Biomechanics and energetics of walking on uneven terrain

Energetics of Walking With a Robotic Knee Exoskeleton

The authors tested 4 young healthy subjects walking with a powered knee exoskeleton to determine if it could reduce the metabolic cost of locomotion. Subjects walked with a backpack loaded and unloaded, on a treadmill with inclinations of 0° and 15°, and outdoors with varied natural terrain. Participants walked at a self-selected speed (average 1.0 m/s) for all conditions, except incline treadmill walking (average 0.5 m/s). The authors hypothesized that the knee exoskeleton would reduce the metabolic cost of walking uphill and with a load compared with walking without the exoskeleton. The knee exoskeleton reduced metabolic cost by 4.2% in the 15° incline with the backpack load. All other conditions had an increase in metabolic cost when using the knee exoskeleton compared with not using the exoskeleton. There was more variation in metabolic cost over the outdoor walking course with the knee exoskeleton than without it. Our findings indicate that powered assistance at the knee is more likely to decrease the metabolic cost of walking in uphill conditions and during loaded walking rather than in level conditions without a backpack load. Differences in positive mechanical work demand at the knee for varying conditions may explain the differences in metabolic benefit from the exoskeleton.

Reducing the metabolic rate of walking and running with a versatile, portable exosuit

Walking and running have fundamentally different biomechanics, which makes developing devices that assist both gaits challenging. We show that a portable exosuit that assists hip extension can reduce the metabolic rate of treadmill walking at 1.5 meters per second by 9.3% and that of running at 2.5 meters per second by 4.0% compared with locomotion without the exosuit. These reduction magnitudes are comparable to the effects of taking off 7.4 and 5.7 kilograms during walking and running, respectively, and are in a range that has shown meaningful athletic performance changes. The exosuit automatically switches between actuation profiles for both gaits, on the basis of estimated potential energy fluctuations of the wearer's center of mass. Single-participant experiments show that it is possible to reduce metabolic rates of different running speeds and uphill walking, further demonstrating the exosuit's versatility.

Effect of Common Pavements on Interjoint Coordination of Walking with and without Robotic Exoskeleton

Background

The analysis and comprehension of the coordination control of a human gait on common grounds benefit the development of robotic exoskeleton for motor recovery.

Objective

This study investigated whether the common grounds effect the interjoint coordination of healthy participants with/without exoskeletons in walking.

Methods

The knee-ankle coordination and hip-knee coordination of 8 healthy participants in a sagittal plane were measured on five kinds of pavements (tiled, carpet, wooden, concrete, and pebbled) with/without exoskeletons, using the continuous relative phase (CRP). The root mean square of CRP (CRP_RMS) over each phase of the gait cycle is used to analyze the magnitude of dephasing between joints, and the standard deviation of CRP (CRP_SD) in the full gait cycle is used to assess the variability of coordination patterns between joints.

Results

The CRP_Hip-Knee/RMS of the carpet pavement with exoskeleton is different from that of other pavements (except the tiled pavement) in the midstance phase. The CRP_Hip-Knee/RMS on the pebble pavement without exoskeleton is less than that on the other pavements in all phases. The CRP_Hip-Knee/SD of the pebble pavement without exoskeleton is smaller than that of other pavements. The CRP_{Knee-Ankle/SD} with/without exoskeleton is similar across all pavements.

Conclusion

The compressive capacity of the pavement and the unevenness of the pavement are important factors that influence interjoint coordination, which can be used as key control elements of gait to adapt different pavements for robotic exoskeleton.

Novelty

We provide a basis of parameter change of kinematics on different common grounds for the design and optimization of robotic exoskeleton for motor recovery.

Locomotion of Ants Walking up Slippery Slopes of Granular Materials

Many insects encounter locomotory difficulties in walking up sand inclines. This is masterfully exploited by some species for building traps from which prey are rarely able to escape, as the antlion and its deadly pit. The aim of this work is to tear apart the relative roles of granular material properties and slope steepness on the insect leg kinematics, gait patterns, and locomotory stability. For this, we used factorial manipulative experiments with different granular media inclines and the ant Aphaenogaster subterranea. Our results show that its locomotion is similar on granular and solid media, while for granular inclined slopes we observe a loss of stability followed by a gait pattern transition from tripod to metachronal. This implies that neither the discrete nature nor the roughness properties of sand alone are sufficient to explain the struggling of ants on sandy slopes: the interaction between sand properties and slope is key. We define an abnormality index that allows us to quantify the locomotory difficulties of insects walking up a granular incline. The probability of its occurrence reveals the local slipping of the granular media as a consequence of the pressure exerted by the ant's legs. Our findings can be extended to other models presenting locomotory difficulties for insects, such as slippery walls of urns of pitcher plants. How small arthropods walking on granular and brittle materials solve their unique stability trade-off will require a thorough understanding of the transfer of energy from leg to substrate at the particle level.

Terrestrial locomotion of the Svalbard rock ptarmigan: comparing field and laboratory treadmill studies

Research into the terrestrial locomotion of birds is often based upon laboratory treadmill experiments. However, it is unclear how transposable these results are for birds moving in the wild. Here, using video recordings, we compared the kinematics of locomotion (stride frequency, stride length, stance phase, swing phase, duty factor) and speed range of Svalbard rock ptarmigan (Lagopus muta hyperborea) under field and laboratory treadmill conditions. Our findings indicate that the kinematics of walking and aerial running are conserved when moving on the treadmill and in the field. Differences, however, were found when grounded running under the two conditions, linked to substrate. Substrate effects were confirmed by analysing trials only moving over very hard snow. In line with laboratory treadmill energetic predictions, wild ptarmigan have a preferred speed during walking and to a lesser extent when aerial running but not when moving with a grounded running gait. The birds were also capable of a higher top speed in the field than that observed during treadmill studies. Our findings demonstrate that laboratory treadmill research provides meaningful information relevant to wild birds while highlighting the importance of understanding the substrate the animals are moving over.

Locomotor patterns change over time during walking on an uneven surface

During walking, uneven surfaces impose new demands for controlling balance and forward progression at each step. It is unknown to what extent walking may be refined given an amount of stride-to-stride unpredictability at the distal level. Here, we explored the effects of an uneven terrain surface on whole-body locomotor dynamics immediately following exposure and after a familiarization period. Eleven young, unimpaired adults walked for 12 min on flat and uneven terrain treadmills. The whole-body center of mass excursion range (COM_exc) and peak velocity (COM_vel), step length and width were estimated. On first exposure to uneven terrain, we saw significant increases in medial-lateral COM_exc and lateral COM_vel, and in the variability of COM_exc, COM_vel and foot placement in both anterior-posterior and medial-lateral directions. Increases in step width and decreases in step length supported the immediate adoption of a cautious, restrictive solution on uneven terrain. After familiarization, step length increased and the variability of anterior-posterior COM_vel and step length reduced, while step width and lateral COM_vel reduced, alluding to a refinement of movement and a reduction of conservative strategies over time. However, the variability of medial-lateral COM_exc and lateral COM_vel increased, consistent with the release of previously constrained degrees of freedom. Despite this increase in variability, a strong relationship between step width and medial-lateral center of mass movement was maintained. Our results indicate that movement strategies of unimpaired adults when walking on uneven terrain can evolve over time with longer exposure to the surface.

Investigation of gait cycle deviation over surface irregularities utilizing muscle activities

BACKGROUND

Human beings regularly walk over even and uneven surfaces during their daily activities. A human being with lower limb disability needs an exoskeleton to walk independently. However, walking surface irregularities increase the risk of falling of exoskeleton users. This falling tendency can be minimized by balancing the exoskeleton on irregular surface profiles against the gait cycle variation. Gait variation is studied using quality EMG signals obtained from the gastrocnemius and hamstring muscle activity during uneven surface walking.

OBJECTIVE

The present study compares the activity of hamstring and gastrocnemius muscles during walking on a treadmill, utilizing both even and uneven planes.

METHODS

Integrated electromyography signals from eight healthy male subjects are collected while walking on a treadmill, even and uneven planes. Muscle activity variation on these planes is studied using two-way ANOVA with replications.

RESULTS

The results show that hamstring muscle activity registers a sound variation in swing phase but has no variation in stance phase over all three planes, whereas gastrocnemius muscle activity changes between swing and stance phases over even and uneven planes during forward walking.

CONCLUSIONS

The results illustrate that the gait cycle variation depends on surface irregularities which indicates the importance of surface consideration.

Dynamics and stability of running on rough terrains

Stability of running on rough terrain depends on the propagation of perturbations due to the ground. We consider stability within the sagittal plane and model the dynamics of running as a two-dimensional body with alternating aerial and stance phases. Stance is modelled as a passive, impulsive collision followed by an active, impulsive push-off that compensates for collisional losses. Such a runner has infinitely many strategies to maintain periodic gaits on flat ground. However, these strategies differ in how perturbations due to terrain unevenness are propagated. Instabilities manifest as tumbling (orientational instability) or failing to maintain a steady speed (translational instability). We find that open-loop strategies that avoid sensory feedback are sufficient to maintain stability on step-like terrains with piecewise flat surfaces that randomly vary in height. However, these open-loop runners lose orientational stability on rough terrains whose slope also varies randomly. The orientational instability is significantly mitigated by minimizing the tangential collision, which typically requires sensory information and anticipatory strategies such as leg retraction. By analysing the propagation of perturbations, we derive a single dimensionless parameter that governs stability. This parameter provides guidelines for the design and control of both biological and robotic runners.

Preferred walking speed on rough terrain; is it all about energetics?

Humans have evolved the ability to walk very efficiently. Further, humans prefer to walk at speeds that approximately minimize their metabolic energy expenditure per unit distance (i.e. gross cost of transport, COT). This has been found in a variety of population groups and other species. However, these studies were mostly performed on smooth, level ground or on treadmills. We hypothesized that the objective function for walking is more complex than only minimizing the COT. To test this idea, we compared the preferred speeds and the relationships between COT and speed for people walking on both a smooth, level floor and a rough, natural terrain trail. Rough terrain presumably introduces other factors, such as stability, to the objective function. 10 healthy men walked on both a straight, flat, smooth floor and on an outdoor trail strewn with rocks and boulders. In both locations, subjects performed 5-7 trials at different speeds relative to their preferred speed. The COT-speed relationships were similarly U-shaped for both surfaces, but the COT values on rough terrain were approximately 115% greater. On the smooth surface, the preferred speed (1.24+/−0.17 m/sec) was found to be statistically not different (p-value =0.09) than the speed that minimized COT (1.34 +/− 0.03 m/sec). On rough terrain, the preferred speed (1.07+/−0.05 m/sec) was slower than the COT minimum speed (1.13 +/− 0.07 m/sec) and was statistical significant (p-value=0.02). Since near the optimum speed the COT function is very shallow, these changes in speed result in small change in COT (0.5%). It appears that the objective function for speed preference when walking on rough terrain includes COT and additional factors such as stability.

Interaction between step-to-step variability and metabolic cost of transport during human walking

Minimizing the metabolic cost of transport can affect selection of the preferred walking speed. While many factors can affect metabolic cost of transport during human walking, its interaction with step-to-step variability is unclear. Here, we aimed to determine the interaction between metabolic cost of transport and step length variability during human walking at different speeds. In particular, two aspects of step length variability were analyzed: the amount of variations ('variations') and the organization of the step-to-step fluctuations ('fluctuations'). Ten healthy, young participants walked on a treadmill at five speeds, ranging from 0.75 to 1.75 m s^-1 Metabolic cost of transport, step length variations (coefficient of variation) and step length fluctuations (quantified via detrended fluctuation analysis) were calculated. A mixed-model ANOVA revealed that variations and walking speed were strong predictors of metabolic cost of transport (R ²=0.917, P<0.001), whereas fluctuations were not. Preferred walking speed (1.05±0.20 m s^-1) was not significantly different from the speed at which metabolic cost of transport was minimized (1.04±0.05 m s^-1; P=0.792), nor from the speed at which fluctuations were most persistent (1.00±0.41 m s^-1; P=0.698). The minimization of variations occurred at a faster speed (1.56±0.17 m s^-1) than the preferred walking speed (P<0.001). Step length variations likely affect metabolic cost of transport because greater variations are indicative of suboptimal, mechanically inefficient steps. Fluctuations have little or no effect on metabolic cost of transport, but still may relate to preferred walking speed.

Linking Gait Dynamics to Mechanical Cost of Legged Locomotion

For millenia, legged locomotion has been of central importance to humans for hunting, agriculture, transportation, sport, and warfare. Today, the same principal considerations of locomotor performance and economy apply to legged systems designed to serve, assist, or be worn by humans in urban and natural environments. Energy comes at a premium not only for animals, wherein suitably fast and economical gaits are selected through organic evolution, but also for legged robots that must carry sufficient energy in their batteries. Although a robot's energy is spent at many levels, from control systems to actuators, we suggest that the mechanical cost of transport is an integral energy expenditure for any legged system—and measuring this cost permits the most direct comparison between gaits of legged animals and robots. Although legged robots have matched or even improved upon total cost of transport of animals, this is typically achieved by choosing extremely slow speeds or by using regenerative mechanisms. Legged robots have not yet reached the low mechanical cost of transport achieved at speeds used by bipedal and quadrupedal animals. Here we consider approaches used to analyze gaits and discuss a framework, termed mechanical cost analysis, that can be used to evaluate the economy of legged systems. This method uses a point mass perspective to evaluate the entire stride as well as to identify individual events that accrue mechanical cost. The analysis of gait began at the turn of the last century with spatiotemporal analysis facilitated by the advent of cine film. These advances gave rise to the “gait diagram,” which plots duty factors and phase separations between footfalls. This approach was supplanted in the following decades by methods using force platforms to determine forces and motions of the center of mass (CoM)—and analytical models that characterize gait according to fluctuations in potential and kinetic energy. Mechanical cost analysis draws from these approaches and provides a unified framework that interprets the spatiotemporal sequencing of leg contacts within the context of CoM dynamics to determine mechanical cost in every instance of the stride. Diverse gaits can be evaluated and compared in biological and engineered systems using mechanical cost analysis.

Optimal regulation of bipedal walking speed despite an unexpected bump in the road

Bipedal locomotion may occur over imperfect surfaces with bumps or other features that disrupt steady gait. An unexpected bump in the road is generally expected to slow down most types of locomotion. On wheels, speed may be regained quite readily with “cruise control” performed in continuous time. But legged locomotion is less straightforward, because the stance leg may be under-actuated, and the continuous-time dynamics are periodically disrupted by discrete ground contact events. Those events may also afford good control opportunities, albeit subject to the delay between discrete opportunities. The regulation of walking speed should ideally use these opportunities to compensate for lost time, and with good economy if possible. However, the appropriate control strategy is unknown. Here we present how to restore speed and make up for time lost going over a bump in the road, through discrete, once-per-step control. We use a simple dynamic walking model to determine the optimal sequence of control actions—pushing off from the leg at the end of each stance phase—for fast response or best economy. A two-step, deadbeat sequence is the fastest possible response, and reasonably economical. Slower responses over more steps are more economical overall, but a bigger difference is that they demand considerably less peak power. A simple, reactive control strategy can thus compensate for an unexpected bump, with explicit trade-offs in time and work. Control of legged locomotion is not as straightforward as with wheels, but discrete control actions also allow for effective and economical reactions to imperfect terrain.

Use of a powered ankle-foot prosthesis reduces the metabolic cost of uphill walking and improves leg work symmetry in people with transtibial amputations

People with transtibial amputations (TTAs) who use a powered ankle-foot prosthesis have equivalent metabolic costs and step-to-step transition work for level-ground walking over a range of speeds compared to non-amputees. The effects of using a powered compared to passive-elastic prosthesis for sloped walking are unknown. We sought to understand how the use of passive-elastic compared to powered ankle-foot prostheses affect metabolic cost and step-to-step transition work during sloped walking. Ten people (six M, four F) with TTAs walked 1.25 m s-1 at 0°, ±3°, ±6° and ±9° using their own passive-elastic prosthesis and the BiOM powered ankle-foot prosthesis, while we measured metabolic rates, kinematics and kinetics. We calculated net metabolic power, individual leg step-to-step transition work and individual leg net work symmetry. The net metabolic power was 5% lower during walking on +3° and +6° uphill slopes when subjects used the BiOM compared to their passive-elastic prosthesis (p < 0.05). The use of the BiOM compared to a passive-elastic prosthesis did not affect individual leg step-to-step transition work (p > 0.05), but did improve individual leg net work symmetry on +6° and +9° uphill slopes (p < 0.01). People with TTAs who use a powered ankle-foot prosthesis have the potential to reduce metabolic costs and increase symmetry during walking on uphill slopes.

Gait alterations on irregular surface in people with Parkinson's disease

BACKGROUND

Persons with Parkinson's disease are at high risk for fall-related injuries with a large proportion of falls occurring while walking, especially when the walking environments are complex. The aim of this study was to characterize gait parameters on irregular surface for persons with Parkinson's disease.

METHODS

Three-dimensional gait analysis was conducted for nine persons with Parkinson's disease and nine healthy age-matched adults on both regular and irregular surfaces. Repeated ANOVA and paired t-test were performed to determine the effect of surface and group for spatiotemporal, kinematic and stability variables.

FINDINGS

Individuals with Parkinson's disease showed a larger ratio of reduction for speed, cadence and step length than controls when the surface changed from regular to irregular. The ankle transverse range of motion and root mean square of trunk acceleration increased on irregular surface for both groups. Additionally, individuals with Parkinson's disease demonstrated a decreased knee sagittal range of motion and trunk frontal and transverse range of motion compared with controls, especially on the irregular surface.

INTERPRETATION

The irregular surface posed a greater challenge to maintain balance and stability for individuals with Parkinson's disease. A relatively small knee range of motion in the sagittal plane and large root mean square of trunk acceleration increased the potential fall risk for individuals with Parkinson's disease. This information improves the understanding of parkinsonian gait adaptations on irregular surfaces and may guide gait training and rehabilitation interventions for this high fall-risk population.

Magnitude and variability of gait characteristics when walking on an irregular surface at different speeds

Different modes of perturbations have been used to understand how individuals negotiate irregular surfaces, with a general notion that increased locomotion variability induces a positive training stimulus. Individuals tend to walk slower when initially exposed to such locomotion tasks, potentially influencing the magnitude and variability of biomechanical parameters. This study investigated theeffects of gait speed on lower extremity biomechanics when walking on an irregular (IS) and regular surface (RS). Twenty physically active males walked on a RS and IS at three different speeds (4 km/h, 5 km/h, 6 km/h). Lower extremity kinematics (300 Hz) and surface electromyography (3000 Hz) were recorded during the first 90 s of gait. Two-factor repeated measures ANOVA was used to determine surface and speed effects (p < 0.05). Gait speed influences walking biomechanics (kinematic and muscle activity parameters) the same irrespective of surface condition. As walking speed increased, sagittal shoe-surface angle, maximum ankle inversion, ankle abduction, knee and hip flexion increased during stance phase when walking on the IS and RS (p < 0.05). Increasing walking speed caused increased muscle activity of the tibialis anterior, peroneus longus, gastrocnemius medialis, vastus medialis and biceps femoris (p < 0.05) on the IS and RS during the gait cycle. Increased gait, kinematic and muscle activity variability was reported at lower walking speed on both the IS and RS. Further, irrespective of gait speed, walking on an IS triggers postural adjustments, higher muscle activity and increased gait variability compared to RS walking. Our findings suggest the benefits of training on the irregular surface may be further enhanced at slower gait speeds.

Challenging human locomotion: stability and modular organisation in unsteady conditions

The need to move over uneven terrain is a daily challenge. In order to face unexpected perturbations due to changes in the morphology of the terrain, the central nervous system must flexibly modify its control strategies. We analysed the local dynamic stability and the modular organisation of muscle activation (muscle synergies) during walking and running on an even- and an uneven-surface treadmill. We hypothesized a reduced stability during uneven-surface locomotion and a reorganisation of the modular control. We found a decreased stability when switching from even- to uneven-surface locomotion (p < 0.001 in walking, p = 0.001 in running). Moreover, we observed a substantial modification of the time-dependent muscle activation patterns (motor primitives) despite a general conservation of the time-independent coefficients (motor modules). The motor primitives were considerably wider in the uneven-surface condition. Specifically, the widening was significant in both the early (+40.5%, p < 0.001) and late swing (+7.7%, p = 0.040) phase in walking and in the weight acceptance (+13.6%, p = 0.006) and propulsion (+6.0%, p = 0.041) phase in running. This widening highlighted an increased motor output’s robustness (i.e. ability to cope with errors) when dealing with the unexpected perturbations. Our results confirmed the hypothesis that humans adjust their motor control strategies’ timing to deal with unsteady locomotion.

Foot and Ankle Joint Biomechanical Adaptations to an Unpredictable Coronally Uneven Surface

Coronally uneven terrain, a common yet challenging feature encountered in daily ambulation, exposes individuals to an increased risk of falling. The foot-ankle complex may adapt to improve balance on uneven terrains, a recovery strategy which may be more challenging in patients with foot-ankle pathologies. A multisegment foot model (MSFM) was used to study the biomechanical adaptations of the foot and ankle joints during a step on a visually obscured, coronally uneven surface. Kinematic, kinetic and in-shoe pressure data were collected as ten participants walked on an instrumented walkway with a surface randomly positioned ±15 deg or 0 deg in the coronal plane. Coronally uneven surfaces altered hindfoot–tibia loading, with more conformation to the surface in early than late stance. Distinct loading changes occurred for the forefoot–hindfoot joint in early and late stance, despite smaller surface conformations. Hindfoot–tibia power at opposite heel contact (@OHC) was generated and increased on both uneven surfaces, whereas forefoot–hindfoot power was absorbed and remained consistent across surfaces. Push-off work increased for the hindfoot–tibia joint on the everted surface and for the forefoot–hindfoot joint on the inverted surface. Net work across joints was generated for both uneven surfaces, while absorbed on flat terrain. The partial decoupling and joint-specific biomechanical adaptations on uneven surfaces suggest that multi-articulating interventions such as prosthetic devices and arthroplasty may improve ambulation for mobility-impaired individuals on coronally uneven terrain.

Contextual interference during adaptation to asymmetric split-belt treadmill walking results in transfer of unique gait mechanics

When humans make errors in stepping during walking due to a perturbation, they may adapt their gait as a way to correct for discrepancies between predicted and actual sensory feedback. This study sought to determine if increased contextual interference during acquisition of a novel asymmetric gait pattern would change lower-limb mechanical strategies generalized to different walking contexts. Such knowledge could help to clarify the role of contextual interference in locomotor adaptation, and demonstrate potential use in future gait rehabilitation paradigms. One belt on a split-belt treadmill was driven at a constant velocity while the other was driven at changing velocities according to one of three practice paradigms: serial, random blocked, or random training. Subjects returned to complete one of two different transfer tests. Results indicate that during acquisition, random practice requires unique gait mechanics to adapt to a challenging walking environment. Also, results from one transfer test close to that of the acquisition experience did not seem to demonstrate any contextual interference effect. Finally, random blocked practice resulted in highly unique changes in step length symmetry on a second, more challenging, transfer test. This perhaps indicates that a moderate level of contextual interference causes unique locomotor generalization strategies.

Summary: Specific biomechanical gait adaptations occur as a result of variance of stepping errors during walking. Humans optimize a novel gait pattern based on different levels of error variance.

Lower limb biomechanical analysis during an unanticipated step on a bump reveals specific adaptations of walking on uneven terrains

Although it is clear that walking over different irregular terrain is associated with altered biomechanics, there is little understanding of how we quickly adapt to unexpected variations in terrain. This study aims to investigate which adaptive strategies humans adopt when performing an unanticipated step on an irregular surface, specifically a small bump. Nine healthy male participants walked at their preferred walking speed along a straight walkway during five conditions: four involving unanticipated bumps of two different heights, and one level walking condition. Muscle activation of eight lower limb muscles and three-dimensional gait analysis were evaluated during these testing conditions. Two distinct adaptive strategies were found, which involved no significant change in total lower limb mechanical work or walking speed. An ankle-based strategy was adopted when stepping on a bump with the forefoot, whereas a hip-based strategy was preferred when stepping with the rearfoot. These strategies were driven by a higher activation of the plantarflexor muscles (6-51%), which generated a higher ankle joint moment during the forefoot conditions and by a higher activation of the quadriceps muscles (36-93%), which produced a higher knee joint moment and hip joint power during the rearfoot conditions. These findings provide insights into how humans quickly react to unexpected events and could be used to inform the design of adaptive controllers for wearable robots intended for use in unstructured environments that can provide optimal assistance to the different lower limb joints.

Step-to-Step Ankle Inversion/Eversion Torque Modulation Can Reduce Effort Associated with Balance

Below-knee amputation is associated with higher energy expenditure during walking, partially due to difficulty maintaining balance. We previously found that once-per-step push-off work control can reduce balance-related effort, both in simulation and in experiments with human participants. Simulations also suggested that changing ankle inversion/eversion torque on each step, in response to changes in body state, could assist with balance. In this study, we investigated the effects of ankle inversion/eversion torque modulation on balance-related effort among amputees (N = 5) using a multi-actuated ankle-foot prosthesis emulator. In stabilizing conditions, changes in ankle inversion/eversion torque were applied so as to counteract deviations in side-to-side center-of-mass acceleration at the moment of intact-limb toe off; higher acceleration toward the prosthetic limb resulted in a corrective ankle inversion torque during the ensuing stance phase. Destabilizing controllers had the opposite effect, and a zero gain controller made no changes to the nominal inversion/eversion torque. To separate the balance-related effects of step-to-step control from the potential effects of changes in average mechanics, average ankle inversion/eversion torque and prosthesis work were held constant across conditions. High-gain stabilizing control lowered metabolic cost by 13% compared to the zero gain controller (p = 0.05). We then investigated individual responses to subject-specific stabilizing controllers following an enforced exploration period. Four of five participants experienced reduced metabolic rate compared to the zero gain controller (−15, −14, −11, −6, and +4%) an average reduction of 9% (p = 0.05). Average prosthesis mechanics were unchanged across all conditions, suggesting that improvements in energy economy might have come from changes in step-to-step corrections related to balance. Step-to-step modulation of inversion/eversion torque could be used in new, active ankle-foot prostheses to reduce walking effort associated with maintaining balance.

Subject-specific toe-in or toe-out gait modifications reduce the larger knee adduction moment peak more than a non-personalized approach

The knee adduction moment (KAM) is a surrogate measure for medial compartment knee loading and is related to the progression of knee osteoarthritis. Toe-in and toe-out gait modifications typically reduce the first and second KAM peaks, respectively. We investigated whether assigning a subject-specific foot progression angle (FPA) modification reduces the peak KAM by more than assigning the same modification to everyone. To explore the effects of motor learning on muscle coordination and kinetics, we also evaluated the peak knee flexion moment and quadriceps-hamstring co-contraction during normal walking, when subjects first learned their subject-specific FPA, and following 20 min of training. Using vibrotactile feedback, we trained 20 healthy adults to toe-in and toe-out by 5° and 10° relative to their natural FPA, then identified the subject-specific FPA as the angle where each subject maximally reduced their larger KAM peak. When walking at their subject-specific FPA, 18 subjects significantly reduced their larger KAM peak; 8 by toeing-in and 10 by toeing-out. On average, subjects reduced their larger KAM peak by 18.6 ± 16.2% when walking at their subject-specific FPA, which was more than the reductions achieved when all subjects toed-in by 10° (10.0 ± 17.1%, p = .013) or toed-out by 10° (11.0 ± 18.3%, p = .002). Quadriceps-hamstring co-contraction and the peak knee flexion moment increased when subjects first learned their subject-specific FPA, but only co-contraction returned to baseline levels following training. These findings demonstrate that subject-specific gait modifications reduce the peak KAM more than uniformly assigned modifications and have the potential to slow the progression of medial compartment knee osteoarthritis.

The Biomechanical and Energetic Advantages of a Mediolaterally Wide Pelvis in Women

Here, we argue that two key shifts in thinking are required to more clearly understand the selection pressures shaping pelvis evolution in female hominins: (1) the primary locomotor mode of female hominins was loaded walking in the company of others, and (2) the periodic gait of human walking is most effectively explained as a biomechanically controlled process related to heel-strike collisions that is tuned for economy and stability by properly-timed motor inputs (a model called dynamic walking). In the light of these two frameworks, the evidence supports differences between female and male upper-pelvic morphology being the result of the unique reproductive role of female hominins, which involved moderately paced, loaded walking in groups. Anat Rec, 300:764-775, 2017. © 2017 Wiley Periodicals, Inc.

Optimal pacing and carbohydrate intake strategies for ultramarathons

PURPOSE

The purpose of this research is to determine the pacing and nutrition strategies which minimize completion time and carbohydrate intake for athletes competing in ultramarathon races.

METHODS

We present the formulation of a two-phase optimization model. The first-phase mixed-integer nonlinear program (MINLP) determines the minimum completion time subject to the altitude, terrain, and distance of the race, as well as the mass and cardiovascular fitness of the athlete. The second-phase MINLP determines the minimum carbohydrate intake required for the athlete to achieve the completion time prescribed by the first-phase subject to the flow of carbohydrates through the stomach, liver, and muscles. Consequently, the second-phase model provides the optimal pacing and nutrition strategies for a particular athlete for each kilometer of a particular race.

RESULTS

We validate model results for a wide range of athlete parameters by comparing completion times to those reported for two case-study events. We also compare the kilometer-by-kilometer pacing and nutrition strategies prescribed by the model to those of a particular athlete. In all cases, the model results closely match those witnessed in the actual events.

CONCLUSION

We have developed a baseline metabolic model that provides athletes prescriptive guidance regarding optimal pacing and carbohydrate intake strategies prior to competing in ultramarathon races. Given the highly variable topographical characteristics common to many ultramarathon courses and the potential inexperience of many athletes with such courses, our model provides valuable insight to competitors who might otherwise fail to complete the event due to exhaustion or carbohydrate depletion.

Walking on uneven terrain in healthy adults and the implications for people after stroke

BACKGROUND

One third of individuals after stroke report an inability to walk in the community. Community mobility requires walking adaptability - the ability to adjust one's stepping pattern to meet environmental demands and task goals. Walking on uneven terrain (e.g. grass, gravel) has unique requirements and is a critical component of walking adaptability that has not been investigated in the post-stroke population.

OBJECTIVE

To summarize current knowledge of biomechanical and neuromuscular modifications during uneven terrain negotiation in healthy individuals and discuss implications of post-stroke impairments.

METHODS

Review of eleven studies, identified through a search of relevant literature on PubMed and CINAHL.

RESULTS

On uneven terrain, healthy adults demonstrate numerous gait modifications including a lowered center of mass, increased muscle co-contraction during stance and exaggerated or increased toe clearance during swing. After stroke, changes in muscle activity and limb coordination will likely result in difficulty or inability performing these modifications that healthy adults use to maintain stability and safety when walking on uneven terrain.

CONCLUSIONS

Studies of biomechanical and neuromuscular control of walking on uneven terrain are needed to quantify mobility limitations in adults post-stroke. Such investigations will contribute to the understanding of mobility impairments after stroke and the design of critically important intervention strategies.

Increased Arctic sea ice drift alters adult female polar bear movements and energetics

Recent reductions in thickness and extent have increased drift rates of Arctic sea ice. Increased ice drift could significantly affect the movements and the energy balance of polar bears (Ursus maritimus) which forage, nearly exclusively, on this substrate. We used radio-tracking and ice drift data to quantify the influence of increased drift on bear movements, and we modeled the consequences for energy demands of adult females in the Beaufort and Chukchi seas during two periods with different sea ice characteristics. Westward and northward drift of the sea ice used by polar bears in both regions increased between 1987-1998 and 1999-2013. To remain within their home ranges, polar bears responded to the higher westward ice drift with greater eastward movements, while their movements north in the spring and south in fall were frequently aided by ice motion. To compensate for more rapid westward ice drift in recent years, polar bears covered greater daily distances either by increasing their time spent active (7.6%-9.6%) or by increasing their travel speed (8.5%-8.9%). This increased their calculated annual energy expenditure by 1.8%-3.6% (depending on region and reproductive status), a cost that could be met by capturing an additional 1-3 seals/year. Polar bears selected similar habitats in both periods, indicating that faster drift did not alter habitat preferences. Compounding reduced foraging opportunities that result from habitat loss; changes in ice drift, and associated activity increases, likely exacerbate the physiological stress experienced by polar bears in a warming Arctic.

Closed-loop EMG-informed model-based analysis of human musculoskeletal mechanics on rough terrains

This work aims at estimating the musculoskeletal forces acting in the human lower extremity during locomotion on rough terrains. We employ computational models of the human neuro-musculoskeletal system that are informed by multi-modal movement data including foot-ground reaction forces, 3D marker trajectories and lower extremity electromyograms (EMG). Data were recorded from one healthy subject locomoting on rough grounds realized using foam rubber blocks of different heights. Blocks arrangement was randomized across all locomotion trials to prevent adaptation to specific ground morphology. Data were used to generate subject-specific models that matched an individual's anthropometry and force-generating capacity. EMGs enabled capturing subject- and ground-specific muscle activation patterns employed for walking on the rough grounds. This allowed integrating realistic activation patterns in the forward dynamic simulations of the musculoskeletal system. The ability to accurately predict the joint mechanical forces necessary to walk on different terrains have implications for our understanding of human movement but also for developing intuitive human machine interfaces for wearable exoskeletons or prosthetic limbs that can seamlessly adapt to different mechanical demands matching biological limb performance.

Kinematics and metabolic cost of running on an irregular treadmill surface

The purpose of this study was to investigate the kinematic and metabolic effects of running on an irregular surface. We also examined how altering the frontal plane foot angle (inversion/eversion) at contact using real-time visual feedback would affect these other variables. Sixteen participants completed three running bouts lasting 5-7 minutes each on an irregular surface (IS) treadmill, a traditional smooth surface (SS) treadmill, and on SS while receiving visual feedback of the frontal plane foot angle at contact (SSF) with a goal of matching IS foot angle on SS. Frontal plane foot angle increased 40% from IS to SS (IS: 8.4 ± 4.09°, SS: 11.8 ± 4.52°, P < 0.0001, ES 1.40). Knee flexion angle at contact decreased 33% from IS to SS (IS: 9.2 ± 4.88°, SS: 6.2 ± 5.03°, P < 0.0001, ES 1.30). Rate of oxygen consumption decreased by 10% from IS to SS (IS: 37.9 ± 5.68 ml·kg^-1·min^-1, SS: 34.1 ± 5.07 ml·kg^-1·min^-1, P < 0.0001, ES 3.05). PSD of leg accelerations decreased by 38% (IS: 0.17 ± 0.07 g²/Hz, SS: 0.106 ± 0.05 g²/Hz, P < 0.000, ES 1.69). Frontal plane foot angle decreased by 14% from SS to SSF (SS: 11.8 ± 4.52°, SSF: 10.1 ± 4.42°, P = 0.027. ES 0.62) but did not result in significant changes in any other variables. There were no significant differences in shock attenuation between any conditions (IS: -9.8 ± 2.26 dB, SS: -9.5 ± 3.12 dB, SSF: -9.9 ± 2.62 dB, P = 0.671). Running with greater eversion on the irregular surface may be an attempt by runners to reduce the perceived potential of an inversion ankle sprain. As a partial compensation for the decreased foot angle, runners increased knee flexion. This maintained shock attenuation but increased the rate of oxygen consumption. Altering the foot angle at contact using feedback on the SS caused the knee angle at contact to increase, but did not change shock attenuation or metabolic cost.

The critical phase for visual control of human walking over complex terrain

To walk efficiently over complex terrain, humans must use vision to tailor their gait to the upcoming ground surface without interfering with the exploitation of passive mechanical forces. We propose that walkers use visual information to initialize the mechanical state of the body before the beginning of each step so the resulting ballistic trajectory of the walker's center-of-mass will facilitate stepping on target footholds. Using a precision stepping task and synchronizing target visibility to the gait cycle, we empirically validated two predictions derived from this strategy: (1) Walkers must have information about upcoming footholds during the second half of the preceding step, and (2) foot placement is guided by information about the position of the target foothold relative to the preceding base of support. We conclude that active and passive modes of control work synergistically to allow walkers to negotiate complex terrain with efficiency, stability, and precision.

Varying negative work assistance at the ankle with a soft exosuit during loaded walking

BACKGROUND

Only very recently, studies have shown that it is possible to reduce the metabolic rate of unloaded and loaded walking using robotic ankle exoskeletons. Some studies obtained this result by means of high positive work assistance while others combined negative and positive work assistance. There is no consensus about the isolated contribution of negative work assistance. Therefore, the aim of the present study is to examine the effect of varying negative work assistance at the ankle joint while maintaining a fixed level of positive work assistance with a multi-articular soft exosuit.

METHODS

We tested eight participants during walking at 1.5 ms-1 with a 23-kg backpack. Participants wore a version of the exosuit that assisted plantarflexion via Bowden cables tethered to an off-board actuation platform. In four active conditions we provided different rates of exosuit bilateral ankle negative work assistance ranging from 0.015 to 0.037 W kg-1 and a fixed rate of positive work assistance of 0.19 W kg-1.

RESULTS

All active conditions significantly reduced metabolic rate by 11 to 15% compared to a reference condition, where the participants wore the exosuit but no assistance was provided. We found no significant effect of negative work assistance. However, there was a trend (p = .08) toward greater reduction in metabolic rate with increasing negative work assistance, which could be explained by observed reductions in biological ankle and hip joint power and moment.

CONCLUSIONS

The non-significant trend of increasing negative work assistance with increasing reductions in metabolic rate motivates the value in further studies on the relative effects of negative and positive work assistance. There may be benefit in varying negative work over a greater range or in isolation from positive work assistance.

Automatic identification of gait events during walking on uneven surfaces

The accurate detection of gait events is essential for clinical gait analysis. Aside from speed, surface characteristics like planarity and compliance can affect gait kinematics. Therefore detection of kinematic gait events on uneven surfaces may be inaccurate. To date, no study has investigated the possible influence of surface characteristics on gait event detection. Thus, the purpose of this study was to assess and compare the performance of four kinematic-based gait event detection algorithms (horizontal heel-heel displacement, foot velocity, heel/toe-PSIS displacement, peak hip extension) during walking on three surfaces with different degrees of planarity. Kinematic and force plate data were collected on thirteen athletes during two self-selected walking speeds at a normal (1.30±0.03m/s) and fast pace (1.70±0.10m/s). Footstrike and toe-off events were calculated by the algorithms and compared to vertical ground reaction force as a reference. The main findings of the study were: (1) surface configuration had an effect on algorithm accuracy (p<0.010, 0.84<d<2.79); (2) the vertical foot-velocity profile provided the lowest RMSE for footstrike (8.8-14.6ms) during normal walking and toe-off (15.4-24.9ms) during normal and fast walking on all surfaces; (3) horizontal heel-ankle displacement provided the lowest RMSE for footstrike during fast walking on all surfaces (RMSE: 8.9-13.8ms). Overall, the vertical foot-velocity algorithm provided low RMSE for all conditions, is easy to apply and thus recommended for gait event detection.

Determinants of preferred ground clearance during swing phase of human walking

During each step of human walking, the swing foot passes close to the ground with a small but (usually) non-zero clearance. The foot can occasionally scuff against the ground, with some risk of stumbling or tripping. The risk might be mitigated simply by lifting the foot higher, but presumably at increased effort, of unknown amount. Perhaps the normally preferred ground clearance is a trade-off between competing costs, one for lifting the foot higher and one for scuffing it. We tested this by measuring the metabolic energy cost of lifting and scuffing the foot, treating these apparently dissimilar behaviors as part of a single continuum, where scuffing is a form of negative foot lift. We measured young, healthy adults (N=9) lifting or scuffing the foot by various amounts mid-swing during treadmill walking, and observed substantial costs, each well capable of doubling the net metabolic rate for normal walking (gross cost minus that for standing). In relative terms, the cost for scuffing increased over twice as steeply as that for lifting. That relative difference means that the expected value of cost, which takes into account movement variability, occurs at a non-zero mean clearance, approximately matching the preferred clearance we observed. Energy cost alone is only a lower bound on the overall disadvantages of inadvertent ground contact, but it is sufficient to show how human behavior may be determined not only by the separate costs of different trade-offs but also by movement variability, which can influence the average cost actually experienced in practice.

Functional reserve in functionally independent elderly persons: a calculation of gait speed and physiological cost

Abstract Aim: To analyze the behavior of functional reserve percentage (%FR) by gender in functional elderly persons, considering speed and the physiological cost of gait. Methods: A total of 53 self-reliant elderly persons, 40 of whom were women (age 69.4±4.7 years old; BMI 31.2±4.3 kg/m2) and 13 of whom were men (age 70.8±7.2 years old; BMI 28.7±3.5 kg/m2) participated in an observational and cross-sectional study. Participants were requested to walk at comfortable (CG) and maximum gait (MG). Both modalities were performed on a 70-meter elliptical circuit over three minutes. Distance and working heart rate were recorded for the corresponding calculation of average gait speed (AGS), physiological cost index (PCI) and used working heart rate percentage (% uWHR). With this information the FR% was determined by the percentage ratio with AGS, PCI and %uWHR under comfortable versus maximum demand walk conditions. Results: The association between %FR and AGS was significantly higher in males (p=0.017), reaching values of +-20% in most subjects. The physiological %FR for PCI was ≈30% in both men and women (p=0.156), while for % uWHR was ≈40% in females and ≈45% in males (p=0.131). Conclusion: AGS is a critical threshold functionality indicator to determine FR in functionally independent Chilean elderly persons.

Mechanisms for regulating step length while running towards and over an obstacle

The ability to run across uneven terrain with continuous stable movement is critical to the safety and efficiency of a runner. Successful step-to-step stabilization while running may be mediated by minor adjustments to a few key parameters (e.g., leg stiffness, step length, foot strike pattern). However, it is not known to what degree runners in relatively natural settings (e.g., trails, paved road, curbs) use the same strategies across multiple steps. This study investigates how three readily measurable running parameters - step length, foot placement, and foot strike pattern - are adjusted in response to encountering a typical urban obstacle - a sidewalk curb. Thirteen subjects were video-recorded as they ran at self-selected slow and fast paces. Runners targeted a specific distance before the curb for foot placement, and lengthened their step over the curb (p<0.0001) regardless of where the step over the curb was initiated. These strategies of adaptive locomotion disrupt step cycles temporarily, and may increase locomotor cost and muscle loading, but in the end assure dynamic stability and minimize the risk of injury over the duration of a run.

Walking on uneven terrain with a powered ankle prosthesis: A preliminary assessment

A successful walking gait with a powered prosthesis depends heavily on proper timing of power delivery, or push-off. This paper describes a control approach which provides improved walking on uneven terrain relative to previous work intended for use on even (level) terrain. This approach is motivated by an initial healthy subject study which demonstrated less variation in sagittal plane shank angle than sagittal plane ankle angle when walking on uneven terrain relative to even terrain. The latter therefore replaces the former as the control signal used to initiate push-off in the powered prosthesis described herein. The authors demonstrate improvement in consistency for several gait characteristics, relative to healthy, as well as controller characteristics with the new control approach, including a 50% improvement in the consistency of the percentage of stride at which push-off is initiated.

Biomechanical Analysis of Ankle during the Stance Phase of Gait on Various Surfaces: A Literature Review

The purpose of this article is to review the literature that deals with the biomechanical analysis of the ankle during gait stance phase on slopes, on uneven and rock surfaces, on sand, and on grass surfaces, as well as to present the observed differences. Methods. The literature was searched in the databases of PubMed and Google Scholar, for the years of 2005–2015. The keywords were: biomechanics, gait analysis, ankle joint, stance phase, uphill walking, downhill walking, sand surface, uneven surface, grass surface, and ballast. Results. The kinetic and kinematic gait behaviour is directly influenced by the surface on which it is being performed. The uphill or downhill surfaces, the surfaces of stone, sand, grass, and uneven surfaces have a direct impact on the biomechanics on joints of the lower limb, changing the energy cost, muscle activation, the resulting mechanical work, ground reaction forces and balance, and the parameters of the gait cycle. All these changes are raising many questions about the safety and comfort of these surfaces. In the structures of the foot, ankle and lower leg high compressive and rotational forces are transmitted resulting in injuries in these regions. Conclusions. Each surface has its own advantages and disadvantages, changing the biomechanics of the lower extremity and particularly the ankle. According to the purpose that one wants to achieve they can choose a suitable surface. To prevent injuries and falls, we must choose shoes that fit well, are comfortable with cushioning, and have a feeling neither too hard nor too soft, with laces and low collar.

Architecture and functional ecology of the human gastrocnemius muscle-tendon unit

The gastrocnemius muscle-tendon unit (MTU) is central to human locomotion. Structural variation in the human gastrocnemius MTU is predicted to affect the efficiency of locomotion, a concept most often explored in the context of performance activities. For example, stiffness of the Achilles tendon varies among individuals with different histories of competitive running. Such a finding highlights the functional variation of individuals and raises the possibility of similar variation between populations, perhaps in response to specific ecological or environmental demands. Researchers often assume minimal variation in human populations, or that industrialized populations represent the human species as well as any other. Yet rainforest hunter-gatherers, which often express the human pygmy phenotype, contradict such assumptions. Indeed, the human pygmy phenotype is a potential model system for exploring the range of ecomorphological variation in the architecture of human hindlimb muscles, a concept we review here.

How do treadmill speed and terrain visibility influence neuromuscular control of guinea fowl locomotion?

Locomotor control mechanisms must flexibly adapt to both anticipated and unexpected terrain changes to maintain movement and avoid a fall. Recent studies revealed that ground birds alter movement in advance of overground obstacles, but not treadmill obstacles, suggesting context-dependent shifts in the use of anticipatory control. We hypothesized that differences between overground and treadmill obstacle negotiation relate to differences in visual sensory information, which influence the ability to execute anticipatory manoeuvres. We explored two possible explanations: (1) previous treadmill obstacles may have been visually imperceptible, as they were low contrast to the tread, and (2) treadmill obstacles are visible for a shorter time compared with runway obstacles, limiting time available for visuomotor adjustments. To investigate these factors, we measured electromyographic activity in eight hindlimb muscles of the guinea fowl (Numida meleagris, N=6) during treadmill locomotion at two speeds (0.7 and 1.3 m s⁻¹) and three terrain conditions at each speed: (i) level, (ii) repeated 5 cm low-contrast obstacles (<10% contrast, black/black), and (iii) repeated 5 cm high-contrast obstacles (>90% contrast, black/white). We hypothesized that anticipatory changes in muscle activity would be higher for (1) high-contrast obstacles and (2) the slower treadmill speed, when obstacle viewing time is longer. We found that treadmill speed significantly influenced obstacle negotiation strategy, but obstacle contrast did not. At the slower speed, we observed earlier and larger anticipatory increases in muscle activity and shifts in kinematic timing. We discuss possible visuomotor explanations for the observed context-dependent use of anticipatory strategies.

Summary: Guinea fowl (Numida meleagris) show speed-dependent shifts in neuromuscular control during obstacle negotiation, characterized by a greater reliance on anticipatory modulation and stride-to-stride neural adjustments at slow speed, shifting towards feedforward activation and intrinsic mechanical stability at high speed.

How visual perceptual grouping influences foot placement

Everybody would agree that vision guides locomotion; but how does vision influence choice when there are different solutions for possible foot placement? We addressed this question by investigating the impact of perceptual grouping on foot placement in humans. Participants performed a stepping stone task in which pathways consisted of target stones in a spatially regular path of foot falls and visual distractor stones in their proximity. Target and distractor stones differed in shape and colour so that each subset of stones could be easily grouped perceptually. In half of the trials, one target stone swapped shape and colour with a distractor in its close proximity. We show that in these 'swapped' conditions, participants chose the perceptually groupable, instead of the spatially regular, stepping location in over 40% of trials, even if the distance between perceptually groupable steps was substantially larger than normal step width/length. This reveals that the existence of a pathway that could be traversed without spatial disruption to periodic stepping is not sufficient to guarantee participants will select it and suggests competition between different types of visual input when choosing foot placement. We propose that a bias in foot placement choice in favour of visual grouping exists as, in nature, sudden changes in visual characteristics of the ground increase the uncertainty for stability.

Once-per-step control of ankle-foot prosthesis push-off work reduces effort associated with balance during walking

Background

Individuals with below-knee amputation have more difficulty balancing during walking, yet few studies have explored balance enhancement through active prosthesis control. We previously used a dynamical model to show that prosthetic ankle push-off work affects both sagittal and frontal plane dynamics, and that appropriate step-by-step control of push-off work can improve stability. We hypothesized that this approach could be applied to a robotic prosthesis to partially fulfill the active balance requirements of human walking, thereby reducing balance-related activity and associated effort for the person using the device.

Methods

We conducted experiments on human participants (N = 10) with simulated amputation. Prosthetic ankle push-off work was varied on each step in ways expected to either stabilize, destabilize or have no effect on balance. Average ankle push-off work, known to affect effort, was kept constant across conditions. Stabilizing controllers commanded more push-off work on steps when the mediolateral velocity of the center of mass was lower than usual at the moment of contralateral heel strike. Destabilizing controllers enforced the opposite relationship, while a neutral controller maintained constant push-off work regardless of body state. A random disturbance to landing foot angle and a cognitive distraction task were applied, further challenging participants’ balance. We measured metabolic rate, foot placement kinematics, center of pressure kinematics, distraction task performance, and user preference in each condition. We expected the stabilizing controller to reduce active control of balance and balance-related effort for the user, improving user preference.

Results

The best stabilizing controller lowered metabolic rate by 5.5% (p = 0.003) and 8.5% (p = 0.02), and step width variability by 10.0% (p = 0.009) and 10.7% (p = 0.03) compared to conditions with no control and destabilizing control, respectively. Participants tended to prefer stabilizing controllers. These effects were not due to differences in average push-off work, which was unchanged across conditions, or to average gait mechanics, which were also unchanged. Instead, benefits were derived from step-by-step adjustments to prosthesis behavior in response to variations in mediolateral velocity at heel strike.

Conclusions

Once-per-step control of prosthetic ankle push-off work can reduce both active control of foot placement and balance-related metabolic energy use during walking.

Electronic supplementary material

The online version of this article (doi:10.1186/s12984-015-0027-3) contains supplementary material, which is available to authorized users.

Biomechanics and energetics of running on uneven terrain

In the natural world, legged animals regularly run across uneven terrain with remarkable ease. To gain understanding of how running on uneven terrain affects the biomechanics and energetics of locomotion, we studied human subjects (N=12) running at 2.3 m s−1 on an uneven terrain treadmill, with up to a 2.5 cm height variation. We hypothesized that running on uneven terrain would show increased energy expenditure, step parameter variability and leg stiffness compared with running on smooth terrain. Subject energy expenditure increased by 5% (0.68 W kg−1; P&lt;0.05) when running on uneven terrain compared with smooth terrain. Step width and length variability also increased by 27% and 26%, respectively (P&lt;0.05). Positive and negative ankle work decreased on uneven terrain by 22% (0.413 J kg−1) and 18% (0.147 J kg−1), respectively (P=0.0001 and P=0.0008). Mean muscle activity increased on uneven terrain for three muscles in the thigh (P&lt;0.05). Leg stiffness also increased by 20% (P&lt;0.05) during running on uneven terrain compared with smooth terrain. Calculations of gravitational potential energy fluctuations suggest that about half of the energetic increases can be explained by additional positive and negative mechanical work for up and down steps on the uneven surface. This is consistent between walking and running, as the absolute increases in energetic cost for walking and running on uneven terrain were similar: 0.68 and 0.48 W kg−1, respectively. These results provide insight into how surface smoothness can affect locomotion biomechanics and energetics in the real world.