The cost of walking downhill: is the preferred gait energetically optimal?

More than energy cost: multiple benefits of the long Achilles tendon in human walking and running

Elastic strain energy that is stored and released from long, distal tendons such as the Achilles during locomotion allows for muscle power amplification as well as for reduction of the locomotor energy cost: as distal tendons perform mechanical work during recoil, plantar flexor muscle fibres can work over smaller length ranges, at slower shortening speeds, and at lower activation levels. Scant evidence exists that long distal tendons evolved in humans (or were retained from our more distant Hominoidea ancestors) primarily to allow high muscle-tendon power outputs, and indeed we remain relatively powerless compared to many other species. Instead, the majority of evidence suggests that such tendons evolved to reduce total locomotor energy cost. However, numerous additional, often unrecognised, advantages of long tendons may speculatively be of greater evolutionary advantage, including the reduced limb inertia afforded by shorter and lighter muscles (reducing proximal muscle force requirement), reduced energy dissipation during the foot-ground collisions, capacity to store and reuse the muscle work done to dampen the vibrations triggered by foot-ground collisions, reduced muscle heat production (and thus core temperature), and attenuation of work-induced muscle damage. Cumulatively, these effects should reduce both neuromotor fatigue and sense of locomotor effort, allowing humans to choose to move at faster speeds for longer. As these benefits are greater at faster locomotor speeds, they are consistent with the hypothesis that running gaits used by our ancestors may have exerted substantial evolutionary pressure on Achilles tendon length. The long Achilles tendon may therefore be a singular adaptation that provided numerous physiological, biomechanical, and psychological benefits and thus influenced behaviour across multiple tasks, both including and additional to locomotion. While energy cost may be a variable of interest in locomotor studies, future research should consider the broader range of factors influencing our movement capacity, including our decision to move over given distances at specific speeds, in order to understand more fully the effects of Achilles tendon function as well as changes in this function in response to physical activity, inactivity, disuse and disease, on movement performance.

Gait Variability at Different Walking Speeds

Gait variability (GV) is a crucial measure of inconsistency of muscular activities or body segmental movements during repeated tasks. Hence, GV might serve as a relevant and sensitive measure to quantify adjustments of walking control. However, it has not been clarified whether GV is associated with walking speed, a clarification needed to exploit effective better bilateral coordination level. For this aim, fourteen male students (age 22.4 ± 2.7 years, body mass 74.9 ± 6.8 kg, and body height 1.78 ± 0.05 m) took part in this study. After three days of walking 1 km each day at a self-selected speed (SS) on asphalt with an Apple Watch S. 7 (AppleTM, Cupertino, CA, USA), the participants were randomly evaluated on a treadmill at three different walking speed intensities for 10 min at each one, SS - 20%/SS + 20%/ SS, with 5 min of passive recovery in-between. Heart rate (HR) was monitored and normalized as %HRmax, while the rate of perceived exertion (RPE) (CR-10 scale) was asked after each trial. Kinematic analysis was performed, assessing the Contact Time (CT), Swing Time (ST), Stride Length (SL), Stride Cycle (SC), and Gait Variability as Phase Coordination Index (PCI). RPE and HR increased as the walking speed increased (p = 0.005 and p = 0.035, respectively). CT and SC decreased as the speed increased (p = 0.0001 and p = 0.013, respectively), while ST remained unchanged (p = 0.277). SL increased with higher walking speed (p = 0.0001). Conversely, PCI was 3.81 ± 0.88% (high variability) at 3.96 ± 0.47 km·h-1, 2.64 ± 0.75% (low variability) at SS (4.94 ± 0.58 km·h-1), and 3.36 ± 1.09% (high variability) at 5.94 ± 0.70 km·h-1 (p = 0.001). These results indicate that while the metabolic demand and kinematics variables change linearly with increasing speed, the most effective GV was observed at SS. Therefore, SS could be a new methodological approach to choose the individual walking speed, normalize the speed intensity, and avoid a gait pattern alteration.

The value of time in the invigoration of human movements when interacting with a robotic exoskeleton

Time and effort are thought to be subjectively balanced during the planning of goal-directed actions, thereby setting the vigor of volitional movements. Theoretical models predicted that the value of time should then amount to high levels of effort. However, the time-effort trade-off has so far only been studied for a narrow range of efforts. To investigate the extent to which humans can invest in a time-saving effort, we used a robotic exoskeleton to substantially vary the energetic cost associated with a certain vigor during reaching movements. In this situation, minimizing the time-effort trade-off should lead to high and low human efforts for upward and downward movements, respectively. Consistently, all participants expended substantial amounts of energy upward and remained essentially inactive by harnessing the work of gravity downward, while saving time in both cases. A common time-effort trade-off may therefore determine the vigor of reaching movements for a wide range of efforts.

Review: Harnessing extant energy and protein requirement modeling for sustainable beef production

Numerous mathematical nutrition models have been developed in the last sixty years to predict the dietary supply and requirement of farm animals' energy and protein. Although these models, usually developed by different groups, share similar concepts and data, their calculation routines (i.e., submodels) have rarely been combined into generalized models. This lack of mixing submodels is partly because different models have different attributes, including paradigms, structural decisions, inputs/outputs, and parameterization processes that could render them incompatible for merging. Another reason is that predictability might increase due to offsetting errors that cannot be thoroughly studied. Alternatively, combining concepts might be more accessible and safer than combining models' calculation routines because concepts can be incorporated into existing models without changing the modeling structure and calculation logic, though additional inputs might be needed. Instead of developing new models, improving the merging of extant models' concepts might curtail the time and effort needed to develop models capable of evaluating aspects of sustainability. Two areas of beef production research that are needed to ensure adequate diet formulation include accurate energy requirements of grazing animals (decrease methane emissions) and efficiency of energy use (reduce carcass waste and resource use) by growing cattle. A revised model for energy expenditure of grazing animals was proposed to incorporate the energy needed for physical activity, as the British feeding system recommended, and eating and rumination (HjEer) into the total energy requirement. Unfortunately, the proposed equation can only be solved iteratively through optimization because HjEer requires metabolizable energy (ME) intake. The other revised model expanded an existing model to estimate the partial efficiency of using ME for growth (kg) from protein proportion in the retained energy by including an animal degree of maturity and average daily gain (ADG) as used in the Australian feeding system. The revised kg model uses carcass composition, and it is less dependent on dietary ME content, but still requires an accurate assessment of the degree of maturity and ADG, which in turn depends on the kg. Therefore, it needs to be solved iteratively or using one-step delayed continuous calculation (i.e., use the previous day's ADG to compute the current day's kg). We believe that generalized models developed by merging different models' concepts might improve our understanding of the relationships of existing variables that were known for their importance but not included in extant models because of the lack of proper information or confidence at that time.

Minimum effort simulations of split-belt treadmill walking exploit asymmetry to reduce metabolic energy expenditure

Walking on a split-belt treadmill elicits an adaptation response that changes baseline step length asymmetry. The underlying causes of this adaptation, however, are difficult to determine. It has been proposed that effort minimization may drive this adaptation, based on the idea that adopting longer steps on the fast belt, or positive step length asymmetry (SLA), can cause the treadmill to exert net-positive mechanical work on a bipedal walker. However, humans walking on split-belt treadmills have not been observed to reproduce this behavior when allowed to freely adapt. To determine if an effort-minimization motor control strategy would result in experimentally observed adaptation patterns, we conducted simulations of walking on different combinations of belt speeds with a human musculoskeletal model that minimized muscle excitations and metabolic rate. The model adopted increasing amounts of positive SLA and decreased its net metabolic rate with increasing belt speed difference, reaching +42.4% SLA and -5.7% metabolic rate relative to tied-belt walking at our maximum belt speed ratio of 3:1. These gains were primarily enabled by an increase in braking work and a reduction in propulsion work on the fast belt. The results suggest that a purely effort minimization driven split-belt walking strategy would involve substantial positive SLA, and that the lack of this characteristic in human behavior points to additional factors influencing the motor control strategy, such as aversion to excessive joint loads, asymmetry, or instability.NEW & NOTEWORTHY Behavioral observations of split-belt treadmill adaptation have been inconclusive toward its underlying causes. To estimate gait patterns when driven exclusively by one of these possible underlying causes, we simulated split-belt treadmill walking with a musculoskeletal model that minimized its summed muscle excitations. Our model took significantly longer steps on the fast belt and reduced its metabolic rate below tied-belt walking, unlike experimental observations. This suggests that asymmetry is energetically optimal, but human adaptation involves additional factors.

How frequent is the spontaneous occurrence of synchronized walking in daily life?

Experimental work has suggested that individuals walking side by side may frequently synchronize their steps. The present study created video records of pedestrian activity on pedestrian pathways in order to estimate the frequency of continuous synchronization among pairs of walkers going about their daily lives. About 6% of 498 coded pairs were continuously synchronized. Analysis and modeling of the distributions of frequency differences suggested that while different walkers will tend to have different preferred frequencies for a given speed (i.e., a preferred ratio of step length to step frequency, or walk ratio), they may tend to adjust their walk ratios slightly toward one another's even when they are not synchronizing their steps.

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.

Quantification of Gait Stability During Incline and Decline Walking: The Responses of Required Coefficient of Friction and Dynamic Postural Index

This study aims to investigate the gait stability response during incline and decline walking for various surface inclination angles in terms of the required coefficient of friction (RCOF), postural stability index (PSI), and center of pressure (COP)-center of mass (COM) distance. A customized platform with different surface inclinations (0°, 5°, 7.5°, and 10°) was designed. Twenty-three male volunteers participated by walking on an inclined platform for each inclination. The process was then repeated for declined platform as well. Qualysis motion capture system was used to capture and collect the trajectories motion of ten reflective markers that attached to the subjects before being exported to a visual three-dimensional (3D) software and executed in Matlab to obtain the RCOF, PSI, as well as dynamic PSI (DPSI) and COP-COM distance parameters. According to the result for incline walking, during initial contact, the RCOF was not affected to inclination. However, it was affected during peak ground reaction force (GRF) starting at 7.5° towards 10° for both walking conditions. The most affected PSI was found at anterior-posterior PSI (APSI) even as low as 5° inclination during both incline and decline walking. On the other hand, DPSI was not affected during both walking conditions. Furthermore, COP-COM distance was most affected during decline walking in anterior-posterior direction. The findings of this research indicate that in order to decrease the risk of falling and manage the inclination demand, a suitable walking strategy and improved safety measures should be applied during slope walking, particularly for decline and anterior-posterior orientations. This study also provides additional understanding on the best incline walking technique for secure and practical incline locomotion.

Quantifying mechanical and metabolic interdependence between speed and propulsive force during walking

Walking speed is a useful surrogate for health status across the population. Walking speed appears to be governed in part by interlimb coordination between propulsive (F_P) and braking (F_B) forces generated during step-to-step transitions and is simultaneously optimized to minimize metabolic cost. Of those forces, F_P generated during push-off has received significantly more attention as a contributor to walking performance. Our goal was to first establish empirical relations between F_P and walking speed and then to quantify their effects on metabolic cost in young adults. To specifically address any link between F_P and walking speed, we used a self-paced treadmill controller and real-time biofeedback to independently prescribe walking speed or F_P across a range of condition intensities. Walking with larger and smaller F_P led to instinctively faster and slower walking speeds, respectively, with ~80% of variance in walking speed explained by F_P. We also found that comparable changes in either F_P or walking speed elicited predictable and relatively uniform changes in metabolic cost, together explaining ~53% of the variance in net metabolic power and ~14% of the variance in cost of transport. These results provide empirical data in support of an interdependent relation between F_P and walking speed, building confidence that interventions designed to increase F_P will translate to improved walking speed. Repeating this protocol in other populations may identify other relations that could inform the time course of gait decline due to age and disease.

A neuromuscular model of human locomotion combines spinal reflex circuits with voluntary movements

Existing models of human walking use low-level reflexes or neural oscillators to generate movement. While appropriate to generate the stable, rhythmic movement patterns of steady-state walking, these models lack the ability to change their movement patterns or spontaneously generate new movements in the specific, goal-directed way characteristic of voluntary movements. Here we present a neuromuscular model of human locomotion that bridges this gap and combines the ability to execute goal directed movements with the generation of stable, rhythmic movement patterns that are required for robust locomotion. The model represents goals for voluntary movements of the swing leg on the task level of swing leg joint kinematics. Smooth movements plans towards the goal configuration are generated on the task level and transformed into descending motor commands that execute the planned movements, using internal models. The movement goals and plans are updated in real time based on sensory feedback and task constraints. On the spinal level, the descending commands during the swing phase are integrated with a generic stretch reflex for each muscle. Stance leg control solely relies on dedicated spinal reflex pathways. Spinal reflexes stimulate Hill-type muscles that actuate a biomechanical model with eight internal joints and six free-body degrees of freedom. The model is able to generate voluntary, goal-directed reaching movements with the swing leg and combine multiple movements in a rhythmic sequence. During walking, the swing leg is moved in a goal-directed manner to a target that is updated in real-time based on sensory feedback to maintain upright balance, while the stance leg is stabilized by low-level reflexes and a behavioral organization switching between swing and stance control for each leg. With this combination of reflex-based stance leg and voluntary, goal-directed control of the swing leg, the model controller generates rhythmic, stable walking patterns in which the swing leg movement can be flexibly updated in real-time to step over or around obstacles.

Metabolic cost of level, uphill, and downhill running in highly cushioned shoes with carbon-fiber plates

BACKGROUND

Compared to conventional racing shoes, Nike Vaporfly 4% running shoes reduce the metabolic cost of level treadmill running by 4%. The reduction is attributed to their lightweight, highly compliant, and resilient midsole foam and a midsole-embedded curved carbon-fiber plate. We investigated whether these shoes also could reduce the metabolic cost of moderate uphill (+3°) and downhill (-3°) grades. We tested the null hypothesis that, compared to conventional racing shoes, highly cushioned shoes with carbon-fiber plates would impart the same ∼4% metabolic power (W/kg) savings during uphill and downhill running as they do during level running.

METHODS

After familiarization, 16 competitive male runners performed six 5-min trials (2 shoes × 3 grades) in 2 Nike marathon racing-shoe models (Streak 6 and Vaporfly 4%) on a level, uphill (+3°), and downhill (-3°) treadmill at 13 km/h (3.61 m/s). We measured submaximal oxygen uptake and carbon dioxide production during Minutes 4-5 and calculated metabolic power (W/kg) for each shoe model and grade combination.

RESULTS

Compared to the conventional shoes (Streak 6), the metabolic power in the Vaporfly 4% shoes was 3.83% (level), 2.82% (uphill), and 2.70% (downhill) less (all p < 0.001). The percent of change in metabolic power for uphill running was less compared to level running (p = 0.04; effect size (ES) = 0.561) but was not statistically different between downhill and level running (p = 0.17; ES = 0.356).

CONCLUSION

On a running course with uphill and downhill sections, the metabolic savings and hence performance enhancement provided by Vaporfly 4% shoes would likely be slightly less overall, compared to the savings on a perfectly level race course.

Metabolically efficient walking assistance using optimized timed forces at the waist

The metabolic rate of walking can be reduced by applying a constant forward force at the center of mass. It has been shown that the metabolically optimal constant force magnitude minimizes propulsion ground reaction force at the expense of increased braking. This led to the hypothesis that selectively assisting propulsion could lead to greater benefits. We used a robotic waist tether to evaluate the effects of forward forces with different timings and magnitudes. Here, we show that it is possible to reduce the metabolic rate of healthy participants by 48% with a greater efficiency ratio of metabolic cost reduction per unit of net aiding work compared with other assistive robots. This result was obtained using a sinusoidal force profile with peak timing during the middle of the double support. The same timing could also reduce the metabolic rate in patients with peripheral artery disease. A model explains that the optimal force profile accelerates the center of mass into the inverted pendulum movement during single support. Contrary to the hypothesis, the optimal force timing did not entirely coincide with propulsion. Within the field of wearable robotics, there is a trend to use devices to mimic biological torque or force profiles. Such bioinspired actuation can have relevant benefits; however, our results demonstrate that this is not necessarily optimal for reducing metabolic rate.

Energy expenditure does not solely explain step length-width choices during walking

Healthy young adults have a most preferred walking speed, step length and step width that are close to energetically optimal. However, people can choose to walk with a multitude of different step lengths and widths, which can vary in both energy expenditure and preference. Here, we further investigated step length-width preferences and their relationship to energy expenditure. In line with a growing body of research, we hypothesized that people's preferred stepping patterns would not be fully explained by metabolic energy expenditure. To test this hypothesis, we used a two-alternative forced-choice paradigm. Fifteen participants walked on an oversized treadmill. Each trial, participants performed two prescribed stepping patterns and then chose the pattern they preferred. Over time, we adapted the choices such that there was 50% chance of choosing one pattern over another (equally preferred). If people's preferences are based solely on metabolic energy expenditure, then these equally preferred stepping patterns should have equal energy expenditure. In contrast, we found that energy expenditure differed across equally preferred step length-width patterns (P<0.001). On average, longer steps with higher energy expenditure were preferred over shorter and wider steps with lower energy expenditure (P<0.001). We also asked participants to rank a set of shorter, wider and longer steps from most preferred to least preferred, and from most energy expended to least energy expended. Only 7/15 participants had the same rankings for their preferences and perceived energy expenditure. Our results suggest that energy expenditure is not the only factor influencing a person's conscious gait choices.

Can humans perceive the metabolic benefit provided by augmentative exoskeletons?

BACKGROUND

The purpose of augmentative exoskeletons is to help people exceed the limitations of their human bodies, but this cannot be realized unless people choose to use these exciting technologies. Although human walking efficiency has been highly optimized over generations, exoskeletons have been able to consistently improve this efficiency by 10-15%. However, despite these measurable improvements, exoskeletons today remain confined to the laboratory. To achieve widespread adoption, exoskeletons must not only exceed the efficiency of human walking, but also provide a perceivable benefit to their wearers.

METHODS

In this study, we quantify the perceptual threshold of the metabolic efficiency benefit provided during exoskeleton-assisted locomotion. Ten participants wore bilateral ankle exoskeletons during continuous walking. The assistance provided by the exoskeletons was varied in 2 min intervals while participants provided feedback on their metabolic rate. These data were aggregated and used to estimate the perceptual threshold.

RESULTS

Participants were able to detect a change in their metabolic rate of 22.7% (SD: 17.0%) with 75% accuracy. This indicates that in the short term and on average, wearers cannot yet reliably perceive the metabolic benefits of today's augmentative exoskeletons.

CONCLUSIONS

If wearers cannot perceive the benefits provided by these technologies, it will negatively affect their impact, including long-term adoption and product viability. Future exoskeleton researchers and designers can use these methods and results to inform the development of exoskeletons that reach their potential.

Design of an Ankle Exoskeleton that Recycles Energy to Assist Propulsion during Human Walking

AbstractObjective: Active exoskeletons can handle different walking conditions, but require bulky components (e.g., motors) that need a significant source of power to do so. Purely passive exoskeletons are lightweight and energy-neutral, containing energy-recycling mechanisms that capture energy loss during negative power phases and return it as walking assistance. However, they are usually designed for stereotyped gaits (e.g., walking at fixed speed) and thus show poor adaptivity for variable conditions. This study is aimed to overcome these issues.

METHODS

A quasi-passive ankle exoskeleton is designed to integrate the merits of both active and passive exoskeletons, which captures the heel-strike energy loss and recycles it into propulsion. A novel, lightweight, energy-saving clutch and a heel-strike energy-storage mechanism are developed. They are coupled by a series spring that assists users calf muscles. Six healthy subjects walked with the device on level ground and inclined surfaces to validate its functionality.

RESULTS

Level ground studies indicate that the energy-storage mechanism enhances the assistance by increasing the output torque of the exoskeleton. Reductions in metabolic cost (6.4 1.3%, p < 0.05) were observed. During uphill walking, the assistance torque decreased compared with that on level ground, but it still reduced overall metabolic cost compared with baseline walking. During downhill walking, the assistance torque increased, but metabolic cost also slightly increased.

CONCLUSION

These results demonstrate the functionality of the prototype on level ground and its limitations on inclined surfaces.

SIGNIFICANCE

The proposed device highlights the possibility of widening the potential applications of exoskeletons.

Movement control, decision-making, and the building of Roman roads to link them

In science, as in life, one can only hope to both inform others, and be informed by them. The commentaries associated with our book Vigor have highlighted the many ways in which the theory that we proposed can be improved. For example, there are a myriad of factors that need to be considered in a fully encompassing objective function. The neural mechanisms underlying the links between movement and decision-making have yet to be unraveled. The implications of a two-way interaction between movement and decisions at both the individual and social levels remain to be understood. The commentaries outline future questions, and encouragingly highlight the diversity of science communities that may be linked via the concept of vigor.

Factors influencing self-selected walking speed in fibrotic interstitial lung disease

This study aimed to investigate the walking economy and possible factors influencing self-selected walking speed (SSWS) in patients with fibrotic interstitial lung disease (ILD) compared to controls. In this study, 10 patients with ILD (mean age: 63.8 ± 9.2 years, forced expiratory volume in the first second: 56 ± 7% of predicted) and 10 healthy controls underwent resting pulmonary function tests, cardiopulmonary exercise, and submaximal treadmill walking tests at different speeds. The walking economy was assessed by calculating the cost-of-transport (CoT). Dynamic stability was assessed by stride-to-stride fluctuations using video recordings. Patients with ILD showed reduced peak oxygen uptake with a tachypneic breathing pattern and significant oxygen desaturation during exercise. The CoT did not differ between the groups (p = 0.680), but dyspnea and SpO₂ were higher and lower, respectively, in patients with ILD at the same relative speeds. SSWS was reduced in ILD patients (2.6 ± 0.9 vs. 4.2 ± 0.4 km h^-1 p = 0.001) and did not correspond to the energetically optimal walking speed. Dynamic stability was significantly lower in patients with ILD than in healthy controls, mainly at lower speeds. Patients with ILD presented a similar cost of transport compared to healthy controls; however, they chose lower SSWS despite higher walking energy expenditure. Although walking stability and dyspnea were negatively affected, these factors were not associated with the slower walking speed chosen by individuals with ILD.

Speed-related but not detrended gait variability increases with more sensitive self-paced treadmill controllers at multiple slopes

Self-paced treadmills are being used more frequently to study humans walking with their self-selected gaits on a range of slopes. There are multiple options to purchase a treadmill with a built-in controller, or implement a custom written self-paced controller, which raises questions about how self-paced controller affect treadmill speed and gait biomechanics on multiple slopes. This study investigated how different self-paced treadmill controller sensitivities affected gait parameters and variability on decline, level, and incline slopes. We hypothesized that increasing self-paced controller sensitivity would increase gait variability on each slope. We also hypothesized that detrended variability could help mitigate differences in variability that arise from differences in speed fluctuations created by the self-paced controllers. Ten young adults walked on a self-paced treadmill using three controller sensitivities (low, medium, and high) and fixed speeds at three slopes (decline, -10°; level, 0°; incline, +10°). Within each slope, average walking speeds and spatiotemporal gait parameters were similar regardless of self-paced controller sensitivity. With higher controller sensitivities on each slope, speed fluctuations, speed variance, and step length variance increased whereas step frequency variance and step width variance were unaffected. Detrended variance was not affected by controller sensitivity suggesting that detrending variability helps mitigate differences associated with treadmill speed fluctuations. Speed-trend step length variances, however, increased with more sensitive controllers. Further, detrended step length variances were similar for self-paced and fixed speed walking, whereas self-paced walking included substantial speed-trend step length variance not present in fixed speed walking. In addition, regardless of the self-paced controller, subjects walked fastest on the level slope with the longest steps, narrowest steps, and least variance. Overall, our findings suggest that separating gait variability into speed-trend and detrended variability could be beneficial for interpreting gait variability among multiple self-paced treadmill studies and when comparing self-paced walking with fixed speed walking.

Using asymmetry to your advantage: learning to acquire and accept external assistance during prolonged split-belt walking

People can learn to exploit external assistance during walking to reduce energetic cost. For example, walking on a split-belt treadmill affords the opportunity for people to redistribute the mechanical work performed by the legs to gain assistance from the difference in belts' speed and reduce energetic cost. Though we know what people should do to acquire this assistance, this strategy is not observed during typical adaptation studies. We hypothesized that extending the time allotted for adaptation would result in participants adopting asymmetric step lengths to increase the assistance they can acquire from the treadmill. Here, participants walked on a split-belt treadmill for 45 min while we measured spatiotemporal gait variables, metabolic cost, and mechanical work. We show that when people are given sufficient time to adapt, they naturally learn to step further forward on the fast belt, acquire positive mechanical work from the treadmill, and reduce the positive work performed by the legs. We also show that spatiotemporal adaptation and energy optimization operate over different timescales: people continue to reduce energetic cost even after spatiotemporal changes have plateaued. Our findings support the idea that walking with symmetric step lengths, which is traditionally thought of as the endpoint of adaptation, is only a point in the process by which people learn to take advantage of the assistance provided by the treadmill. These results provide further evidence that reducing energetic cost is central in shaping adaptive locomotion, but this process occurs over more extended timescales than those used in typical studies.NEW & NOTEWORTHY Split-belt treadmill adaptation can be seen as a process where people learn to acquire positive work from the treadmill to reduce energetic cost. Though we know what people should do to reduce energetic cost, this strategy is not observed during adaptation studies. We extended the duration of adaptation and show that people continuously adapt their gait to acquire positive work from the treadmill to reduce energetic cost. This process requires longer exposure than traditionally allotted.

Precis of Vigor: Neuroeconomics of movement control

Why do we run toward people we love, but only walk toward others? Why do people in New York seem to walk faster than other cities? Why do our eyes linger longer on things we value more? There is a link between how the brain assigns value to things, and how it controls our movements. This link is an ancient one, developed through shared neural circuits that on one hand teach us how to value things, and on the other hand control the vigor with which we move. As a result, when there is damage to systems that signal reward, like dopamine and serotonin, that damage not only affects our mood and patterns of decision making, but how we move. In this book, we first ask why in principle evolution should have developed a shared system of control between valuation and vigor. We then focus on the neural basis of vigor, synthesizing results from experiments that have measured activity in various brain structures and neuromodulators, during tasks in which animals decide how patiently they should wait for reward, and how vigorously they should move to acquire it. Thus, the way we move unmasks one of our well-guarded secrets: how much we value the thing we are moving toward.

Walkability Index for Elderly Health: A Proposal

Nowadays, the elderly tend to make more trips: Health benefits resulting from their daily walking routines are an important topic in the context of urban renewal processes. Many health organizations and researchers have demonstrated the influence of the urban environment on walkability levels. This article aims to design a multifactor Walkability Index for Elderly Health (WIEH), capable of associating both the adequacy level of public spaces to elderly walkability, and physical exercise benefits while walking. The methodological approach comprised two main parts: Firstly, a literature review of main reports, legislation, and scientific articles was conducted at the intersection of ‘gerontology and physical exercise’ with ‘urban design and mobility’, leading to the selection of four aging-related studies as main contributors to the design of the WIEH; and, secondly, the development of the WIEH was undertaken, based on two premises and designed according to four steps. The first premise defined three systematic areas (urban tissue, urban scene, and safety), variables, and criteria to classify the pedestrian network; and the second premise focused on slopes and stairs in public spaces. The WIEH is divided in four steps: (1) Analyzing public spaces and characterizing their quality for walking, (2) considering the existence of slopes and stairs, (3) calculating different routes for the elderly in their daily routines, or when going to points of interest, and (4) selecting the “heart-friendly route” for elderly people. Adequate walking paths for the elderly can be identified through this innovative approach, with the aim of achieving direct health benefits during their daily routines. Ultimately, the WIEH is capable of supporting decision makers and designers in creating inclusive and age-friendly spaces.

Does surface slope affect dual task performance and gait? An exploratory study in younger and older adults

An increased risk of falling is associated with changes in gait while dual-tasking. The degree to which gait stability is altered during walking is influenced by an individual's cognitive and postural capacity, and the difficulty of the presented tasks. However, it is unknown how greater walking task difficulty affects gait stability in younger and older adults when dual-tasking. The purpose of the current study was to determine the effect of walking task difficulty on gait stability in younger and older adults while performing a difficult audiospatial task. Ten younger [mean (SD) age 30.8 (6.6) years; 5 women] and 10 older [66.8 (5.7) years; 6 women] healthy adults walked on a treadmill at their preferred walking speed [younger 4.8 (0.4) ms^-1, older 4.5 (0.5) ms^-1) on either a level, or downhill slope both with and without responding to an audiospatial task. Step width, step width SD and mediolateral centre of mass displacement were calculated to determine changes in gait, and response time and accuracy were calculated to determine secondary task performance. Results indicated that older adults displayed a consistently greater step width (p ≤ 0.015) and maintained their mediolateral centre of mass displacement (p > 0.05) while walking downhill and responding to the audiospatial task, compared to downhill walking only. In contrast, younger adults maintained a regular step width during both level and downhill dual-tasking compared to level and downhill walking only (p > 0.05), however displayed a lower mediolateral centre of mass displacement during level dual-task walking compared to level walking only (p = 0.013). When the difficulty of the walking task was greater, older adults increased their step width, which increased their stability.

A hinge-free, non-restrictive, lightweight tethered exosuit for knee extension assistance during walking

In individuals with motor impairments such as those post-stroke or with cerebral palsy, the function of the knee extensors may be affected during walking, resulting in decreased mobility. We have designed a lightweight, hinge-free wearable robot combining soft textile exosuit components with integrated rigid components, which assists knee extension when needed but is otherwise highly transparent to the wearer. The exosuit can apply a wide range of assistance profiles using a flexible multi-point reference trajectory generator. Additionally, we implemented a controller safety limit to address the risk of hyperextension stemming from the hinge-free design. The exosuit was evaluated on six healthy participants walking uphill and downhill on a treadmill at a 10° slope with a set of joint power-inspired assistance profiles. A comparison of sagittal plane joint angles between no exosuit and exosuit unpowered conditions validated the device transparency. With positive power assistance, we observed reduction in average positive knee biological power during uphill walking (left: 17.5 ± 3.21%, p = 0.005; right: 23.2 ± 3.54%, p = 0.008). These initial findings show promise for the assistive capability of the device and its potential to improve the quality of gait and increase mobility in clinical populations.

Seeing Gravity: Gait Adaptations to Visual and Physical Inclines - A Virtual Reality Study

Using advanced virtual reality technology, we demonstrate that exposure to virtual inclinations visually simulating inclined walking induces gait modulations in a manner consistent with expected gravitational forces (i.e., acting upon a free body), suggesting vision-based perception of gravity. The force of gravity critically impacts the regulation of our movements. However, how humans perceive and incorporate gravity into locomotion is not well understood. In this study, we introduce a novel paradigm for exposing humans to incongruent sensory information under conditions constrained by distinct gravitational effects, facilitating analysis of the consistency of human locomotion with expected gravitational forces. Young healthy adults walked under conditions of actual physical inclinations as well as virtual inclinations. We identify and describe ‘braking’ and ‘exertion’ effects – locomotor adaptations accommodating gravito-inertial forces associated with physical inclines. We show that purely visual cues (from virtual inclinations) induce consistent locomotor adaptations to counter expected gravity-based changes, consistent with indirect prediction mechanisms. Specifically, downhill visual cues activate the braking effect in anticipation of a gravitational boost, whereas uphill visual cues promote an exertion effect in anticipation of gravitational deceleration. Although participants initially rely upon vision to accommodate environmental changes, a sensory reweighting mechanism gradually reprioritizes body-based cues over visual ones. A high-level neural model outlines a putative pathway subserving the observed effects. Our findings may be pivotal in designing virtual reality-based paradigms for understanding perception and action in complex environments with potential translational benefits.

Effects of downslope walking on Soleus H-reflexes and walking function in individuals with multiple sclerosis: A preliminary study

BACKGROUND

Downslope walking (DSW) is an eccentric-based exercise intervention that promotes neuroplasticity of spinal reflex circuitry by inducing depression of Soleus Hoffman (H)-reflexes in young, neurologically unimpaired adults.

OBJECTIVE

The objective of the study was to evaluate the effects of DSW on spinal excitability (SE) and walking function (WF) in people with multiple sclerosis (PwMS).

METHODS

Our study comprised two experiments on 12 PwMS (11 women; 45.3±11.8 years). Experiment 1 evaluated acute effects of a single 20-minute session of treadmill walking at three different walking grades on SE, 0% or level walking (LW), - 7.5% DSW, and - 15% DSW. Experiment 2 evaluated the effects of 6 sessions of DSW, at - 7.5% DSW (with second session being - 15% DSW) on SE and WF.

RESULTS

Experiment 1 showed significantly greater acute % H-reflex depression following - 15% DSW compared to LW (p = 0.02) and - 7.5% DSW (p = 0.05). Experiment 2 demonstrated significant improvements in WF. PwMS who showed greater acute H-reflex depression during the - 15% DSW session also demonstrated greater physical activity, long-distance WF, and the ability to have greater H-reflex depression after DSW training. Significant changes were not observed in regards to SE.

CONCLUSIONS

Though significant changes were not observed in SE after DSW training, we observed an improvement in WF which merits further investigation of DSW in PwMS.

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.

Taking advantage of external mechanical work to reduce metabolic cost: the mechanics and energetics of split-belt treadmill walking

KEY POINTS

The neuromotor system generates flexible motor patterns that can adapt to changes in our bodies or environment and also take advantage of assistance provided by the environment. We ask how energy minimization influences adaptive learning during human locomotion to improve economy when walking on a split-belt treadmill. We use a model-based approach to predict how people should adjust their walking pattern to take advantage of the assistance provided by the treadmill, and we validate these predictions empirically. We show that adaptation to a split-belt treadmill can be explained as a process by which people reduce step length asymmetry to take advantage of the work performed by the treadmill to reduce metabolic cost. Our results also have implications for the evaluation of devices designed to reduce effort during walking, as locomotor adaptation may serve as a model approach to understand how people learn to take advantage of external assistance.

ABSTRACT

In everyday tasks such as walking and running, we often exploit the work performed by external sources to reduce effort. Recent research has focused on designing assistive devices capable of performing mechanical work to reduce the work performed by muscles and improve walking function. The success of these devices relies on the user learning to take advantage of this external assistance. Although adaptation is central to this process, the study of adaptation is often done using approaches that seem to have little in common with the use of external assistance. We show in 16 young, healthy participants that a common approach for studying adaptation, split-belt treadmill walking, can be understood from a perspective in which people learn to take advantage of mechanical work performed by the treadmill. Initially, during split-belt walking, people step further forward on the slow belt than the fast belt which we measure as a negative step length asymmetry, but this asymmetry is reduced with practice. We demonstrate that reductions in asymmetry allow people to extract positive work from the treadmill, reduce the positive work performed by the legs, and reduce metabolic cost. We also show that walking with positive step length asymmetries, defined by longer steps on the fast belt, minimizes metabolic cost, and people choose this pattern after guided experience of a wide range of asymmetries. Our results suggest that split-belt adaptation can be interpreted as a process by which people learn to take advantage of mechanical work performed by an external device to improve economy.

Is the 10 metre walk test on sloped surfaces associated with age and physical activity in healthy adults?

Background

Preferred walking speed is considered an important indicator of health in older adults and is measured on level ground. However, this may not represent the complex demands of community ambulation such as walking on sloped surfaces. Performing a 10 m walk test on a sloped surface is a novel test, and may be a more sensitive measure of walking capacity which may better discriminate age or health-related changes in gait speed compared to a traditional level 10 m walk test. The purpose of this investigation was to determine healthy adults’ performance in the 10 m walk test across various inclines and speeds, and which version of the 10 m walk test would be best at discriminating age-related changes in walking speed. Further, this study aimed to determine whether measures of general health and physical activity are associated with the performance of each test.

Methods

Healthy Adults (n = 181) aged 20–80 years completed the 10 m walk test on level, downhill and uphill surfaces (8° inclination) at fastest and preferred speeds. Descriptive statistics were calculated for walking speed for males and females across each decade of life. Repeated measures ANOVA was performed to discriminate age-related changes in gait speed by decade, for the 10 m walk test at each speed and slope. Multiple linear regression analyses were conducted to examine the association between waist to height ratio, resting heart rate, age and self-reported physical activity upon preferred and fastest walking speeds at each incline (level/downhill/uphill).

Results

The 10 m walk test best discriminated age-related changes in gait speed when performed at fastest speeds on each slope, or at a preferred speed on an uphill slope. Waist to height ratio, age and the physical activity index were all significantly associated with fastest walking speeds over each incline and preferred uphill speed. Only waist to height ratio was associated with preferred walking speed on level and downhill surfaces.

Conclusions

The 10 m walk test has the greatest ability to discriminate age- and health-related changes in gait speed when it is performed at a fastest speed on any slope, or uphill at a preferred speed. The normative data reported in this study may be used to compare the performance of the 10 m walk test to that of healthy adults at preferred and fastest speeds on sloped surfaces.

Electronic supplementary material

The online version of this article (10.1186/s11556-019-0219-0) contains supplementary material, which is available to authorized users.

Differential activation of lumbar and sacral motor pools during walking at different speeds and slopes

Organization of spinal motor output has become of interest for investigating differential activation of lumbar and sacral motor pools during locomotor tasks. Motor pools are associated with functional grouping of motoneurons of the lower limb muscles. Here we examined how the spatiotemporal organization of lumbar and sacral motor pool activity during walking is orchestrated with slope of terrain and speed of progression. Ten subjects walked on an instrumented treadmill at different slopes and imposed speeds. Kinetics, kinematics, and electromyography of 16 lower limb muscles were recorded. The spinal locomotor output was assessed by decomposing the coordinated muscle activation profiles into a small set of common factors and by mapping them onto the rostrocaudal location of the motoneuron pools. Our results show that lumbar and sacral motor pool activity depend on slope and speed. Compared with level walking, sacral motor pools decrease their activity at negative slopes and increase at positive slopes, whereas lumbar motor pools increase their engagement when both positive and negative slope increase. These findings are consistent with a differential involvement of the lumbar and the sacral motor pools in relation to changes in positive and negative center of body mass mechanical power production due to slope and speed.NEW & NOTEWORTHY In this study, the spatiotemporal maps of motoneuron activity in the spinal cord were assessed during walking at different slopes and speeds. We found differential involvement of lumbar and sacral motor pools in relation to changes in positive and negative center of body mass power production due to slope and speed. The results are consistent with recent findings about the specialization of neuronal networks located at different segments of the spinal cord for performing specific locomotor tasks.

Is natural variability in gait sufficient to initiate spontaneous energy optimization in human walking?

In new walking contexts, the nervous system can adapt preferred gaits to minimize energetic cost. During treadmill walking, this optimization is not usually spontaneous but instead requires experience with the new energetic cost landscape. Experimenters can provide subjects with the needed experience by prescribing new gaits or instructing them to explore new gaits. Yet in familiar walking contexts, people naturally prefer energetically optimal gaits: the nervous system can optimize cost without an experimenter's guidance. Here we test the hypothesis that the natural gait variability of overground walking provides the nervous system with sufficient experience with new cost landscapes to initiate spontaneous minimization of energetic cost. We had subjects walk over paths of varying terrain while wearing knee exoskeletons that penalized walking as a function of step frequency. The exoskeletons created cost landscapes with minima that were, on average, 8% lower than the energetic cost at the initially preferred gaits and achieved at walking speeds and step frequencies that were 4% lower than the initially preferred values. We found that our overground walking trials amplified gait variability by 3.7-fold compared with treadmill walking, resulting in subjects gaining greater experience with new cost landscapes, including frequent experience with gaits at the new energetic minima. However, after 20 min and 2.0 km of walking in the new cost landscapes, we observed no consistent optimization of gait, suggesting that natural gait variability during overground walking is not always sufficient to initiate energetic optimization over the time periods and distances tested in this study. NEW & NOTEWORTHY While the nervous system can continuously optimize gait to minimize energetic cost, what initiates this optimization process during every day walking is unknown. Here we tested the hypothesis that the nervous system leverages the natural variability in gait experienced during overground walking to converge on new energetically optimal gaits created using exoskeletons. Contrary to our hypothesis, we found that participants did not adapt toward optimal gaits: natural variability is not always sufficient to initiate spontaneous energy optimization.

Gait variability following abrupt removal of external stabilization decreases with practice in incomplete spinal cord injury but increases in non-impaired individuals

Background

Individuals with incomplete spinal cord injury (iSCI) exhibit considerable lateral center of mass (COM) movement variability during gait transitions from a stabilizing to unassisted environment, while non-impaired individuals do not. To understand how iSCI influences gait adaption, we examined persons with and without iSCI performing repeated locomotor transitions. We hypothesized that, with practice, individuals with iSCI would prioritize COM control performance during the transition as exhibited by a reduction in kinematic variability. In, contrast, we hypothesized that non-impaired individuals would prioritize control effort by decreasing muscular activity.

Methods

Thirteen participants with iSCI and 12 non-impaired participants performed five treadmill-walking trials. During some trials, a cable-robot applied stabilizing lateral forces to the pelvis proportional in magnitude and opposite in direction to real-time lateral COM velocity. Each trial consisted of 300 continuous steps with or without a transition. During the first and last trials, no forces were applied and no transitions occurred (Null trials). During trials 2–4 (transition trials), the first 200 steps occurred in the stabilizing force field, forces were then abruptly removed, and 100 more unassisted steps were performed. We analyzed COM and step width variability, and hip abductor muscle activity during transitions (force removal until gait returned to steady state).

Results

Participants with iSCI displayed large COM movement variability during the first transition but reduced variability with practice. During the first transition, lateral COM speed, lateral COM excursion, and step width were all more variable than during the first Null trial (p < 0.05). By the third transition, no metric was different from Null trials (p > 0.05).

In contrast, non-impaired participants’ movement variability during the first transition was not different from Null trials (p > 0.05). With practice, movement variability increased: lateral COM excursion was more variable during Transitions 2 and 3 versus the first Null trial (p < 0.05). Non-impaired participants decreased hip abductor activity from Transition 1 to 3 (p < 0.05).

Conclusions

Individuals with iSCI demonstrated rapid motor savings. By the third transition, individuals with iSCI reduced locomotor variability to baseline levels. In contrast, non-impaired participants prioritized control effort over control performance. With practice transitioning, non-impaired participants increased locomotor variability and decreased muscular effort.

Electronic supplementary material

The online version of this article (10.1186/s12984-018-0475-7) contains supplementary material, which is available to authorized users.

The motor repertoire of older adult fallers may constrain their response to balance perturbations

Older adults are at a high risk of falls, and most falls occur during locomotor activities like walking. This study aimed to improve our understanding of changes in neuromuscular control associated with increased risk of falls in older adults in the presence of dynamic balance challenges during walking. Motor module (also known as muscle synergy) analyses identified changes in the neuromuscular recruitment of leg muscles during walking with and without perturbations designed to elicit the visual perception of lateral instability. During normal walking we found that a history of falls (but not age) was associated with reduced motor module complexity and that age (but not a history of falls) was associated with increased step-to-step variability of module recruitment timing. Furthermore, motor module complexity was unaltered in the presence of optical flow perturbations. The specific effects of a history of falls on leg muscle recruitment included an absence and/or inability to independently recruit motor modules normally recruited to perform biomechanical functions important for walking balance control. These results suggest that fallers do not recruit the appropriate motor modules necessary for well-coordinated walking balance control even in the presence of perturbations. The identified changes in the modular control of walking balance in older fallers may either represent a neural deficit that leads to poor balance control or a prior history of falls that results in a compensatory motor adaptation. In either case, our study provides initial evidence that a reduced motor repertoire in older adult fallers may be a constraint on their ability to appropriately respond to balance challenges during walking. NEW & NOTEWORTHY This is the first study to demonstrate a reduced motor repertoire during walking in older adults with a history of falls but without any overt neurological deficits. Furthermore, using virtual reality during walking to elicit the visual perception of lateral instability, we provide initial evidence that a reduced motor repertoire in older adult fallers may be a constraint on their ability to appropriately respond to balance challenges during walking.

Kinematic patterns while walking on a slope at different speeds

During walking, the elevation angles of the thigh, shank, and foot (i.e., the angle between the segment and the vertical) covary along a characteristic loop constrained on a plane. Here, we investigate how the shape of the loop and the orientation of the plane, which reflect the intersegmental coordination, change with the slope of the terrain and the speed of progression. Ten subjects walked on an inclined treadmill at different slopes (between -9° and +9°) and speeds (from 0.56 to 2.22 m/s). A principal component analysis was performed on the covariance matrix of the thigh, shank, and foot elevation angles. At each slope and speed, the variance accounted for by the two principal components was >99%, indicating that the planar covariation is maintained. The two principal components can be associated to the limb orientation (PC1*) and the limb length (PC2*). At low walking speeds, changes in the intersegmental coordination across slopes are characterized mainly by a change in the orientation of the covariation plane and in PC2* and to a lesser extent, by a change in PC1*. As speed increases, changes in the intersegmental coordination across slopes are more related to a change in PC1 *, with limited changes in the orientation of the plane and in PC 2*. Our results show that the kinematic patterns highly depend on both slope and speed. NEW & NOTEWORTHY In this paper, changes in the lower-limb intersegmental coordination during walking with slope and speed are linked to changes in the trajectory of the body center of mass. Modifications in the kinematic pattern with slope depend on speed: at slow speeds, the net vertical displacement of the body during each step is related to changes in limb length and orientation. When speed increases, the vertical displacement is mostly related to a change in limb orientation.

A Novel Downhill Gait-Training Program Following a Total Knee Arthroplasty: A Case Report Highlighting the Impacts of Self-selected Speed on Gait Symmetry

Study Design Case report. Background Walking plays an essential role in activities of daily living and has varied health benefits. Studies report that gait speed and symmetry are impacted in individuals following total knee arthroplasty (TKA). Unfortunately, abnormal gait patterns persist in individuals after TKA. Downhill walking may provide a simple and feasible exercise regimen to improve gait patterns. The purpose of this case report was to describe the application of a downhill treadmill training program and the subsequent changes in gait patterns in an individual following a unilateral TKA. Case Description The participant was a 59-year-old woman following a right TKA. Downhill gait training was initiated 1 month post TKA and completed for 5 weeks. Outcomes were assessed using questionnaires, mobility tests, strength of quadriceps, and gait patterns. The treadmill speed was determined by the participant's self-selected gait speed on a level surface. Outcomes The participant's eccentric quadriceps strength in the operated limb significantly increased after the gait training. Her physical function recovered to a level similar to that of previous reports. Postintervention gait analysis was conducted at 2 self-selected speeds, due to an increase in the participant's self-selected gait speed between sessions. The participant demonstrated a more symmetrical gait pattern when walking slower and a more asymmetrical gait pattern at the faster speed. Discussion After completion of downhill gait training in conjunction with therapeutic exercises, the participant showed an increase in quadriceps strength and improved physical function. This case report describes the utilization and potential feasibility of downhill gait training in conjunction with outpatient physical therapy for an individual following unilateral TKA. Level of Evidence Therapy, level 5. J Orthop Sports Phys Ther 2018;48(2):104-110. Epub 7 Nov 2017. doi:10.2519/jospt.2018.7374.

Proprioceptive feedback contributes to the adaptation toward an economical gait pattern

Humans generally prefer gait patterns with a low metabolic cost, but it is unclear how such patterns are chosen. We have previously proposed that humans may use proprioceptive feedback to identify economical movement patterns. The purpose of the present experiments was to investigate the role of plantarflexor proprioception in the adaptation toward an economical gait pattern. To disrupt proprioception in some trials, we applied noisy vibration (randomly varying between 40-120Hz) over the bilateral Achilles tendons while participants stood quietly or walked on a treadmill. For all 10min walking trials, the treadmill surface was initially level before slowly increasing to a 2.5% incline midway through the trial without participant knowledge. During standing posture, noisy vibration increased sway, indicating decreased proprioception accuracy. While walking on a level surface, vibration did not significantly influence stride period or metabolic rate. However, vibration had clear effects for the first 2-3min after the incline increase; vibration caused participants to walk with shorter stride periods, reduced medial gastrocnemius (MG) activity during mid-stance (30-65% stance), and increased MG activity during late-stance (65-100% stance). Over time, these metrics gradually converged toward the gait pattern without vibration. Likely as a result of this delayed adaptation to the new mechanical context, the metabolic rate when walking uphill was significantly higher in the presence of noisy vibration. These results may be explained by the disruption of proprioception preventing rapid identification of muscle activation patterns which allow the muscles to operate under favorable mechanical conditions with low metabolic demand.

Downhill walking gait pattern discriminates between types of knee arthroplasty: improved physiological knee functionality in UKA versus TKA

PURPOSE

To determine whether downhill walking gait pattern discriminates between different types of knee arthroplasty.

METHODS

Nineteen unicompartmental knee arthroplasty (UKA) and fourteen total knee arthroplasty (TKA) patients who were well matched demographically and with high Oxford knee scores (OKS) for their operation type were evaluated at a minimum 1 year after their operation with downhill gait analysis. Nineteen healthy young subjects were used as controls. Downhill gait analysis was carried out on an instrumented treadmill that was ramped at the rear to produce a declination of 7°. All subjects after a period of habituation were tested for preferred and top downhill walking speed with associated ground reaction and temporospatial measurements.

RESULTS

The UKA group had higher mean OKS (44.8 ± 2.9 vs 41.9 ± 4.7, p = 0.03) as predicted. The UKA group walked downhill 15% faster than the TKA group (1.75 ± 0.14 vs 1.52 ± 0.13 m/s, p < 0.0001) despite having the same cadence (134.9 ± 8.0 vs 133.9 ± 9.6 steps/min). This 15% difference in speed appeared largely due to a 15% increase in stride length (173 ± 14 vs 150 ± 17 cm, p = 0.0007) and normal weight acceptance, both of which were similar to the controls.

CONCLUSION

Using an instrumented treadmill to test a commonly performed task, stride length when walking downhill highlights the functional differences between arthroplasty groups. Near normal restoration of physiological gait pattern was found in unicompartmentals as compared to total knee replacements.

LEVEL OF EVIDENCE

Retrospective comparative study, Level III.

On voluntary rhythmic leg movement behaviour and control during pedalling

The overall purpose of the present dissertation was to contribute to the understanding of voluntary human rhythmic leg movement behaviour and control. This was achieved by applying pedalling as a movement model and exposing healthy and recreationally active individuals as well as trained cyclists to for example cardiopulmonary and mechanical loading, fatiguing exercise, and heavy strength training. As a part of the background, the effect of pedalling frequency on diverse relevant biomechanical, physiological, and psychophysiological variables as well as on performance was initially explored. Freely chosen pedalling frequency is considerably higher than the energetically optimal pedalling frequency. This has been shown by others and was confirmed in the present work. As a result, pedal force is relatively low while rates of VO2 and energy turnover are relatively high during freely chosen pedalling as compared to a condition where a lower and more efficient pedalling frequency is imposed. The freely chosen pedalling frequency was in the present work, and by others, found to most likely be less advantageous than the lower energetically optimal pedalling frequency with respect to performance during intensive cycling following prolonged submaximal cycling. This stimulates the motivation to understand the behaviour and control of the freely chosen pedalling frequency during cycling. Freely chosen pedalling frequency was in the present work shown to be highly individual. In addition, the pedalling frequency was shown to be steady in a longitudinal perspective across 12 weeks. Further, it was shown to be unaffected by both fatiguing hip extension exercise and hip flexion exercise as well as by increased loading on the cardiopulmonary system at constant mechanical loading, and vice versa. Based on this, the freely chosen pedalling frequency is considered to be characterised as a highly individual, steady, and robust innate voluntary motor rhythm under primary influence of central pattern generators. The last part of the characterisation is largely based on, and supported by, work of other researchers in the field. Despite the robustness of the freely chosen pedalling frequency, it may be affected by some particular factors. As an example from the present work, freely chosen pedalling frequency during treadmill cycling increased by on average 15 to 17 rpm when power output was increased from a value corresponding to 86% and up to 165% of Wmax . This phenomenon is supported by other studies. As another example from the present work, freely chosen pedalling frequency decreased by on average 9 to 14 rpm following heavy strength training that involved both hip extension and hip flexion. Further, the present work suggested that the latter phenomenon occurred within the first week of training and was caused by in particular the hip extension strength training rather than the hip flexion strength training. The fast response to the strength training indicated that neural adaptations presumably caused the observed changes in movement behaviour. The internal organisation of the central pattern generator is by some other researchers in the field considered to be functionally separated into two components, in which, one is responsible for movement frequency and another is responsible for movement pattern. For the present dissertation, the freely chosen pedalling frequency was considered to reflect the rhythmic movement frequency of the voluntary rhythmic leg movement of pedalling. The tangential pedal force profile was considered to reflect the rhythmic movement pattern. The present work showed that fatiguing hip flexion exercise in healthy and recreationally active individuals modified the tangential pedal force profile during cycling at a pre-set target pedalling frequency in a way that the minimum tangential pedal force became more negative, the maximum tangential pedal force increased, and the phase with negative tangential pedal force increased. In other words, the legs were "actively lifted" to a lesser extent in the upstroke phase. Fatiguing hip extension exercise did not have that effect. And none of the fatiguing exercises affected the freely chosen pedalling frequency. The present work furthermore showed that the primary effect of hip extension strength training was that it decreased the freely chosen pedalling frequency. An interpretation of this could be that the hip extension strength training, in particular, influenced the output from the component of the central pattern generator that may be responsible for rhythmic movement frequency.

Subjective valuation of cushioning in a human drop landing task as quantified by trade-offs in mechanical work

Humans can perform motor tasks in a variety of ways, yet often favor a particular strategy. Some factors governing the preferred strategy may be objective and quantifiable, (e.g. metabolic energy or mechanical work) while others may be more subjective and less measurable, (e.g. discomfort, pain, or mental effort). Subjectivity can make it challenging to explain or predict preferred movement strategies. We propose that subjective factors might nevertheless be characterized indirectly by their trade-offs against more objective measures such as work. Here we investigated whether subjective costs that influence human movement during drop landings could be indirectly assessed by quantifying mechanical work performed. When landing on rigid ground, humans typically absorb much of the collision actively by bending their knees, perhaps to avoid the discomfort of stiff-legged landings. We measured how work performed by healthy adults (N=8) changed as a function of surface cushioning for drop landings (fixed at about 0.4m) onto varying amounts of foam. Landing on more foam dissipated more energy passively in the surface, thus reducing the net dissipation required of subjects, due to relatively fixed landing energy. However, subjects actually performed even less work in the dissipative collision, as well as in the subsequent active, positive work to return to upright stance (approximately linear decrease of about 1.52 J per 1 cm of foam thickness). As foam thickness increased, there was also a corresponding reduction in center-of-mass vertical displacement after initial impact by up to 43%. Humans appear to subjectively value cushioning, revealed by the extra work they perform landing without it. Cushioning is thus worth more than the energy it dissipates, in an amount that indicates the subjective discomfort of stiff landings.

Slope walking causes short-term changes in soleus H-reflex excitability

The purpose of this study was to test the hypothesis that downslope treadmill walking decreases spinal excitability. Soleus H-reflexes were measured in sixteen adults on 3 days. Measurements were taken before and twice after 20 min of treadmill walking at 2.5 mph (starting at 10 and 45 min post). Participants walked on a different slope each day [level (Lv), upslope (Us) or downslope (Ds)]. The tibial nerve was electrically stimulated with a range of intensities to construct the M-response and H-reflex curves. Maximum evoked responses (Hmax and Mmax) and slopes of the ascending limbs (Hslp and Mslp) of the curves were evaluated. Rate-dependent depression (RDD) was measured as the % depression of the H-reflex when measured at a rate of 1.0 Hz versus 0.1 Hz. Heart rate (HR), blood pressure (BP), and ratings of perceived exertion (RPE) were measured during walking. Ds and Lv walking reduced the Hmax/Mmax ratio (P = 0.001 & P = 0.02), although the reduction was larger for Ds walking (29.3 ± 6.2% vs. 6.8 ± 5.2%, P = 0.02). The reduction associated with Ds walking was correlated with physical activity level as measured via questionnaire (r = -0.52, P = 0.04). Us walking caused an increase in the Hslp/Mslp ratio (P = 0.03) and a decrease in RDD (P = 0.04). These changes recovered by 45 min. Exercise HR and BP were highest during Us walking. RPE was greater during Ds and Us walking compared to Lv walking, but did not exceed "Fairly light" for Ds walking. In conclusion, in healthy adults treadmill walking has a short-term effect on soleus H-reflex excitability that is determined by the slope of the treadmill surface.

Age-related differences when walking downhill on different sloped terrains

Despite the common situation of walking on different sloped terrains, previous work on gait has focused on level terrain. This study aims to assess whether any age-related differences exist in spatiotemporal and stability parameters when walking downhill on three different sloped walkways. Two tri-axial accelerometers were used at the levels of head and pelvis to investigate spatiotemporal parameters, magnitude (root mean square, RMS), harmonic content of accelerations (harmonic ratios, HR) and attenuation between body levels (ATT) in 35 older adults (OA, 69 ± 4.5 y.o.) and 22 young adults (YA, 22.1 ± 1.9 y.o.). Older adults walked at the same speed and cadence as young adults in flat terrain (FL, 0%) and moderate hill (MH, 8%). In the highest slope (PH, 20%), older adults reduced speed and step length and both groups increased cadence. Age had no effect on attenuation and RMS profiles. RMS increased with slope in all directions at both head and pelvis, except, for medio-lateral direction (ML), with similar head RMS in all slopes. There is an important shift in ATT from anteroposterior direction (AP) to ML at the highest slope, resulting in smaller antero-posterior attenuation and greater medio-lateral attenuation. Age differences appeared in the smoothness (HR) at the flat terrain, with increased vertical and antero-posterior values for young adults. As slope increased, group differences disappeared and HR decreased for all directions of motion. In general, spatiotemporal adaptations to increased slope seem to be part of a mechanism to improve ML attenuation, in both young and old adults.

Influence of physical capacities of males with transtibial amputation on gait adjustments on sloped surfaces

The aim of the study was to investigate how kinematic and kinetic adjustments between level and slope locomotion of persons with transtibial amputation are related to their individual muscular and functional capacities. A quantified gait analysis was conducted on flat and slope surfaces for seven patients with transtibial amputation and a control group of eight subjects to obtain biomechanical parameters. In addition, maximal isometric muscular strength (knee and hip extensors) and functional scores were measured. The results of this study showed that most of the persons with transtibial amputation could adapt to ramp ascent either by increasing ankle, knee, and hip flexion angles of the residual limb and/or by recruiting their hip extensors to guarantee enough hip extension power during early stance. Besides, 6-minute walk test score was shown to be a good predictor of adaptation capacities to slope ascent. In ramp descent, the increase of knee flexion moment was correlated with knee extensor strength and residual-limb length. However, no correlation was observed with functional parameters. Results show that the walking strategy adopted by persons with transtibial amputation to negotiate ramp locomotion mainly depends on their muscular capacities. Therefore, muscular strengthening should be a priority during rehabilitation.

Downhill walking to improve lower limb strength in healthy young adults

Walking is the most natural physical activity to maintain and improve fitness and health. Walking downhill is usefully adopted to plan training programmes to improve the strength, particularly in older adults. The present research was aimed to evaluate the influence of downhill walking on leg strength in young adult. A total of 32 females (age 26 ± 4 years; height 1.64 ± 0.05 m; body mass 57.6 ± 5.6 kg) were divided into four groups and they carried out an exercise intervention consisting of three sessions per week for 6 weeks, each lasting 30 minutes. Groups were defined at several workloads characterised by treadmill inclination (%) and walking speed (m · s(-1)): Level Walking at treadmill inclination 0% and walking speed 1.0; Uphill Walking at +20%, 0.75; Downhill Walking (DW) at -20%, 1.36; and Mixed Walking at +20%, 0.75 and -20%, 1.36 each lasting 15 minutes. Maximum voluntary contraction (MVC) developed by the Quadriceps Femoris and Endurance Time at 60% MVC were evaluated before and after experimental period. At the end of each session, Borg's scale and Visual Analogue Scale (VAS) were adopted in order to evaluate perception of rate exertion and pain. Statistical analysis showed significant only in MVC for DW in both right and left legs. Borg's scale and VAS described light activity free of pain. Present findings showed how an eccentric exercise, short lasting and at a low workload, can be useful in inducing improvements in leg strength.

Energetic consequences of using a prosthesis with adaptive ankle motion during slope walking in persons with a transtibial amputation

BACKGROUND

Technological advances in prosthetic design include the use of microprocessors that adapt device performance based on user motion. The Proprio ankle unit prepositions the foot to adjust for walking on slopes and increases foot clearance during swing to minimize gait deviations.

STUDY DESIGN

Comparative analysis.

OBJECTIVES

To investigate the effect of a prosthesis with adaptive ankle motion on physiological gait performance during slope walking.

METHODS

Six persons with a unilateral transtibial amputation completed treadmill walking tests at three slopes (-5°, 0°, and 5°). The participants were tested wearing a customary device, active Proprio (Pon), and an identical inactivated Proprio (Poff).

RESULTS

Metabolic energy expenditure, energy cost for walking, and rating of walking difficulty were not statistically different between the Pon and Poff for all tested slopes. However, for slope descent, energy expenditure and energy cost for walking improved significantly by an average of 10%-14% for both the Pon and Poff compared to the customary limb. Rating of walking difficulty also showed an improvement with slope descent for both the Pon and Poff compared to the customary device. An improvement with slope ascent was found for Pon compared to the customary limb only.

CONCLUSIONS

Adaptive ankle motion provided no meaningful physiological benefit during slope walking. The Proprio was, however, less demanding than the customary device for slope descent. Differences in the mechanical properties of the prosthetic feet likely contributed to the changes.

CLINICAL RELEVANCE

While the adaptive ankle motion did not affect metabolic energy expenditure or energy cost for walking, the results suggest close attention should be paid to the mechanical properties of the foot component. Assessment of gait on nonlevel surfaces is recommended to better understand the implications of different prosthetic design features.