Characterizing the Mechanical Properties of Running-Specific Prostheses

Development and Preliminary Evaluation of a Bimodal Foot Prosthesis for Walking and Running

People often alternate between bouts of walking and running, for instance, when adults participate in recreational activities. Transitioning between activities can be challenging for prosthesis users because existing prosthetic feet are not well-suited for both tasks. Meanwhile, switching between prostheses for different tasks is often impractical. Collectively, these challenges can present barriers to physical activity participation for people with limb loss, which can negatively impact social or physical health. This work describes the development and evaluation of a passive bimodal prosthetic foot prototype with different configurations and stiffnesses for walking and running. Users rated the bimodal prosthesis higher for standing and walking compared to a running prosthesis (+2.3 for both tasks on a seven-point Likert scale). Users rated the bimodal prosthesis higher for running compared to a walking prosthesis (+1.7 and +0.5 for 2.0 and 2.5 m/s running, respectively). Changing from walking to running mode increased the device's stiffness by 23-84%, depending on the user's preference. Users could switch between bimodal prosthesis walking and running modes quickly (21.3 ± 12.0 s). Overall, the preliminary results were encouraging in terms of user satisfaction, stiffness change between modes, and mode-switching speed. These findings motivate future exploration of this bimodal prosthesis concept.

Effects of prosthetic stiffness and added mass on metabolic power and asymmetry in female runners with a leg amputation

Similar to nonamputees, female athletes with unilateral transtibial amputation (TTA) using running-specific leg prostheses (RSPs) may have worse running economy and higher rates of running-related injury than male athletes. Optimizing RSP configuration for female athletes could improve running economy and minimize biomechanical asymmetry, which has been associated with running-related injury. Nine females with a TTA ran at 2.5 m/s while we measured metabolic rates and ground reaction forces. Subjects used an RSP with a manufacturer-recommended stiffness category, one category less stiff and two categories less stiff than recommended. Use of an RSP two categories less stiff resulted in 3.0% lower net metabolic power (P = 0.04), 7.8% lower affected leg stiffness (P = 6.01 × 10-4), increased contact time asymmetry (P = 0.04), and decreased stance average vertical ground reaction force asymmetry (P = 0.04) compared with a recommended stiffness category RSP. Lower RSP stiffness (kN/m) values were associated with lower net metabolic power (P = 0.02), lower affected leg stiffness (P = 1.36 × 10-4), longer affected leg contact time (P = 1.46 × 10-4), and similar affected leg peak and stance-average vertical ground reaction force compared with higher RSP stiffness values. Subjects then used the RSP stiffness category that elicited the lowest net metabolic power with 100 g, 200 g, and 300 g added distally. We found no significant effects of added mass on net metabolic power, biomechanics, or asymmetry. These results suggest that female runners with a TTA could decrease metabolic power during running while minimizing biomechanical asymmetries, which have been associated with running-related injury, by using an RSP two categories less stiff than manufacturer recommended.NEW & NOTEWORTHY Females with unilateral transtibial amputation can improve running performance through reductions in net metabolic power by using a running-specific prosthesis (RSP) that is less stiff than manufacturer-recommended. Lower RSP stiffness values are associated with greater leg stiffness and contact time asymmetry, and lower stance-average vertical ground reaction force asymmetry. However, we found that adding mass to the RSP did not affect net metabolic power and stance-phase biomechanical asymmetries during running.

A review of evidence on mechanical properties of running specific prostheses and their relationship with running performance

Background

Although mechanical properties of running specific prostheses (RSPs) can affect running performance, manufacturers do not consistently report them. This study aimed to review existing literature on RSP mechanical and structural properties and their relationship with running performance.

Methods

A comprehensive search was conducted using keywords related to mechanical properties of RSPs and running performance. Search terms included stiffness and hysteresis, as well as performance outcomes including metabolic cost and running speed. Non-peer-reviewed and non-English publications were excluded.

Results

Twenty articles were included in the review. Sixteen studies used a material testing machine to measure RSP material properties, and four articles used other techniques including 2D/3D video capture and force platforms. Both measurement techniques and reporting of outcomes were inconsistent, which limits the ability to draw broad conclusions. Additionally, several studies did not report the numerical data for material properties despite measuring them. Relatively few articles measured both material properties and running performance and assessed correlations.

Conclusion

Several articles connected prosthesis properties to running performance. However, inconsistent measurement and reporting of mechanical properties, along with the multifactorial nature of the athlete-prosthesis system, limit the ability to draw broad conclusions regarding the relationship between material and structural properties and athlete performance. Current evidence may be useful for clinicians seeking ways to optimize RSP stiffness in a case-by-case basis; however, clinicians would benefit from more consistent and systematic comparisons of the attributes of different RSPs and their role in performance.

Running-specific prosthesis' performance characterization by dynamic finite element approach

INTRODUCTION

Composite running-specific prostheses (RSP) are widely used by athletes with lower limb amputations to simulate the spring-like behavior of biological legs. However, the effect of these devices on the biomechanics of athletes with transtibial amputations remains uncertain.

MODELING METHOD DESCRIPTION

To address this issue, this study proposes a time-dependent finite element model that uses angles and dynamic loads during ground contact to evaluate RSP performance parameters such as stiffness and energy efficiency. The study also examines the impact of running speed and RSP geometry on performance.

NUMERICAL SIMULATION AND MODEL VERIFICATION

The in-silico characterization approach used in this study takes into account both the intrinsic characteristics of the RSP and the athlete's biomechanics to identify the influence of fundamental geometric variables of the RSP on performance. The model is verified by comparing its results with experimental data.

RESULTS AND DISCUSSION

The study found that as running speed increases, RSP stiffness, vertical ground reaction force (vGRF), and contact time decrease. The force-displacement profiles of RSP are nonlinear, but a linear function can be used to accurately represent their behavior at high sprinting speeds. Using higher RSP reduces energy efficiency and vGRF due to their lower stiffness. J-curve RSP result in higher stiffness, vGRF, and strain energy, while C-curve RSP are associated with longer contact times and higher energy efficiency.

The contribution of lower-limb joint quasi-stiffness to theoretical leg stiffness during level, uphill and downhill running at different speeds

Humans change joint quasi-stiffness (k joint) and leg stiffness (kleg) when running at different speeds on level ground and during uphill and downhill running. These mechanical properties can inform device designs for running such as footwear, exoskeletons and prostheses. We measured kinetics and kinematics from 17 runners (10 M; 7 F) at three speeds on 0°, ±2°, ±4° and ±6° slopes. We calculated ankle and knee k joint, the quotient of change in joint moment and angular displacement, and theoretical leg stiffness (klegT) based on the joint external moment arms and k joint. Runners increased k ankle at faster speeds (p < 0.01). Runners increased and decreased the ankle and knee contributions to klegT, respectively, by 2.89% per 1° steeper uphill slope (p < 0.01) during the first half of stance. Runners decreased and increased ankle and knee joint contributions to klegT, respectively, by 3.68% during the first half and 0.86% during the second half of stance per 1° steeper downhill slope (p < 0.01). Thus, biomimetic devices require stiffer k ankle for faster speeds, and greater ankle contributions and greater knee contributions to klegT during the first half of stance for steeper uphill and downhill slopes, respectively.

Equivalent running leg lengths require prosthetic legs to be longer than biological legs during standing

We aimed to determine a method for prescribing a standing prosthetic leg length (ProsL) that results in an equivalent running biological leg length (BioL) for athletes with unilateral (UTTA) and bilateral transtibial amputations (BTTA). We measured standing leg length of ten non-amputee (NA) athletes, ten athletes with UTTA, and five athletes with BTTA. All athletes performed treadmill running trials from 3 m/s to their maximum speed. We calculated standing and running BioL and ProsL lengths and assessed the running-to-standing leg length ratio (L_ratio) at three instances during ground contact: touchdown, mid-stance, and take-off. Athletes with UTTA had 2.4 cm longer standing ProsL than BioL length (p = 0.030), but their ProsL length were up to 3.3 cm shorter at touchdown and 4.1 cm shorter at mid-stance than BioL, at speed 3-11.5 m/s. At touchdown, mid-stance, and take-off, athletes with BTTA had 0.01-0.05 lower L_ratio at 3 m/s (p < 0.001) and 0.03-0.07 lower L_ratio at 10 m/s (p < 0.001) in their ProsL compared to the BioL of NA athletes. During running, ProsL were consistently shorter than BioL. To achieve equivalent running leg lengths at touchdown and take-off, athletes with UTTA should set their running-specific prosthesis height so that their standing ProsL length is 2.8-4.5% longer than their BioL length, and athletes with BTTA should set their running-specific prosthesis height so that their standing ProsL lengths are at least 2.1-3.9% longer than their presumed BioL length. Setting ProsL length to match presumed biological dimensions during standing results in shorter legs during running.

A novel characterisation protocol of mechanical interactions between the ground and a tibial prosthesis for long jump

The mechanical study of Running Specific Prostheses (RSPs) is often limited to the blade. The setup developed and presented herein is a simple experiment, based on a mechanical testing machine and a camera, that assesses two indicators relevant to coaches and athletes in the field of athletics: secant stiffness and energy dissipation. The influence of four parameters on global prosthesis behaviour is evaluated: the load line offset, the prosthesis-ground angle, the sole type and the flooring type. The load line offset and the flooring type have little to no influence on their behaviour. The prosthesis-ground angle impacts the stiffness: an increase in the angle brings a significant decrease in stiffness, which strongly impacts the performance. The type of sole modifies the kinematics of the blade tip's interaction with the ground. However, this effect is less likely to enhance the sports practice since athletics imposes the use of spikes. The camera images allow assessing the local behaviour of the sole, thus enabling to follow its strain through the compression process.

Measurement and reporting of footwear characteristics in running biomechanics: A systematic search and narrative synthesis of contemporary research methods

This review aimed to synthesise the methods for assessing and reporting footwear characteristics among studies evaluating the effect of footwear on running biomechanics. Electronic searches of Scopus®, EBSCO, PubMed®, ScienceDirect®, and Web of Science® were performed to identify original research articles of the effect of running footwear on running biomechanics published from 1^st January 2015 to 7^th October 2020. Risk of bias among included studies was not assessed. Results were presented via narrative synthesis. Eligible studies compared the effect of two or more footwear conditions in adult runners on a biomechanical parameter. Eighty-seven articles were included and data from 242 individual footwear were extracted. Predominantly, studies reported footwear taxonomy (i.e., classification) and manufacturer information, however omitted detail regarding the technical specifications of running footwear and did not use validated footwear reporting tools. There is inconsistency among contemporary studies in the methods by which footwear characteristics are assessed and reported. These findings point towards a need for consensus regarding the reporting of these characteristics within biomechanical studies to facilitate the conduct of systematic reviews and meta-analyses pertaining to the effect of running footwear on running biomechanics.

Walking asymmetry and its relation to patient-reported and performance-based outcome measures in individuals with unilateral lower limb loss

Gait asymmetry persists for most people after lower limb amputation and is associated with slower walking speeds. However, the relationship between gait asymmetry and patient-reported function remains unclear because they are not commonly assessed together. The purpose of this study was to determine relationships between gait asymmetries in people with lower limb loss and (1) patient-reported outcomes and (2) performance-based prosthetic functional measures. This cross-sectional analysis included nine people with unilateral limb loss aged 48.2 ± 13.1 years of mixed amputation etiology. Patient-reported outcomes included the Prosthetic Evaluation Questionnaire mobility subscale and Activities-specific Balance Confidence scale. Performance outcomes included the Berg Balance Scale and the 30-second sit-to-stand test. Walking performance measures included the 2-Minute Walk Test, during which APDM Opal sensors recorded spatiotemporal gait parameters, and daily step-counts from StepWatch4 activity monitors. The study found that the most asymmetric gait symmetry ratios (prosthetic-limb divided by intact-limb) could be attributed to prosthetic foot dorsiflexion-plantarflexion and rotation motion limitations: prosthetic-limb trailing double support (0.789 ± 0.052), toe-off (0.760 ± 0.068) and toe-out angle (0.653 ± 0.256). Single limb stance, and stance and swing phase durations were most strongly associated with balance and walking performance measures. Notably, no symmetry ratio was significantly associated with patient-reported prosthetic function (unadjusted Pearson correlation coefficients r < 0.50, P > 0.05). More gait symmetry was associated with better balance and walking performance but had no significant relationship with patient-reported function. Although achieving gait symmetry after lower limb loss is a common walking goal, symmetry was unrelated to the perception of functional mobility for people with lower limb loss.

Emulating the Effective Ankle Stiffness of Commercial Prosthetic Feet Using a Robotic Prosthetic Foot Emulator

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<.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 a PFE could be an effective tool for emulating commercial prosthetic feet, enabling prosthesis users to quickly trial different feet and provide experiential input as part of a prosthetic foot prescription.

Running-specific prosthesis model, stiffness and height affect biomechanics and asymmetry of athletes with unilateral leg amputations across speeds

Athletes with transtibial amputation (TTA) use running-specific prostheses (RSPs) to run. RSP configuration likely affects the biomechanics of such athletes across speeds. We determined how the use of three RSP models (Catapult, Sprinter and Xtend) with three stiffness categories (recommended, ±1), and three heights (recommended, ±2 cm) affected contact length (L_c ), stance average vertical ground reaction force (F _avg), step frequency (f _step) and asymmetry between legs for 10 athletes with unilateral TTA at 3-7 m s^-1. The use of the Xtend versus Catapult RSP decreased L_c (p = 2.69 × 10^-7) and F _avg asymmetry (p = 0.032); the effect on L_c asymmetry diminished with faster speeds (p = 0.0020). The use of the Sprinter versus Catapult RSP decreased F _avg asymmetry (p = 7.00 × 10^-5); this effect was independent of speed (p = 0.90). The use of a stiffer RSP decreased L_c asymmetry (p ≤ 0.00033); this effect was independent of speed (p ≥ 0.071). The use of a shorter RSP decreased L_c (p = 5.86 × 10^-6), F _avg (p = 8.58 × 10^-6) and f _step asymmetry (p = 0.0011); each effect was independent of speed (p ≥ 0.15). To minimize asymmetry, athletes with unilateral TTA should use an Xtend or Sprinter RSP with 2 cm shorter than recommended height and stiffness based on intended speed.

Metabolic Cost and Performance of Athletes With Lower Limb Amputation and Nonamputee Matched Controls During Running: A Systematic Review

ABSTRACT

The elastic function of running-specific prostheses likely contributes to a lower metabolic cost of running. However, it remains unclear whether running-specific prostheses provide advantages concerning the metabolic cost of running in relationship with nonamputee runners. This study aimed to systematically review the scientific literature to examine the peak performance (peak oxygen consumption-VO2peak and peak speed) and the metabolic cost between paired amputees and nonamputees during running and between amputee runners with traditional prostheses and running-specific prostheses. A literature search on three databases (MEDLINE/PubMed, Scopus, and Web of Science) was conducted using the following key words: (amputation OR amputee) AND (run OR running OR runner) AND (prosthesis OR prosthetics), resulting in 2060 records and 4 studies within the inclusion criteria. A methodological quality assessment was carried out using a modified version of the Downs and Black checklist. VO2peak of the amputees athletes (54 ± 2 mL kg-1 min-1) is similar (mean difference = -0.80 mL kg-1 min-1, confidence interval = -4.63 to 3.03) to nonamputees athletes (55 ± 2 mL kg-1 min-1). The average metabolic cost of the paired amputee athletes (4.94 ± 1.19 J kg-1 m-1) also does not differ (mean difference = 0.73 J kg-1 m-1, confidence interval = -0.74 to 2.20) from nonamputee runners (4.21 ± 0.16 J kg-1 m-1). The research on running in amputee and nonamputee athletes is limited. The few existing studies have limited methodological quality. The metabolic cost data from amputee athletes running with running-specific prostheses are within the range of nonamputee data.

Prosthetic forefoot and heel stiffness across consecutive foot stiffness categories and sizes

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.

Effects of shear force reduction during mechanical testing and day-to-day variation on stiffness of commercial prosthetic feet: a technical note

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.

Biomechanical factors affecting individuals with lower limb amputations running using running-specific prostheses: A systematic review

BACKGROUND

Running-specific prostheses (RSPs) are biomechanically designed to enable individuals with lower limb amputations to engage in high level sports.

RESEARCH QUESTION

What is the influence of RSP use on the running biomechanics of individuals with lower limb amputations?

METHODS

An article search was conducted in six databases since their inception to July 2021. Two independent reviewers assessed the title, abstract and full texts in the review process. The quality of the papers was appraised. The review included a total of 35 articles.

RESULTS

Main findings indicate force production is a limitation of RSPs. Individuals with lower limb absence employ a variety of compensatory strategies such as adjusting their step frequency, contact length and joint kinetics to improve their running performance. Leg stiffness modulation and external factors relating to the RSP design and fitting play important roles in RSP biomechanics. For individuals with unilateral amputations, the increased loading of the intact limb could increase the risk of acute injury or chronic joint degradation.

SIGNIFICANCE

To improve their running performance, runners with lower limb amputations employ various compensatory strategies, such as altering the spatiotemporal and kinetic parameters. Factors relating to RSP height, stiffness, shape, and alignment also play an important role in terms of running biomechanics and should be considered in RSP design and fitting. Future studies should focus on the use of RSPs for recreation, in pediatric populations, with certain amputation levels, as well as the impact of training and running techniques.

External Mechanical Work in Runners With Unilateral Transfemoral Amputation

Carbon-fiber running-specific prostheses have enabled individuals with lower extremity amputation to run by providing a spring-like leg function in their affected limb. When individuals without amputation run at a constant speed on level ground, the net external mechanical work is zero at each step to maintain a symmetrical bouncing gait. Although the spring-like “bouncing step” using running-specific prostheses is considered a prerequisite for running, little is known about the underlying mechanisms for unilateral transfemoral amputees. The aim of this study was to investigate external mechanical work at different running speeds for unilateral transfemoral amputees wearing running-specific prostheses. Eight unilateral transfemoral amputees ran on a force-instrumented treadmill at a range of speeds (30, 40, 50, 60, 70, and 80% of the average speed of their 100-m personal records). We calculated the mechanical energy of the body center of mass (COM) by conducting a time-integration of the ground reaction forces in the sagittal plane. Then, the net external mechanical work was calculated as the difference between the mechanical energy at the initial and end of the stance phase. We found that the net external work in the affected limb tended to be greater than that in the unaffected limb across the six running speeds. Moreover, the net external work of the affected limb was found to be positive, while that of the unaffected limb was negative across the range of speeds. These results suggest that the COM of unilateral transfemoral amputees would be accelerated in the affected limb’s step and decelerated in the unaffected limb’s step at each bouncing step across different constant speeds. Therefore, unilateral transfemoral amputees with passive prostheses maintain their bouncing steps using a limb-specific strategy during running.

Sprinting with prosthetic versus biological legs: insight from experimental data

Running-prostheses have enabled exceptional athletes with bilateral leg amputations to surpass Olympic 400 m athletics qualifying standards. Due to the world-class performances and relatively fast race finishes of these athletes, many people assume that running-prostheses provide users an unfair advantage over biologically legged competitors during long sprint races. These assumptions have led athletics governing bodies to prohibit the use of running-prostheses in sanctioned non-amputee (NA) competitions, such as at the Olympics. However, here we show that no athlete with bilateral leg amputations using running-prostheses, including the fastest such athlete, exhibits a single 400 m running performance metric that is better than those achieved by NA athletes. Specifically, the best experimentally measured maximum running velocity and sprint endurance profile of athletes with prosthetic legs are similar to, but not better than those of NA athletes. Further, the best experimentally measured initial race acceleration (from 0 to 20 m), maximum velocity around curves, and velocity at aerobic capacity of athletes with prosthetic legs were 40%, 1-3% and 19% slower compared to NA athletes, respectively. Therefore, based on these 400 m performance metrics, use of prosthetic legs during 400 m running races is not unequivocally advantageous compared to the use of biological legs.

Adapting Semi-Active Prostheses to Real-World Movements: Sensing and Controlling the Dynamic Mean Ankle Moment Arm with a Variable-Stiffness Foot on Ramps and Stairs

(1) Background: Semi-active prosthetic feet can provide adaptation in different circumstances, enabling greater function with less weight and complexity than fully powered prostheses. However, determining how to control semi-active devices is still a challenge. The dynamic mean ankle moment arm (DMAMA) provides a suitable biomechanical metric, as its simplicity matches that of a semi-active device. However, it is unknown how stiffness and locomotion modes affect DMAMA, which is necessary to create closed-loop controllers for semi-active devices. In this work, we develop a method to use only a prosthesis-embedded load sensor to measure DMAMA and classify locomotion modes, with the goal of achieving mode-dependent, closed-loop control of DMAMA using a variable-stiffness prosthesis. We study how stiffness and ground incline affect the DMAMA, and we establish the feasibility of classifying locomotion modes based exclusively on the load sensor. (2) Methods: Human subjects walked on level ground, ramps, and stairs while wearing a variable-stiffness prosthesis in low-, medium-, and high-stiffness settings. We computed DMAMA from sagittal load sensor data and prosthesis geometric measurements. We used linear mixed-effects models to determine subject-independent and subject-dependent sensitivity of DMAMA to incline and stiffness. We also used a machine learning model to classify locomotion modes using only the load sensor. (3) Results: We found a positive linear sensitivity of DMAMA to stiffness on ramps and level ground. Additionally, we found a positive linear sensitivity of DMAMA to ground slope in the low- and medium-stiffness conditions and a negative interaction effect between slope and stiffness. Considerable variability suggests that applications of DMAMA as a control input should look at the running average over several strides. To examine the efficacy of real-time DMAMA-based control systems, we used a machine learning model to classify locomotion modes using only the load sensor. The classifier achieved over 95% accuracy. (4) Conclusions: Based on these findings, DMAMA has potential for use as a closed-loop control input to adapt semi-active prostheses to different locomotion modes.

The Impact of Blade Technology on Paralympic Sprint Performance Between 1996 and 2016: Bilateral Amputees' Competitive Advantage

It is known that high-performance sprinters with unilateral and bilateral prosthetic lower limbs run at different speeds using different spatiotemporal strategies. Historically, these athletes still competed together in the same races, but 2018 classification rule revisions saw the separation of these two groups. This study sought to compare Paralympic sprint performance between all-comer (i.e., transfemoral and transtibial) unilateral and bilateral amputee sprinters using a large athlete sample. A retrospective analysis of race speed among Paralympic sprinters between 1996 and 2016 was conducted. In total, 584 published race results from 161 sprinters revealed that unilateral and bilateral lower-extremity amputee sprinters had significantly different race speeds in all three race finals (100 m, p value <.001; 200 m, <.001; 400 m, <.001). All-comer bilateral amputee runners ran faster than their unilateral counterparts; performance differences increased with race distance. These data support current classification criteria in amputee sprinting, which may create more equal competitive fields in the future.

How Can Biomechanics Improve Physical Preparation and Performance in Paralympic Athletes? A Narrative Review

Recent research in Paralympic biomechanics has offered opportunities for coaches, athletes, and sports practitioners to optimize training and performance, and recent systematic reviews have served to summarize the state of the evidence connecting biomechanics to Paralympic performance. This narrative review serves to provide a comprehensive and critical evaluation of the evidence related to biomechanics and Paralympic performance published since 2016. The main themes within this review focus on sport-specific body posture: the standing, sitting, and horizontal positions of current summer Paralympic sports. For standing sports, sprint and jump mechanics were assessed in athletes with cerebral palsy and in lower-limb amputee athletes using running-specific prostheses. Our findings suggest that running and jumping-specific prostheses should be ‘tuned’ to each athlete depending on specific event demands to optimize performance. Standing sports were also inclusive to athletes with visual impairments. Sitting sports comprise of athletes performing on a bike, in a wheelchair (WC), or in a boat. WC configuration is deemed an important consideration for injury prevention, mobility, and performance. Other sitting sports like hand-cycling, rowing, and canoeing/kayaking should focus on specific sitting positions (e.g., arm-crank position, grip, or seat configuration) and ways to reduce aero/hydrodynamic drag. Para-swimming practitioners should consider athlete-specific impairments, including asymmetrical anthropometrics, on the swim-start and free-swim velocities, with special considerations for drag factors. Taken together, we provide practitioners working in Paralympic sport with specific considerations on disability and event-specific training modalities and equipment configurations to optimize performance from a biomechanical perspective.

Hip Joint Contact Loading and Muscle Forces During Running With a Transtibial Amputation

People with unilateral transtibial amputations (TTA) have greater risks of bilateral hip osteoarthritis, related to asymmetric biomechanics compared to people without TTA. Running is beneficial for physical health and is gaining popularity. However, people with TTA may not have access to running-specific prostheses (RSPs), which are designed for running, and may instead run using their daily-use prosthesis (DUP). Differences in joint loading may result from prosthesis choice; thus, it is important to characterize changes in peak and impulsive hip joint contact loading during running. Six people with and without TTA ran at 3.5 m/s while ground reaction forces, kinematics, and electromyography were collected. People with TTA ran using their own RSP and DUP. Musculoskeletal models incorporating prosthesis type of each individual were used to quantify individual muscle forces and hip joint contact forces (HJCFs) during running. People using RSPs had smaller bilateral peak hip joint contact forces compared to when wearing DUPs during stance and swing, and a smaller impulse over the entire gait cycle. Greater amputated leg peak hip joint contact forces for people wearing DUPs compared to RSPs occurred with greater forces from the ipsilateral gluteus maximus during stance. People with TTA also had greater bilateral peak hip joint contact forces during swing compared to people without TTA, which occurred with greater peak gluteus medius forces. Running with more compliant RSPs may be beneficial for long-term joint health by reducing peak and impulsive hip loading compared to DUPs.

Running-specific prostheses reduce lower-limb muscle activity compared to daily-use prostheses in people with unilateral transtibial amputations

People with unilateral transtibial amputation (TTA) have biomechanical differences between the amputated and intact legs and compared to people without TTA during running. Additional biomechanical differences emerge between running with running-specific (RSPs) and daily-use prostheses (DUPs), but the associated underlying muscle activity is unclear. We collected surface electromyography from the biceps femoris long head, rectus femoris, vastus lateralis, and gastrocnemius as well as body kinematics and ground reaction forces in six people with and six people without TTA. We compared stance phase muscle activity and peak activation timing in people with and without TTA and between people using RSPs compared to DUPs during running at 3.5 m/s. Peak amputated leg hamstring activity occurred 34% (RSP) and 31% (DUP) earlier in stance phase compared to the intact leg. Peak amputated leg rectus femoris activity of people wearing DUPs occurred 8% and 9% later in stance phase than the intact leg of people wearing DUPs and amputated leg of people wearing RSPs, respectively. People with TTA had 45% (DUP) and 61% (RSP) smaller peak amputated leg knee extension moments compared to people without TTA, consistent with observations of quadriceps muscle activity. Using RSPs decreased overall muscle activity compared to DUPs.

Development of Instrumented Running Prosthetic Feet for the Collection of Track Loads on Elite Athletes

Knowledge of loads acting on running specific prostheses (RSP), and in particular on running prosthetic feet (RPF), is crucial for evaluating athletes' technique, designing safe feet, and biomechanical modelling. The aim of this work was to develop a J-shaped and a C-shaped wearable instrumented running prosthetic foot (iRPF) starting from commercial RPF, suitable for load data collection on the track. The sensing elements are strain gauge bridges mounted on the foot in a configuration that allows decoupling loads parallel and normal to the socket-foot clamp during the stance phase. The system records data on lightweight athlete-worn loggers and transmits them via Wi-Fi to a base station for real-time monitoring. iRPF calibration procedure and static and dynamic validation of predicted ground-reaction forces against those measured by a force platform embedded in the track are reported. The potential application of this wearable system in estimating determinants of sprint performance is presented.

Investigation of The Effects of Prosthetic Knee Condition for Individuals with Transfemoral Amputation During Attempted Running

BACKGROUND

A number of individuals with unilateral transfemoral amputation (TFA) run in a prosthesis with an unlocked prosthetic knee, while others choose to run with a locked prosthetic knee to increase stability. Research regarding running with an unlocked knee (UK) versus a locked knee (LK), with respect to energy efficiency, is limited and might be enhanced by characterization of the impact of knee condition on kinematics.

OBJECTIVES

To investigate the effect of an UK versus LK on hip kinematics, energy efficiency, and running speed.

METHODOLOGY

Five male novice runners with unilateral TFA completed one three-minute self-selected running speed (SSRS) trial and three peak speed trials per knee condition. Hip kinematics, energy efficiency, and running speed were compared between conditions.

FINDINGS

Four of the five subjects exhibited a fast walk, rather than a consistent run. Hip flexion increased for all subjects and hip abduction decreased for four subjects during swing phase for the UK condition. Hip kinematic asymmetry was reduced for the UK condition in the sagittal plane for four individuals; hip kinematic asymmetry was also reduced in the frontal plane for the UK condition for three of these individuals. Mean energy efficiency was better for the UK condition (UK: 0.282 mLO2/kg/m, LK: 0.328 mLO2/kg/m). Peak running speed did not differ significantly between knee conditions (UK: 1.47m/s, LK:1.32m/s).

CONCLUSIONS

For novice recreational runners with unilateral transfemoral amputation, the UK condition resulted in improved energy efficiency and enhanced kinematic symmetry, despite comparable peak speed relative to the LK condition. Therefore the UK condition may be advantageous for mid-range distance running.

Are consumers satisfied with the use of prosthetic sports feet and the provision process? A mixed-methods study

BACKGROUND

Special feet connected to a prosthesis, prosthetic sports feet, enable athletes with a lower limb amputation to run. The selection of a prosthetic sports feet is usually based on body weight and preferred sports performance. The selection of a prosthetic sport feet is also based on clinicians who likely have limited experience due to a small number of athletes with a lower limb amputation.

HYPOTHESIS

Athletes with a lower limb amputation are not satisfied with the use and service associated with prosthetic sports feet due to a lack of prosthetic sports feet provision guidelines, poorer function of prosthetic sports feet compared to the anatomical foot and ankle, and limited experience of clinicians.

EVALUATION OF HYPOTHESIS

A mixed-methods study in 16 athletes with a lower limb amputation using a prosthetic sport foot from Össur or Otto Bock, included semi-structured interviews and quantitative analysis. Three dimensions of prosthetic sports feet were investigated: 1) use, 2) provision process, and 3) cosmetics. Qualitative data were analyzed to identify factors influencing consumer satisfaction. Quantitative data were analyzed to investigate satisfaction and perceived relative importance of the dimensions.

RESULTS

Participants were satisfied with the prosthetic sports feet use. However, they were not satisfied with the process prior to provision. The prosthetic sport feet use was perceived as the most important dimension. Sports performance was the critical element in the prosthetic sports feet use and was influenced by stability, confidence and fear, safety, focus, energy return, and comfort. Cosmetics were unimportant. Motivation to purchase the prosthetic sports feet was the key element for the prosthetic sports foot acquisition. Satisfaction about the process prior to provision was negatively influenced by poor support of professionals during rehabilitation, the complexity and duration of the purchase process, and lack of information and accessibility of prosthetic sports feet.

CONCLUSION

The most important dimension of the prosthetic sports feet was its use, which was directly influenced by performance. To further increase the satisfaction with prosthetic sports feet, clinicians should establish how to meet the desired sports performance level of athletes with a lower limb amputation. Improving the process prior to the provision process may increase satisfaction. We suggest increasing the support of professionals during rehabilitation and training through cooperation between involved services, organizing prosthetic sports feet try-out sessions, and increase the accessibility of the prosthetic sports feet. In this way, individuals with a lower limb amputation may become and stay more physically active and participate in sports.

Effect of step frequency on leg stiffness during running in unilateral transfemoral amputees

Spring-like leg behavior is a general feature of mammalian bouncing gaits, such as running and hopping. Although increases in step frequency at a given running speed are known to increase the stiffness of the leg spring (k_leg) in non-amputees, little is known about stiffness regulation in unilateral transfemoral amputees. In this study, we investigated stiffness regulation at different step frequencies at a given running speed in unilateral transfemoral amputees. We recruited nine unilateral transfemoral amputees wearing running-specific prostheses. They were asked to perform the action of running across a range of step frequencies (±20, ±15, ±10, ±5, and 0% of their preferred step frequency) at a given speed on an instrumented treadmill. The k_leg values were calculated using ground reaction force data in both the affected and unaffected limbs. It was found that k_leg increased with increasing step frequency for the unaffected limb, but not for the affected limb. Consequently, the unilateral transfemoral amputees attained the desired step frequency in the unaffected limb, but were unable to match the three highest step frequencies using their affected limbs. These results suggest that the stiffness regulation strategy during running differs between the affected and unaffected limbs.

Prosthetic shape, but not stiffness or height, affects the maximum speed of sprinters with bilateral transtibial amputations

Running-specific prostheses (RSPs) have facilitated an athlete with bilateral transtibial amputations to compete in the Olympic Games. However, the performance effects of using RSPs compared to biological legs remains controversial. Further, the use of different prosthetic configurations such as shape, stiffness, and height likely influence performance. We determined the effects of using 15 different RSP configurations on the maximum speed of five male athletes with bilateral transtibial amputations. These athletes performed sets of running trials up to maximum speed using three different RSP models (Freedom Innovations Catapult FX6, Össur Flex-Foot Cheetah Xtend and Ottobock 1E90 Sprinter) each with five combinations of stiffness category and height. We measured ground reaction forces during each maximum speed trial to determine the biomechanical parameters associated with different RSP configurations and maximum sprinting speeds. Use of the J-shaped Cheetah Xtend and 1E90 Sprinter RSPs resulted in 8.3% and 8.0% (p<0.001) faster maximum speeds compared to the use of the C-shaped Catapult FX6 RSPs, respectively. Neither RSP stiffness expressed as a category (p = 0.836) nor as kN·m-1 (p = 0.916) affected maximum speed. Further, prosthetic height had no effect on maximum speed (p = 0.762). Faster maximum speeds were associated with reduced ground contact time, aerial time, and overall leg stiffness, as well as with greater stance-average vertical ground reaction force, contact length, and vertical stiffness (p = 0.015 for aerial time, p<0.001 for all other variables). RSP shape, but not stiffness or height, influences the maximum speed of athletes with bilateral transtibial amputations.

Prosthetic model, but not stiffness or height, affects maximum running velocity in athletes with unilateral transtibial amputations

The running-specific prosthetic (RSP) configuration used by athletes with transtibial amputations (TTAs) likely affects performance. Athletes with unilateral TTAs are prescribed C- or J-shaped RSPs with a manufacturer-recommended stiffness category based on body mass and activity level, and height based on unaffected leg and residual limb length. We determined how 15 different RSP model, stiffness, and height configurations affect maximum running velocity (v_max) and the underlying biomechanics. Ten athletes with unilateral TTAs ran at 3 m/s to v_max on a force-measuring treadmill. v_max was 3.8-10.7% faster when athletes used J-shaped versus C-shaped RSP models (p < 0.05), but was not affected by stiffness category, actual stiffness (kN/m), or height (p = 0.72, p = 0.37, and p = 0.11, respectively). v_max differences were explained by vertical ground reaction forces (vGRFs), stride kinematics, leg stiffness, and symmetry. While controlling for velocity, use of J-shaped versus C-shaped RSPs resulted in greater stance average vGRFs, slower step frequencies, and longer step lengths (p < 0.05). Stance average vGRFs were less asymmetric using J-shaped versus C-shaped RSPs (p < 0.05). Contact time and leg stiffness were more asymmetric using the RSP model that elicited the fastest v_max (p < 0.05). Thus, RSP geometry (J-shape versus C-shape), but not stiffness or height, affects v_max in athletes with unilateral TTAs.

Use of Dynamic FEA for Design Modification and Energy Analysis of a Variable Stiffness Prosthetic Foot

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.

The Effect of Prosthetic Alignment on Prosthetic and Total Leg Stiffness While Running With Simulated Running-Specific Prostheses

Running-specific prostheses (RSP) are designed to replicate the spring-like behavior of the biological leg in people with a lower limb amputation. Running performance strongly depends on stiffness of the RSP. The aim of this study was to investigate the effects of angle of alignment of the RSP on its stiffness, and how this affects total leg stiffness and the gait pattern during running. Ten able-bodied athletes performed eight trials on a treadmill with running-specific prosthetic simulators, while the alignment of the blades relative to the socket was set in four different angles (0, 5, 10, and 15°) during two different step frequency conditions (free and imposed). RSP stiffness, total leg stiffness, residual leg stiffness, and spatiotemporal parameters were measured. In both step frequency conditions, the RSP stiffness decreased linearly with increasing angle of alignment. Able bodied athletes were able to compensate for the decreased RSP stiffness, and keep total leg stiffness almost invariant, by increasing residual leg stiffness through a more straight the knee at initial contact. This study confirms that alignment is an important factor to take into account when optimizing the RSP. Whether the observed compensations are feasible in amputee athletes needs further investigation.

Methods for Describing and Characterizing the Mechanical Behavior of Running-Specific Prosthetic Feet

Current methods for describing and characterizing the mechanical behavior of running-specific prosthetic feet are incomplete, and this has limited our understanding of how design parameters impact athlete performance. Deflections induced by most ground reaction forces consist of vertical, horizontal, and angular components, but previous work has focused only on the vertical component. Furthermore, the deflection depends heavily on the direction of the force, which changes throughout stance phase of running. In this paper, we introduce several methods that can be used to more precisely describe and characterize the mechanics of running-specific prosthetic feet. We use a custom finite element model to simulate these methods, and validate them with a series of tests using a prototype foot in a universal testing machine.

Stiffness and energy storage characteristics of energy storage and return prosthetic feet

BACKGROUND

Mechanical properties of prosthetic feet can significantly influence amputee gait, but how they vary with respect to limb loading and orientation is infrequently reported.

OBJECTIVE

The objective of this study is to measure stiffness and energy storage characteristics of prosthetic feet across limb loading and a range of orientations experienced in typical gait.

STUDY DESIGN

This study included mechanical testing.

METHODS

Force-displacement data were collected at combinations of 15 sagittal and 5 coronal orientations and used to calculate stiffness and energy storage across prosthetic feet, stiffness categories, and heel wedge conditions.

RESULTS

Stiffness and energy storage were highly non-linear in both the sagittal and coronal planes. Across all prosthetic feet, stiffness decreased with greater heel, forefoot, medial, and lateral orientations, while energy storage increased with forefoot, medial, and lateral loading orientations. Stiffness category was proportional to stiffness and inversely proportional to energy storage. Heel wedge effects were prosthetic foot dependent.

CONCLUSION

Orientation, manufacturer, stiffness category, and heel wedge inclusion greatly influenced stiffness and energy storage characteristics.

CLINICAL RELEVANCE

These results and an available graphical user interface tool may help improve clinical prescriptions by providing prosthetists with quantitative measures to compare prosthetic feet.

Age-Related Changes in Para and Wheelchair Racing Athlete's Performances

During the last decades, all para-athletes with disabilities have significantly increased their performance level due to technological progress and human investment, through better training or recovery protocols, medical care and nutritional monitoring. Among these elements, the athlete’s age is one of the determining factors in performance. The aim of this study was to investigate the effect of age on maximal performances for para-athletes and wheelchair racing athletes, scaled on able-bodied records. We collected 53,554 results including athlete’s best performance of the year, event, age and disability classification from the International Paralympic Committee competitions between 2009 and 2017 for both female and male para-athletics and wheelchair racing disciplines for a total of 472 sport events in Track and Field (considering each impairment type for each event) and gathered the all-time able-bodied records from the International Association of Athletics Federations (IAAF) at the end of 2017. Maximal performance by age was fitted with the Moore function for each para-athletics and wheelchair racing event. This study finds a similar age-related pattern in maximal performance among para-athletes and wheelchair racing athletes. The age at peak performance varies according to sex, impairment type and event and increases gradually from sprint to endurance events. The best Top 100 performances include a large age range suggesting that performance has probably not been optimized yet for most elite para-athletes and wheelchair racers. The next Paralympic Games of Tokyo 2020 and Paris 2024 will certainly offer exceptional performance.

An Overview on Principles for Energy Efficient Robot Locomotion

Despite enhancements in the development of robotic systems, the energy economy of today's robots lags far behind that of biological systems. This is in particular critical for untethered legged robot locomotion. To elucidate the current stage of energy efficiency in legged robotic systems, this paper provides an overview on recent advancements in development of such platforms. The covered different perspectives include actuation, leg structure, control and locomotion principles. We review various robotic actuators exploiting compliance in series and in parallel with the drive-train to permit energy recycling during locomotion. We discuss the importance of limb segmentation under efficiency aspects and with respect to design, dynamics analysis and control of legged robots. This paper also reviews a number of control approaches allowing for energy efficient locomotion of robots by exploiting the natural dynamics of the system, and by utilizing optimal control approaches targeting locomotion expenditure. To this end, a set of locomotion principles elaborating on models for energetics, dynamics, and of the systems is studied.

A Comparison of the Energetic Cost of Running in Marathon Racing Shoes

Background

Reducing the energetic cost of running seems the most feasible path to a sub-2-hour marathon. Footwear mass, cushioning, and bending stiffness each affect the energetic cost of running. Recently, prototype running shoes were developed that combine a new highly compliant and resilient midsole material with a stiff embedded plate.

Objective

The aim of this study was to determine if, and to what extent, these newly developed running shoes reduce the energetic cost of running compared with established marathon racing shoes.

Methods

18 high-caliber athletes ran six 5-min trials (three shoes × two replicates) in prototype shoes (NP), and two established marathon shoes (NS and AB) during three separate sessions: 14, 16, and 18 km/h. We measured submaximal oxygen uptake and carbon dioxide production during minutes 3–5 and averaged energetic cost (W/kg) for the two trials in each shoe model.

Results

Compared with the established racing shoes, the new shoes reduced the energetic cost of running in all 18 subjects tested. Averaged across all three velocities, the energetic cost for running in the NP shoes (16.45 ± 0.89 W/kg; mean ± SD) was 4.16 and 4.01% lower than in the NS and AB shoes, when shoe mass was matched (17.16 ± 0.92 and 17.14 ± 0.97 W/kg, respectively, both p < 0.001). The observed percent changes were independent of running velocity (14–18 km/h).

Conclusion

The prototype shoes lowered the energetic cost of running by 4% on average. We predict that with these shoes, top athletes could run substantially faster and achieve the first sub-2-hour marathon.

Spatiotemporal parameters in sprinters with unilateral and bilateral transfemoral amputations and functional impairments

PURPOSE

Although sprinters with unilateral (UTF) and bilateral transfemoral (BTF) amputations and functional impairments (FIs) without amputation were allocated into different classifications because of the recent revision of the International Paralympic Committee Athletics Rules and Regulations, it is unclear whether running mechanics differ among the three groups. The aim of this study was to investigate the differences in the spatiotemporal parameters of the three groups during 100-m sprint in official competitions.

METHODS

Using publicly available Internet broadcasts, we analyzed 11 elite-level sprinters with UTF amputation, 4 sprinters with BTF amputation, and 5 sprinters with FI without amputation. The best personal times for nearly all individuals were included. For each sprinter's race, the average speed, step frequency, and step length were calculated using the number of steps in conjunction with the official race time.

RESULTS

Although there were no significant differences in the average speed among the UTF, BTF, and FI groups (7.95 ± 0.22, 7.90 ± 0.42, and 7.93 ± 0.14 m/s, respectively, p = 0.87), those with BTF amputation showed significantly lower step frequency (UTF: 4.20 ± 0.20 Hz, BTF: 3.71 ± 0.32 Hz, FI: 4.20 ± 0.10 Hz, p < 0.05) and longer step length (UTF: 1.90 ± 0.08 m, BTF: 2.14 ± 0.02 m, FI: 1.89 ± 0.06 m, p < 0.05) than the other two groups.

CONCLUSION

These results suggest that the step characteristics during sprinting are not the same among sprinters with UTF amputation, BTF amputations, or FI without amputations.

The biomechanics of the fastest sprinter with a unilateral transtibial amputation

People have debated whether athletes with transtibial amputations should compete with nonamputees in track events despite insufficient information regarding how the use of running-specific prostheses (RSPs) affect athletic performance. Thus, we sought to quantify the spatiotemporal variables, ground reaction forces, and spring-mass mechanics of the fastest athlete with a unilateral transtibial amputation using an RSP to reveal how he adapts his biomechanics to achieve elite running speeds. Accordingly, we measured ground reaction forces during treadmill running trials spanning 2.87 to 11.55 m/s of the current male International Paralympic Committee T44 100- and 200-m world record holder. To achieve faster running speeds, the present study’s athlete increased his affected leg (AL) step lengths ( P < 0.001) through longer contact lengths ( P < 0.001) and his unaffected leg (UL) step lengths ( P < 0.001) through longer contact lengths ( P < 0.001) and greater stance average vertical ground reaction forces ( P < 0.001). At faster running speeds, step time decreased for both legs ( P < 0.001) through shorter ground contact and aerial times ( P < 0.001). Unlike athletes with unilateral transtibial amputations, this athlete maintained constant AL and UL stiffness across running speeds ( P ≥ 0.569). Across speeds, AL step lengths were 8% longer ( P < 0.001) despite 16% lower AL stance average vertical ground reaction forces compared with the UL ( P < 0.001). The present study’s athlete exhibited biomechanics that differed from those of athletes with bilateral and without transtibial amputations. Overall, we present the biomechanics of the fastest athlete with a unilateral transtibial amputation, providing insight into the functional abilities of athletes with transtibial amputations using running-specific prostheses.

NEW & NOTEWORTHY The present study’s athlete achieved the fastest treadmill running trial ever attained by an individual with a leg amputation (11.55 m/s). From 2.87 to 11.55 m/s, the present study’s athlete maintained constant affected and unaffected leg stiffness, which is atypical for athletes with unilateral transtibial amputations. Furthermore, the asymmetric vertical ground reaction forces of athletes with unilateral transtibial amputations during running may be the result of leg length discrepancies.

How do prosthetic stiffness, height and running speed affect the biomechanics of athletes with bilateral transtibial amputations?

Limited available information describes how running-specific prostheses and running speed affect the biomechanics of athletes with bilateral transtibial amputations. Accordingly, we quantified the effects of prosthetic stiffness, height and speed on the biomechanics of five athletes with bilateral transtibial amputations during treadmill running. Each athlete performed a set of running trials with 15 different prosthetic model, stiffness and height combinations. Each set of trials began with the athlete running on a force-measuring treadmill at 3 m s^-1, subsequent trials incremented by 1 m s^-1 until they achieved their fastest attainable speed. We collected ground reaction forces (GRFs) during each trial. Prosthetic stiffness, height and running speed each affected biomechanics. Specifically, with stiffer prostheses, athletes exhibited greater peak and stance average vertical GRFs (β = 0.03; p < 0.001), increased overall leg stiffness (β = 0.21; p < 0.001), decreased ground contact time (β = -0.07; p < 0.001) and increased step frequency (β = 0.042; p < 0.001). Prosthetic height inversely associated with step frequency (β = -0.021; p < 0.001). Running speed inversely associated with leg stiffness (β = -0.58; p < 0.001). Moreover, at faster running speeds, the effect of prosthetic stiffness and height on biomechanics was mitigated and unchanged, respectively. Thus, prosthetic stiffness, but not height, likely influences distance running performance more than sprinting performance for athletes with bilateral transtibial amputations.

Reduced prosthetic stiffness lowers the metabolic cost of running for athletes with bilateral transtibial amputations

Inspired by the springlike action of biological legs, running-specific prostheses are designed to enable athletes with lower-limb amputations to run. However, manufacturer’s recommendations for prosthetic stiffness and height may not optimize running performance. Therefore, we investigated the effects of using different prosthetic configurations on the metabolic cost and biomechanics of running. Five athletes with bilateral transtibial amputations each performed 15 trials on a force-measuring treadmill at 2.5 or 3.0 m/s. Athletes ran using each of 3 different prosthetic models (Freedom Innovations Catapult FX6, Össur Flex-Run, and Ottobock 1E90 Sprinter) with 5 combinations of stiffness categories (manufacturer’s recommended and ± 1) and heights (International Paralympic Committee’s maximum competition height and ± 2 cm) while we measured metabolic rates and ground reaction forces. Overall, prosthetic stiffness [fixed effect (β) = 0.036; P = 0.008] but not height ( P ≥ 0.089) affected the net metabolic cost of transport; less stiff prostheses reduced metabolic cost. While controlling for prosthetic stiffness (in kilonewtons per meter), using the Flex-Run (β = −0.139; P = 0.044) and 1E90 Sprinter prostheses (β = −0.176; P = 0.009) reduced net metabolic costs by 4.3–4.9% compared with using the Catapult prostheses. The metabolic cost of running improved when athletes used prosthetic configurations that decreased peak horizontal braking ground reaction forces (β = 2.786; P = 0.001), stride frequencies (β = 0.911; P < 0.001), and leg stiffness values (β = 0.053; P = 0.009). Remarkably, athletes did not maintain overall leg stiffness across prosthetic stiffness conditions. Rather, the in-series prosthetic stiffness governed overall leg stiffness. The metabolic cost of running in athletes with bilateral transtibial amputations is influenced by prosthetic model and stiffness but not height.

NEW & NOTEWORTHY We measured the metabolic rates and biomechanics of five athletes with bilateral transtibial amputations while running with different prosthetic configurations. The metabolic cost of running for these athletes is minimized by using an optimal prosthetic model and reducing prosthetic stiffness. The metabolic cost of running was independent of prosthetic height, suggesting that longer legs are not advantageous for distance running. Moreover, the in-series prosthetic stiffness governs the leg stiffness of athletes with bilateral leg amputations.

Prosthetic model, but not stiffness or height, affects the metabolic cost of running for athletes with unilateral transtibial amputations

Running-specific prostheses enable athletes with lower limb amputations to run by emulating the spring-like function of biological legs. Current prosthetic stiffness and height recommendations aim to mitigate kinematic asymmetries for athletes with unilateral transtibial amputations. However, it is unclear how different prosthetic configurations influence the biomechanics and metabolic cost of running. Consequently, we investigated how prosthetic model, stiffness, and height affect the biomechanics and metabolic cost of running. Ten athletes with unilateral transtibial amputations each performed 15 running trials at 2.5 or 3.0 m/s while we measured ground reaction forces and metabolic rates. Athletes ran using three different prosthetic models with five different stiffness category and height combinations per model. Use of an Ottobock 1E90 Sprinter prosthesis reduced metabolic cost by 4.3 and 3.4% compared with use of Freedom Innovations Catapult [fixed effect (β) = -0.177; P < 0.001] and Össur Flex-Run (β = -0.139; P = 0.002) prostheses, respectively. Neither prosthetic stiffness (P ≥ 0.180) nor height (P = 0.062) affected the metabolic cost of running. The metabolic cost of running was related to lower peak (β = 0.649; P = 0.001) and stance average (β = 0.772; P = 0.018) vertical ground reaction forces, prolonged ground contact times (β = -4.349; P = 0.012), and decreased leg stiffness (β = 0.071; P < 0.001) averaged from both legs. Metabolic cost was reduced with more symmetric peak vertical ground reaction forces (β = 0.007; P = 0.003) but was unrelated to stride kinematic symmetry (P ≥ 0.636). Therefore, prosthetic recommendations based on symmetric stride kinematics do not necessarily minimize the metabolic cost of running. Instead, an optimal prosthetic model, which improves overall biomechanics, minimizes the metabolic cost of running for athletes with unilateral transtibial amputations.NEW & NOTEWORTHY The metabolic cost of running for athletes with unilateral transtibial amputations depends on prosthetic model and is associated with lower peak and stance average vertical ground reaction forces, longer contact times, and reduced leg stiffness. Metabolic cost is unrelated to prosthetic stiffness, height, and stride kinematic symmetry. Unlike nonamputees who decrease leg stiffness with increased in-series surface stiffness, biological limb stiffness for athletes with unilateral transtibial amputations is positively correlated with increased in-series (prosthetic) stiffness.