick and effective simple manual tuning procedure for identifying preferred prosthetic foot parameters. Conclusions Emulator-based approaches could improve individualization of prosthetic foot prescription. The present results motivate future clinical studies of the validity, efficacy, and economics of the approach across larger and more diverse subject populations. Clinical Relevance Today, emulator technology is being used to accelerate research and development of novel prosthetic and orthotic devices. In the future, after further refinement and validation, this technology could benefit clinical practice by providing a means for rapid test-driving and optimal selection of clinically available prosthetic feet.

Amputees using passive ankle-foot prostheses tend to expend more metabolic energy during walking than non-amputees, and reducing this cost has been a central motivation for the development of active ankle-foot prostheses. Increased push-off work at the end of stance has been proposed as a way to reduce metabolic energy use, but the effects of push-off work have not been tested in isolation. In this experiment, participants with unilateral transtibial amputation (N=6) walked on a treadmill at a constant speed while wearing a powered prosthesis emulator. The prosthesis delivered different levels of ankle push-off work across conditions, ranging from the value for passive prostheses to double the value for non-amputee walking, while all other prosthesis mechanics were held constant. Participants completed six acclimation sessions prior to a data collection in which metabolic rate, kinematics, kinetics, muscle activity and user satisfaction were recorded. Metabolic rate was not affected by net prosthesis work rate (p=0.5; R2=0.007). Metabolic rate, gait mechanics and muscle activity varied widely across participants, but no participant had lower metabolic rate with higher levels of push-off work. User satisfaction was affected by push-off work (p=0.002), with participants preferring values of ankle push-off slightly higher than in non-amputee walking, possibly indicating other benefits. Restoring or augmenting ankle push-off work is not sufficient to improve energy economy for lower-limb amputees. Additional necessary conditions might include alternate timing or control, individualized tuning, or particular subject characteristics.

Recently-developed robotic prostheses have demonstrated that it is possible to design a prosthesis which makes it easier for unilateral transtibial amputees to walk. Unfortunately, it is unclear which design features are most important and which users will benefit most from these advanced technologies that increase prosthesis cost by an order of magnitude. I developed a novel experimental approach to resolving these design and prescription uncertainties. Candidate prosthetic feet are emulated during treadmill walking experiments using a high-performance off-board actuated and controlled lightweight robotic prosthesis. Prosthesis behavior is systematically varied while users’ walking economy, performance, and satisfaction are measured. This process thereby determines unambiguous relationships between device behavior and outcomes of interest. In Chapter 1 of this thesis I motivate the approach. In Chapter 2 I detail the design and evaluation of the novel prosthesis emulator system. Then, in Chapter 3, I detail an experiment in which I test the simple walking model prediction that increasing prosthetic ankle push-off work will lessen leading limb collision, thereby reducing users’ metabolic energy consumption. I demonstrate that increased push-off instead seems to primarily reduce energy consumption by aiding in the acceleration of the swing leg. In Chapter 4, I emulate the behavior of off-the-shelf prostheses, giving patients the opportunity to test-drive candidate devices prior to purchase, and enabling prescriptions to be justified by predictive experimental data. Finally, in Chapter 5, I demonstrate a human-in-the-loop prosthesis design optimization scheme that enables the manufacture of user-customized prostheses, which could ultimately supersede the need for prosthesis selection.

Lower-limb prostheses are designed based on observations of how users on average respond to different design features. Prostheses are then marketed on the basis that certain features are appropriate for certain types of individuals, with few options for user customization. This process is unlikely to provide individual users with devices that best suit their needs since it is unclear how to best categorize users and which design features are most important [1, 2]. This process is also slow to accommodate disruptive technologies since it requires time to develop a body of observations about new devices that practitioners are hesitant to prescribe. Prosthesis designs could instead be optimized for individual users, producing customized designs that are likely to be preferable to off-theshelf designs. Using a traditional design approach, this would require costly rapid prototyping and evaluation of candidate designs. Instead, we demonstrate the use of a tethered robotic ankle-foot prosthesis [3] as a tool for rapid exploration of candidate designs. We devised a strategy for systematically exploring a space of possible device behaviors and identifying which are user preferred. The resultant optimized designs could then be sent to prosthesis manufacturers for physical implementation using traditional processes.

Robotic ankle-foot prostheses that provide net positive push-off work can reduce the metabolic rate of walking for individuals with amputation, but benefits might be sensitive to push-off timing. Simple walking models suggest that preemptive push-off reduces center-of-mass work, possibly reducing metabolic rate. Studies with bilateral exoskeletons have found that push-off beginning before leading leg contact minimizes metabolic rate, but timing was not varied independently from push-off work, and the effects of push-off timing on biomechanics were not measured. Most lower-limb amputations are unilateral, which could also affect optimal timing. The goal of this study was to vary the timing of positive prosthesis push-off work in isolation and measure the effects on energetics, mechanics and muscle activity.

Robotic prostheses can improve walking performance for amputees, but prescription of these devices has been hindered by their high cost and uncertainty about the degree to which individuals will benefit. The typical prescription process cannot well predict how an individual will respond to a device they have never used because it bases decisions on subjective assessment of an individual's current activity level. We propose a new approach in which individuals 'test drive' candidate devices using a prosthesis emulator while their walking performance is quantitatively assessed and results are distilled to inform prescription. In this system, prosthesis behavior is controlled by software rather than mechanical implementation, so users can quickly experience a broad range of devices. To test the viability of the approach, we developed a prototype emulator and assessment protocol, leveraging hardware and methods we previously developed for basic science experiments. We demonstrated emulations across the spectrum of commercially available prostheses, including traditional (e.g. SACH), dynamic-elastic (e.g. FlexFoot), and powered robotic (e.g. BiOM® T2) prostheses. Emulations exhibited low error with respect to reference data and provided subjectively convincing representations of each device. We demonstrated an assessment protocol that differentiated device classes for each individual based on quantitative performance metrics, providing feedback that could be used to make objective, personalized device prescriptions.

Robotic ankle-foot prostheses can improve walking performance for amputees compared to conventional designs [1], but prescription has been hindered by their high cost and uncertainty about the degree to which individuals will benefit. The typical prescription process cannot well predict how an individual will respond to a device they have never used, as it bases decisions on subjective assessment of an individual's current activity level [2]. We propose a new approach to prescription in which individuals ‘test drive’ candidate devices using a prosthesis emulator while their walking performance is quantitatively assessed and results are distilled to inform prescription decisions [3]. Figure 1: The emulator consists of a lightweight prosthesis actuated through a tether by a powerful motor and controller. Metabolic and heart rate, maximum walking speed, and user satisfaction are measured to assess the emulated behaviors.

The current process of prescribing prosthetic feet is hampered by imprecise classifications based on self-assessment, recommendations based on subjective prediction, burdensome justification requirements, and slow, costly testing of devices. These problems have been exacerbated by the introduction of robotic prostheses, which can improve gait performance for some individuals, but are very expensive. We propose an alternative process, in which a versatile robotic emulator is used to preview patient interactions with a range of prostheses, while objective data related to effort, stability, speed and preference are collected, all prior to prescription. Results from pilot testing with a prototype emulator system demonstrate accurate haptic rendering of a wide range of prosthesis classes and differentiation of user performance across these classes. Eventually, emulation-based prescription could reduce bias, cost and waste in the prescription process, while simultaneously improving patient outcomes.

Prosthesis push-off timing, isolated from push-off work, had a strong effect on metabolic cost, with optimal push-off occurring at or after opposite leg heel contact. This result roughly concurs with findings from simulations, exoskeleton experiments, and normal walking, although the optimal timing was slightly later. It is possible that a different result would be found if the prosthesis were worn by amputees, as they would not have the additional weight and leg length and are more acclimated to walking with prostheses [11]. These results confirm that the prosthesis emulator used here allows experimental tests of the influence of push-off parameters on metabolic cost. In the future a similar experimental approach could be used to optimize the assistance that amputees can get from a constrained amount of battery energy.

Robotic prostheses have the potential to significantly improve mobility for people with lower-limb amputation. Humans exhibit complex responses to mechanical interactions with these devices, however, and computational models are not yet able to predict such responses meaningfully. Experiments therefore play a critical role in development, but have been limited by the use of product-like prototypes, each requiring years of development and specialized for a narrow range of functions. Here we describe a robotic ankle-foot prosthesis system that enables rapid exploration of a wide range of dynamical behaviors in experiments with human subjects. This emulator comprises powerful off-board motor and control hardware, a flexible Bowden cable tether, and a lightweight instrumented prosthesis, resulting in a combination of low mass worn by the human (0.96 kg) and high mechatronic performance compared to prior platforms. Benchtop tests demonstrated closed-loop torque bandwidth of 17 Hz, peak torque of 175 Nm, and peak power of 1.0 kW. Tests with an anthropomorphic pendulum "leg" demonstrated low interference from the tether, less than 1 Nm about the hip. This combination of low worn mass, high bandwidth, high torque, and unrestricted movement makes the platform exceptionally versatile. To demonstrate suitability for human experiments, we performed preliminary tests in which a subject with unilateral transtibial amputation walked on a treadmill at 1.25 ms-1 while the prosthesis behaved in various ways. These tests revealed low torque tracking error (RMS error of 2.8 Nm) and the capacity to systematically vary work production or absorption across a broad range (from -5 to 21 J per step). These results support the use of robotic emulators during early stage assessment of proposed device functionalities and for scientific study of fundamental aspects of human-robot interaction. The design of simple, alternate end-effectors would enable studies at other joints or with additional degrees of freedom.

Individuals with unilateral below-knee amputation expend more energy than non-amputees during walking and exhibit reduced push-off work and increased hip work in the affected limb. Simple dynamic models of walking suggest a possible solution, predicting that increasing prosthetic ankle push-off should decrease leading limb collision, thereby reducing overall energy requirements. We conducted a rigorous experimental test of this idea wherein ankle-foot prosthesis push-off work was incrementally varied in isolation from one-half to two-times normal levels while subjects with simulated amputation walked on a treadmill at 1.25 m·s −1 . Increased prosthesis push-off significantly reduced metabolic energy expenditure, with a 14% reduction at maximum prosthesis work. In contrast to model predictions, however, collision losses were unchanged, while hip work during swing initiation was decreased. This suggests that powered ankle push-off reduces walking effort primarily through other mechanisms, such as assisting leg swing, which would be better understood using more complete neuromuscular models.

Biomechatronic devices show promise for restoring human performance, but development has been made inefficient by the need for specialized autonomous devices prior to testing benefits of proposed functionalities. This has severely limited exploration within and across intervention strategies. We have developed a laboratory testbed suitable for emulating and rapidly assessing wearable robot designs. The testbed is comprised of powerful off-board motor and control hardware, a flexible tether, and lightweight instrumented end-effectors worn by a person. We performed a series of benchtop tests to gauge mechatronic performance, and found significant improvements over prior candidate testbed platforms. In particular, this system has an unusual combination of low worn mass (less than 1 kg), high closed-loop torque bandwidth (17 Hz), and high peak torque (175 N·m), key to emulating specialized devices. We also performed walking trials to gauge dynamic torque control and versatility. Walking trials with a prosthesis end-effector demonstrated precise torque tracking (4 N·m RMS error), both in time and joint-angle space, and versatile mechanical behavior through systematic changes in high-level control law parameters. For example, we widely varied net ankle work (from -3 J to 9 J per step) using an impedance law relating joint angle and velocity to desired torque. These results suggest such testbeds could be used to emulate and evaluate novel assistive robot concepts prior to laborious product design.

1. Motivation Powered ankle-foot prostheses can increase preferred walking speed and reduce metabolic rate as compared to conventional passive-elastic devices [1]. In these devices, motors and batteries are typically used to provide net positive work; to increase net work requires an increase in device mass [2]. Added mass to the lower limbs increases metabolic rate [3], so a design tradeoff between assistive power and mass must be made. Quantifying the relationship between prosthesis work and metabolic rate would enable design choices that provide the most benefit for the user.