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.

Combining an Artificial Gastrocnemius and Powered Ankle Prosthesis: Effects on Transtibial Prosthesis User Gait.

Walking is more difficult for transtibial prosthesis users, partly due to a lack of calf muscle function. Powered ankle prostheses can partially restore calf muscle function, specifically push-off power from the soleus. But one limitation of a powered ankle is that emulating the soleus does not restore the multi-articular function of the gastrocnemius. This missing function may explain elevated hip and knee muscle demands observed in individuals walking on powered ankles. These elevated demands can make walking more fatiguing and impact mobility. Adding an Artificial Gastrocnemius to a powered ankle might improve gait for prosthesis users by reducing the prosthesis-side hip and knee demands. This work investigates if an Artificial Gastrocnemius reduced prosthesis-side hip or knee demands for individuals walking with a powered ankle providing high levels of push-off. We performed two case series studies that examined the effects that a passive elastic Artificial Gastrocnemius has on joint moment-impulses when prosthesis users walked with a powered ankle. We found that hip moment-impulse was reduced during stance when walking with an Artificial Gastrocnemius for six of seven participants. The Artificial Gastrocnemius effects on knee kinetics were variable and subject-specific, but in general, it did not reduce the knee flexor or extensor demands. The Artificial Gastrocnemius should be further explored to determine if reduced hip demands improve mobility or the user's quality of life by increasing the distance they can walk, increasing walking economy, or leading to increased physical activity or community engagement.

Humans trade off whole-body energy cost to avoid overburdening muscles while walking.

Metabolic cost minimization is thought to underscore the neural control of locomotion. Yet, avoiding high muscle activation, a cause of fatigue, often outperforms energy minimization in computational predictions of human gait. Discerning the relative importance of these criteria in human walking has proved elusive, in part, because they have not been empirically decoupled. Here, we explicitly decouple whole-body metabolic cost and 'fatigue-like' muscle activation costs (estimated from electromyography) by pitting them against one another using two distinct gait tasks. When experiencing these competing costs, participants (n = 10) chose the task that avoided overburdening muscles (fatigue avoidance) at the expense of higher metabolic power (p < 0.05). Muscle volume-normalized activation more closely models energy use and was also minimized by the participants' decision (p < 0.05), demonstrating that muscle activation was, at best, an inaccurate signal for metabolic energy. Energy minimization was only observed when there was no adverse effect on muscle activation costs. By decoupling whole-body metabolic and muscle activation costs, we provide among the first empirical evidence of humans embracing non-energetic optimality in favour of a clearly defined neuromuscular objective. This finding indicates that local muscle fatigue and effort may well be key factors dictating human walking behaviour and its evolution.

Is conservation of center of mass mechanics a priority in human walking? Insights from leg-length asymmetry experiments.

Center of mass (COM) control has been proposed to serve economy- and stability-related locomotor task objectives. However, given the lack of evidence supporting direct sensing and/or regulation of the COM, it remains unclear whether COM mechanics are prioritized in the control scheme of walking. We posit that peripheral musculoskeletal structures, e.g. muscle, are more realistic control targets than the COM, given their abundance of sensorimotor receptors and ability to influence whole-body energetics. As a first test of this hypothesis, we examined whether conservation of stance-phase joint mechanics is prioritized over COM mechanics in a locomotor task where simultaneous conservation of COM and joint mechanics is not feasible: imposed leg-length asymmetry. Positive joint mechanical cost of transport (work per distance traveled; COTJNT) was maintained at values closer to normal walking than COM mechanical cost of transport (COTCOM; P<0.05, N=15). Furthermore, compared with our measures of COM mechanics (COTCOM, COM displacement), joint-level variables (COTJNT, integrated total support moment) also displayed stronger conservation (less change from normal walking) when the participants' self-selected gait was assessed against other possible gait solutions. We conclude that when walking humans are exposed to an asymmetric leg-length perturbation, control of joint mechanics is prioritized over COM mechanics. Our results suggest that mechanical and metabolic effort is likely regulated via control of peripheral structures and not directly at the level of the COM. Joint mechanics may provide a more accurate representation of the underlying locomotor control targets and may prove advantageous in informing predictive models of human walking.

Pole Dancing for Fitness: The Physiological and Metabolic Demand of a 60-Minute Class.

Nicholas, JC, McDonald, KA, Peeling, P, Jackson, B, Dimmock, JA, Alderson, JA, and Donnelly, CJ. Pole dancing for fitness: The physiological and metabolic demand of a 60-minute class. J Strength Cond Res 33(10): 2704-2710, 2019-Little is understood about the acute physiological or metabolic demand of pole dancing classes. As such, the aims of this study were to quantify the demands of a standardized recreational pole dancing class, classifying outcomes according to American College of Sports Medicine (ACSM) exercise-intensity guidelines, and to explore differences in physiological and metabolic measures between skill- and routine-based class components. Fourteen advanced-level amateur female pole dancers completed three 60-minute standardized pole dancing classes. In one class, participants were fitted with a portable metabolic analysis unit. Overall, classes were performed at a mean VO2 of 16.0 ml·kg·min, total energy cost (EC) of 281.6 kcal (4.7 kcal·min), metabolic equivalent (METs) of 4.6, heart rate of 131 b·min, rate of perceived exertion (RPE) of 6.3/10, and blood lactate of 3.1 mM. When comparing skill- and routine-based components of the class, EC per minute (4.4 vs. 5.3 kcal·min), peak VO2 (21.5 vs. 29.6 ml·kg·min), METs (4.3 vs. 5.2), and RPE (7.2 vs. 8.4) were all greater in the routine-based component (p < 0.01), indicating that classes with an increased focus on routine-based training, as compared to skill-based training, may benefit those seeking to exercise at a higher intensity level, resulting in greater caloric expenditure. In accordance with ASCM guidelines, an advanced-level 60-minute pole dancing class can be classified as a moderate-intensity cardiorespiratory exercise; when completed for ≥30 minutes, ≥5 days per week (total ≥150 minutes) satisfies the recommended level of exercise for improved health and cardiorespiratory fitness.

A comparison of haemolytic responses in fore-foot and rear-foot distance runners.

This study examined the haemolytic effects of an interval-based running task in fore-foot and rear-foot striking runners. Nineteen male distance runners (10 fore-foot, 9 rear-foot) completed 8 × 3 min repeats at 90% vVO2peak on a motorised treadmill. Pre- and post-exercise venous blood samples were analysed for serum haptoglobin to quantify the haemolytic response to running. Vertical ground reaction forces were also captured via a force plate beneath the treadmill belt. Haptoglobin levels were significantly decreased following exercise (P = 0.001) in both groups (but not between groups), suggesting that the running task created a haemolytic stress. The ground reaction force data showed strong effect sizes for a greater peak force (d = 1.20) and impulse (d = 1.37) in fore-foot runners, and a greater rate of force development (d = 2.74) in rear-foot runners. The lack of difference in haptoglobin response between groups may be explained by the trend for fore-foot runners to experience greater peak force and impulse during the stance phase of their running gait, potentially negating any impact of the greater rate of force development occurring from the rear-foot runners' heel strike. Neither type of runner (fore-foot or rear-foot) appears more susceptible to technique-related foot-strike haemolysis.

Lower-limb dominance does not explain subject-specific foot kinematic asymmetries observed during walking and running.

Studies of human locomotion have observed asymmetries in lower-limb kinematics, especially at the more distal joints. However, it is unclear whether these asymmetries are related to functional differences between the dominant and non-dominant limb. This study aimed to determine the effect of lower-limb dominance on foot kinematics during human locomotion. Range of motion for the metatarsophalangeal joint (MPJ) and medial longitudinal arch (MLA), as well as time duration of windlass mechanism engagement, were recorded from healthy young adults (N = 12) across a range of treadmill walking and running speeds. On the group level, there were no differences in MPJ or MLA range of motion, or windlass engagement timing, between the dominant and non-dominant limb (p > 0.05). While not explained by limb dominance, between-limb differences in MPJ and MLA ranges of motion were observed for individual participants on the order of ∼2-6°, which could be clinically relevant or impact interpretation of research data.

Biomechanical effects of adding an articulating toe joint to a passive foot prosthesis for incline and decline walking.

Walking on sloped surfaces is challenging for many lower limb prosthesis users, in part due to the limited ankle range of motion provided by typical prosthetic ankle-foot devices. Adding a toe joint could potentially benefit users by providing an additional degree of flexibility to adapt to sloped surfaces, but this remains untested. The objective of this study was to characterize the effect of a prosthesis with an articulating toe joint on the preferences and gait biomechanics of individuals with unilateral below-knee limb loss walking on slopes. Nine active prosthesis users walked on an instrumented treadmill at a +5° incline and -5° decline while wearing an experimental foot prosthesis in two configurations: a Flexible toe joint and a Locked-out toe joint. Three participants preferred the Flexible toe joint over the Locked-out toe joint for incline and decline walking. Eight of nine participants went on to participate in a biomechanical data collection. The Flexible toe joint decreased prosthesis Push-off work by 2 Joules during both incline (p = 0.008; g = -0.63) and decline (p = 0.008; g = -0.65) walking. During incline walking, prosthetic limb knee flexion at toe-off was 3° greater in the Flexible configuration compared to the Locked (p = 0.008; g = 0.42). Overall, these results indicate that adding a toe joint to a passive foot prosthesis has relatively small effects on joint kinematics and kinetics during sloped walking. This study is part of a larger body of work that also assessed the impact of a prosthetic toe joint for level and uneven terrain walking and stair ascent/descent. Collectively, toe joints do not appear to substantially or consistently alter lower limb mechanics for active unilateral below-knee prosthesis users. Our findings also demonstrate that user preference for passive prosthetic technology may be both subject-specific and task-specific. Future work could investigate the inter-individual preferences and potential benefits of a prosthetic toe joint for lower-mobility individuals.

Unilateral transtibial prosthesis users load their intact limb more than their prosthetic limb during sit-to-stand, squatting, and lifting.

BACKGROUND: Lower limb prosthesis users exhibit high rates of joint pain and disease, such as osteoarthritis, in their intact limb. Overloading of their intact limb during daily activities may be a contributing factor. Limb loading biomechanics have been extensively studied during walking, but fewer investigations into limb loading during other functional movements exist. The purpose of this study was to characterize the lower limb loading of transtibial prosthesis users during three common daily tasks: sit-to-stand, squatting, and lifting. METHODS: Eight unilateral transtibial prosthesis users performed sit-to-stand (from three chair heights), squatting, and lifting a 10 kg box. Peak vertical ground reaction forces and peak knee flexion moments were computed for each limb (intact and prosthetic) to characterize limb loading and asymmetry. Ranges of motion of the intact and prosthetic ankles were also quantified. FINDINGS: Users had greater peak ground reaction forces and knee flexion moments in their intact limb for all tasks (p < 0.02). On average, the intact limb had 36-48% greater peak ground reaction forces and 168-343% greater peak knee flexion moments compared to the prosthetic limb. The prosthetic ankle provided <10° of ankle range of motion for all tasks, less than half the range of motion provided by the intact ankle. INTERPRETATION: Prosthesis users overloaded their intact limb during all tasks. This asymmetric loading may lead to an accumulation of damage to the intact limb joints, such as the knee, and may contribute to the development of osteoarthritis. Prosthetic design and rehabilitation interventions that promote more symmetric loading should be investigated for these tasks.

Transtibial prosthetic socket fitting: Australian prosthetist perspectives on primary challenges, management strategies, and opportunities for workflow and technological innovation.

BACKGROUND: Following transtibial amputation, a custom-built socket is the most common interface between the prosthesis and residual limb. Desire from both prosthetists and prosthesis users for improved socket fitting processes have been well documented. However, there is currently limited information available about prosthetists' experiences of how prosthetic manufacturing workflow can contribute to socket fit problems. OBJECTIVES: This study aims to determine how socket fit problems are currently detected and managed by prosthetists and to identify challenges, management strategies, and opportunities for workflow and technological innovation during prosthesis manufacture and socket fitting. STUDY DESIGN: Mixed-method (quantitative and qualitative) survey. METHODS: An online survey was developed and piloted in consultation with members of the Australian Orthotic Prosthetic Association. The final 25-question survey was distributed through their membership database. Mixed methods were used to analyze survey items. Qualitative items were grouped and coded under themes relating to challenges, management strategies, and opportunities. Quantitative data were analyzed using nonparametric descriptive methods. RESULTS: Twenty-three respondents with a range of experience completed the survey. Seven of eight major Australian states/territories were represented. Primary workflow stages presenting challenges with limited strategies/solutions available to the prosthetists were roll-on liner selection, mold or cast modifications, communication with the client, and check socket fitting. Suggested solutions included improved socket-limb interface monitoring technology. CONCLUSIONS: This study provides the first insights into prosthetist-identified challenges and limitations at different stages of the socket workflow and presents a starting point for more targeted research into innovation that may assist in these processes.

Biomechanical effects of an articulating prosthetic toe joint during stair navigation for individuals with unilateral, below-knee limb loss.

Stair navigation is an essential and demanding form of locomotion. During stair ascent and descent, persons with lower limb loss exhibit gait characteristics which may increase their risk of falls and joint degeneration of the intact limb. To reduce deviations from typically-able-bodied gait and overloading of the intact limb for this population, one potential intervention involves modifying passive prosthetic feet by incorporating a flexible toe joint that simulates the biological metatarsophalangeal joint. In this study, we aimed to assess the user preferences and biomechanical effects of a flexible prosthetic toe joint during stair ascent and descent for persons with unilateral lower-limb loss. Nine participants with unilateral lower-limb loss were recruited (Male; Medicare Functional Classification Level: eight K4, one K3; age: 41 ± 11 years; mass: 95 ± 13 kg; height: 1.84 ± 0.05 m; mean ± SD). No significant changes in lower-limb joint mechanics were identified. Five of nine participants preferred the unmodified prosthesis with a standard carbon fiber keel for both stair ascent and descent. Varied user preferences and inconsistent changes in lower-limb joint parameters between participants highlight the importance of subject-specific analyses and individualized device prescription.

International Women in Biomechanics: Promoting, supporting, and sustaining the careers of women in biomechanics.

Gender biases and inequities are prevalent across many scientific fields and biomechanics is likely no exception. While progress has been made to support women in the field, especially at biomechanics society conferences, the recent COVID-19 pandemic has exacerbated professional isolation. The International Women in Biomechanics (IWB) community started in July 2020 with the mission of fostering an environment for women and other under-represented genders in biomechanics to gain year-round support, visibility, and allyship. Nearly 700 biomechanists have joined the IWB community from over 300 universities/organizations and 33 countries. Our community ranges in career stages and professions and interacts through a forum-style platform, teleconference meetings, and social media. In 2021, we conducted a survey to identify the needs, concerns, and issues faced by individuals in the IWB community. We received 144 responses from members in 16 countries. Our survey revealed three primary needs for women in biomechanics: supportive working environments, career planning support, and addressing workplace gender bias. These results, in conjunction with scientific evidence on workforce gender bias, helped us identify three key areas to meet our mission: Member Support, Community Outreach, and Empowering Allyship. Several levels of support are required in these three areas to ensure a lasting, positive, and sustainable impact on gender equity in biomechanics. We conclude by providing our perspectives on an evidence-based call to action to continue addressing gender bias and inequity at the individual, institutional, and scientific society levels. These actions can collectively enhance our allyship for women in the field of biomechanics.

Unilateral below-knee prosthesis users walking on uneven terrain: The effect of adding a toe joint to a passive prosthesis.

Lower limb prosthesis users cite uneven terrain as a challenging surface to walk on. We sought to determine whether adding a Flexible toe joint to a prosthetic foot would be preferred by unilateral below-knee prosthesis users relative to a Locked (non-articulating) toe joint for walking on uneven terrain. We also quantified lower limb joint kinetics for the Locked and Flexible toe joint configurations. Five of our nine participants preferred the Flexible toe joint when walking on uneven terrain, yet from a biomechanical standpoint, the reason for this is unclear. All participants exhibited reductions in prosthetic limb net positive hip joint work when walking with a Flexible toe joint (11%; p < 0.05). For other parameters (e.g., prosthetic side knee joint moments) we observed high inter-subject variation, which adds to a growing body of literature highlighting the need for subject-specific data analyses in lower limb prosthetics research.

Adding a toe joint to a prosthesis: walking biomechanics, energetics, and preference of individuals with unilateral below-knee limb loss.

Toe joints play an important functional role in able-bodied walking; however, for prosthesis users, the effect of adding a toe joint to a passive prosthetic foot remains largely unknown. The current study explores the kinematics, kinetics, rate of oxygen consumption and user preference of nine individuals with below-knee limb loss. Participants walked on a passive prosthetic foot in two configurations: with a Flexible, articulating toe joint and with a Locked-out toe joint. During level treadmill gait, participants exhibited a decrease in Push-Off work when using the Flexible toe joint prosthesis versus the Locked toe joint prosthesis: 16% less from the prosthesis (p = 0.004) and 10% less at the center of mass level (p = 0.039). However, between configurations, participants exhibited little change in other gait kinematics or kinetics, and no apparent or consistent difference in the rate of oxygen consumption (p = 0.097). None of the traditional biomechanical or metabolic outcomes seemed to explain user preference. However, an unexpected and intriguing observation was that all participants who wore the prosthesis on their dominant limb preferred the Flexible toe joint, and every other participant preferred the Locked configuration. Although perhaps coincidental, such findings may suggest a potential link between user preference and limb dominance, offering an interesting avenue for future research.

Characterizing the comfort limits of forces applied to the shoulders, thigh and shank to inform exosuit design.

Recent literature emphasizes the importance of comfort in the design of exosuits and other assistive devices that physically augment humans; however, there is little quantitative data to aid designers in determining what level of force makes users uncomfortable. To help close this knowledge gap, we characterized human comfort limits when applying forces to the shoulders, thigh and shank. Our objectives were: (i) characterize the comfort limits for multiple healthy participants, (ii) characterize comfort limits across days, and (iii) determine if comfort limits change when forces are applied at higher vs. lower rates. We performed an experiment (N = 10) to quantify maximum tolerable force pulling down on the shoulders, and axially along the thigh and shank; we termed this force the comfort limit. We applied a series of forces of increasing magnitude, using a robotic actuator, to soft sleeves around their thigh and shank, and to a harness on their shoulders. Participants were instructed to press an off-switch, immediately removing the force, when they felt uncomfortable such that they did not want to feel a higher level of force. On average, participants exhibited comfort limits of ~0.9-1.3 times body weight on each segment: 621±245 N (shoulders), 867±296 N (thigh), 702±220 N (shank), which were above force levels applied by exosuits in prior literature. However, individual participant comfort limits varied greatly (~250-1200 N). Average comfort limits increased over multiple days (p<3e-5), as users habituated, from ~550-700 N on the first day to ~650-950 N on the fourth. Specifically, comfort limits increased 20%, 35% and 22% for the shoulders, thigh and shank, respectively. Finally, participants generally tolerated higher force when it was applied more rapidly. These results provide initial benchmarks for exosuit designers and end-users, and pave the way for exploring comfort limits over larger time scales, within larger samples and in different populations.

Multi-objective control in human walking: insight gained through simultaneous degradation of energetic and motor regulation systems.

Minimization of metabolic energy is considered a fundamental principle of human locomotion, as demonstrated by an alignment between the preferred walking speed (PWS) and the speed incurring the lowest metabolic cost of transport. We aimed to (i) simultaneously disrupt metabolic cost and an alternate acute task requirement, namely speed error regulation, and (ii) assess whether the PWS could be explained on the basis of either optimality criterion in this new performance and energetic landscape. Healthy adults (N = 21) walked on an instrumented treadmill under normal conditions and, while negotiating a continuous gait perturbation, imposed leg-length asymmetry. Oxygen consumption, motion capture data and ground reaction forces were continuously recorded for each condition at speeds ranging from 0.6 to 1.8 m s-1, including the PWS. Both metabolic and speed regulation measures were disrupted by the perturbation (p < 0.05). Perturbed PWS selection did not exhibit energetic prioritization (although we find some indication of energy minimization after motor adaptation). Similarly, PWS selection did not support prioritization of speed error regulation, which was found to be independent of speed in both conditions. It appears that, during acute exposure to a mechanical gait perturbation of imposed leg-length asymmetry, humans minimize neither energetic cost nor speed regulation errors. Despite the abundance of evidence pointing to energy minimization during normal, steady-state gait, this may not extend acutely to perturbed gait. Understanding how the nervous system acutely controls gait perturbations requires further research that embraces multi-objective control paradigms.

Unholey shoes: Experimental considerations when estimating ankle joint complex power during walking and running.

For studies that aim to assess biological ankle function, calculating ankle joint complex (AJC) power between the calcaneus and shank is recommended over conventional inverse dynamics estimates between a rigid-body foot and shank. However, when designing a new experiment, it remains unclear whether holes should be cut in footwear to permit motion tracking via skin-mounted markers, or whether marker placement locations should be tightly controlled across conditions. Here we provide data to assist researchers in answering these questions. We performed a gait analysis study of walking (0.8, 1.2, 1.6 m·s-1) and running (2.6, 2.8, 3.0 m·s-1) while subjects (N = 10) wore custom-modified footwear, which allowed markers to be placed either on the shoe, or on the skin via cut-out windows in the shoes. First, we compared foot markers affixed to the skin vs. on the same locations on the shoe. Using statistical non-parametric mapping techniques, we discovered that skin vs. shoe markers had no statistically significant effect on net AJC power estimates throughout stance phase, for all walking and running speeds. Second, we compared calcaneal markers in the nominal shoe configuration vs. markers in a nearby location (∼27 mm below) on the shoe. We observed significant differences when marker placement on the shoe was varied, which may be relevant to repeated-measures study designs. The results suggest that when computing AJC power for walking and running, you may want to put down the scissors (i.e., forego cutting holes in your footwear), and instead pick up a Sharpie® (pen) or use a template, to maintain consistent marker placement across trials and conditions.

The Role of Arch Compression and Metatarsophalangeal Joint Dynamics in Modulating Plantar Fascia Strain in Running.

Elastic energy returned from passive-elastic structures of the lower limb is fundamental in lowering the mechanical demand on muscles during running. The purpose of this study was to investigate the two length-modulating mechanisms of the plantar fascia, namely medial longitudinal arch compression and metatarsophalangeal joint (MPJ) excursion, and to determine how these mechanisms modulate strain, and thus elastic energy storage/return of the plantar fascia during running. Eighteen runners (9 forefoot and 9 rearfoot strike) performed three treadmill running trials; unrestricted shod, shod with restricted arch compression (via an orthotic-style insert), and barefoot. Three-dimensional motion capture and ground reaction force data were used to calculate lower limb kinematics and kinetics including MPJ angles, moments, powers and work. Estimates of plantar fascia strain due to arch compression and MPJ excursion were derived using a geometric model of the arch and a subject-specific musculoskeletal model of the plantar fascia, respectively. The plantar fascia exhibited a typical elastic stretch-shortening cycle with the majority of strain generated via arch compression. This strategy was similar in fore- and rear-foot strike runners. Restricting arch compression, and hence the elastic-spring function of the arch, was not compensated for by an increase in MPJ-derived strain. In the second half of stance the plantar fascia was found to transfer energy between the MPJ (energy absorption) and the arch (energy production during recoil). This previously unreported energy transfer mechanism reduces the strain required by the plantar fascia in generating useful positive mechanical work at the arch during running.

The Foot's Arch and the Energetics of Human Locomotion.

The energy-sparing spring theory of the foot's arch has become central to interpretations of the foot's mechanical function and evolution. Using a novel insole technique that restricted compression of the foot's longitudinal arch, this study provides the first direct evidence that arch compression/recoil during locomotion contributes to lowering energy cost. Restricting arch compression near maximally (~80%) during moderate-speed (2.7 ms(-1)) level running increased metabolic cost by + 6.0% (p < 0.001, d = 0.67; unaffected by foot strike technique). A simple model shows that the metabolic energy saved by the arch is largely explained by the passive-elastic work it supplies that would otherwise be done by active muscle. Both experimental and model data confirm that it is the end-range of arch compression that dictates the energy-saving role of the arch. Restricting arch compression had no effect on the cost of walking or incline running (3°), commensurate with the smaller role of passive-elastic mechanics in these gaits. These findings substantiate the elastic energy-saving role of the longitudinal arch during running, and suggest that arch supports used in some footwear and orthotics may increase the cost of running.