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

Estimating the mechanical cost of transport in human walking with a simple kinematic data-driven mechanical model

This work utilizes a simplified, streamlined approach to study the mechanical cost of transport in human walking. Utilizing the kinematic motion data of the center of mass, velocities and accelerations are determined using kinematic analysis; the applied force is then obtained using inverse dynamics. We calculate the mechanical cost of transport per step from both synthetic and measured data, using a very simple mechanical model of walking. The approach studied can serve as an informative gait characteristic to monitor rehabilitation in human walking.

Enhanced motor learning and motor savings after acute intermittent hypoxia are associated with a reduction in metabolic cost

Breathing mild bouts of low oxygen air (i.e. acute intermittent hypoxia, AIH) has been shown to improve locomotor function in humans after a spinal cord injury. How AIH-induced gains in motor performance are achieved remains unclear. We examined the hypothesis that AIH augments motor learning and motor retention during a locomotor adaptation task. We further hypothesized that gains in motor learning and retention will be associated with reductions in net metabolic power, consistent with the acquisition of energetically favourable mechanics. Thirty healthy individuals were randomly allocated into either a control group or an AIH group. We utilized a split-belt treadmill to characterize adaptations to an unexpected belt speed perturbation of equal magnitude during an initial exposure and a second exposure. Adaptation was characterized by changes in spatiotemporal step asymmetry, anterior-posterior force asymmetry, and net metabolic power. While both groups adapted by reducing spatial asymmetry, only the AIH group achieved significant reductions in double support time asymmetry and propulsive force asymmetry during both the initial and the second exposures to the belt speed perturbation. Net metabolic power was also significantly lower in the AIH group, with significant reductions from the initial perturbation exposure to the second. These results provide the first evidence that AIH mediates improvements in both motor learning and retention. Further, our results suggest that reductions in net metabolic power continue to be optimized upon subsequent learning and are driven by more energetically favourable temporal coordination strategies. Our observation that AIH facilitates motor learning and retention can be leveraged to design rehabilitation interventions that promote functional recovery. KEY POINTS: Brief exposures to low oxygen air, known as acute intermittent hypoxia (AIH), improves locomotor function in humans after a spinal cord injury, but it remains unclear how gains in motor performance are achieved. In this study, we tested the hypothesis that AIH induces enhancements in motor learning and retention by quantifying changes in interlimb coordination, anterior-posterior force symmetry and metabolic cost during a locomotor adaptation task. We show the first evidence that AIH improves both motor learning and savings of newly learned temporal interlimb coordination strategies and force asymmetry compared to untreated individuals. We further demonstrate that AIH elicits greater reductions in metabolic cost during motor learning that continues to be optimized upon subsequent learning. Our findings suggest that AIH-induced gains in locomotor performance are facilitated by enhancements in motor learning and retention of more energetically favourable coordination strategies.

Habitual exercise evokes fast and persistent adaptation during split-belt walking

Changing movement patterns in response to environmental perturbations is a critical aspect of gait and is related to reducing the energetic cost of the movement. Exercise improves energetic capacity for submaximal exercise and may affect how people adapt movement to reach an energetic minimum. The purpose of this study was to determine whether self-reported exercise behavior influences gait adaptation in young adults. Young adults who met the optimal volume of exercise according to the Physical Activity Guidelines for Americans (MOVE; n = 19) and young adults who did not meet the optimal volume of exercise (notMOVE; n = 13) walked on a split-belt treadmill with one belt moving twice the speed of the other belt for 10 minutes. Step length asymmetry (SLA) and mechanical work done by each leg were measured. Nonlinear mixed effects models compared the time course of adaptation between MOVE and notMOVE, and t-tests compared net work at the end of adaptation between MOVE and notMOVE. Compared to notMOVE, MOVE had a faster initial response to the split belt treadmill, and continued to adapt over the duration of split-belt treadmill walking. Young adults who engage in sufficient amounts of exercise responded more quickly to the onset of a perturbation, and throughout the perturbation they continued to explore movement strategies, which might be related to reduction of energetic cost. Our findings provide insights into the multisystem positive effects of exercise, including walking adaptation.

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.

Fascicle dynamics of the tibialis anterior muscle reflect whole-body walking economy

Humans can inherently adapt their gait pattern in a way that minimizes the metabolic cost of transport, or walking economy, within a few steps, which is faster than any known direct physiological sensor of metabolic energy. Instead, walking economy may be indirectly sensed through mechanoreceptors that correlate with the metabolic cost per step to make such gait adaptations. We tested whether velocity feedback from tibialis anterior (TA) muscle fascicles during the early stance phase of walking could potentially act to indirectly sense walking economy. As participants walked within a range of steady-state speeds and step frequencies, we observed that TA fascicles lengthen on almost every step. Moreover, the average peak fascicle velocity experienced during lengthening reflected the metabolic cost of transport of the given walking condition. We observed that the peak TA muscle activation occurred earlier than could be explained by a short latency reflex response. The activation of the TA muscle just prior to heel strike may serve as a prediction of the magnitude of the ground collision and the associated energy exchange. In this scenario, any unexpected length change experienced by the TA fascicle would serve as an error signal to the nervous system and provide additional information about energy lost per step. Our work helps provide a biomechanical framework to understand the possible neural mechanisms underlying the rapid optimization of walking economy.

Bilateral asymmetric hip stiffness applied by a robotic hip exoskeleton elicits kinematic and kinetic adaptation

Wearable robotic exoskeletons hold great promise for gait rehabilitation as portable, accessible tools. However, a better understanding of the potential for exoskeletons to elicit neural adaptation-a critical component of neurological gait rehabilitation-is needed. In this study, we investigated whether humans adapt to bilateral asymmetric stiffness perturbations applied by a hip exoskeleton, taking inspiration from asymmetry augmentation strategies used in split-belt treadmill training. During walking, we applied torques about the hip joints to repel the thigh away from a neutral position on the left side and attract the thigh toward a neutral position on the right side. Six participants performed an adaptation walking trial on a treadmill while wearing the exoskeleton. The exoskeleton elicited time-varying changes and aftereffects in step length and propulsive/braking ground reaction forces, indicating behavioral signatures of neural adaptation. These responses resemble typical responses to split-belt treadmill training, suggesting that the proposed intervention with a robotic hip exoskeleton may be an effective approach to (re)training symmetric gait.

Manual stabilization reveals a transient role for balance control during locomotor adaptation

A fundamental feature of human locomotor control is the need to adapt walking patterns in response to changes in the environment. For example, when people walk on a split-belt treadmill, which has belts that move at different speeds, they adapt to the asymmetric speed constraints by reducing spatiotemporal asymmetry. Here, we aim to understand the role of balance control as a potential factor driving this adaptation process. We recruited 24 healthy, young adults to adapt to walking on a split-belt treadmill while either holding on to a handrail or walking with free arm swing. We measured whole body angular momentum and step length asymmetry as measures of dynamic balance and spatiotemporal asymmetry, respectively. To understand how changes in intersegmental coordination influenced whole body angular momentum, we also measured segmental angular momenta and the coefficient of cancellation. When participants were initially exposed to the asymmetry in belt speeds, we observed an increase in whole body angular momentum that was due to both an increase in the momentum of individual segments and a reduction in the coefficient of cancellation. Holding on to a handrail reduced the perturbation to asymmetry during the early phase of adaptation and resulted in a smaller aftereffect during early postadaptation. In addition, the stabilization provided by holding on to a handrail led to reductions in the coupling between angular momentum and asymmetry. These results suggest that regulation of dynamic balance is most important during the initial, transient phase of adaptation to walking on a split-belt treadmill.NEW & NOTEWORTHY We investigated the role of dynamic balance during adaptation to a split-belt treadmill by measuring whole body angular momentum with or without holding on to a handrail. The initial step length asymmetry and associations between balance and asymmetry reduced when holding on to a handrail during early adaptation. These findings indicate that dynamic balance mostly contributes to the initial phase of adaptation when people are exposed to an asymmetric walking constraint.

The split-belt rimless wheel

Split-belt treadmill walking, in which the two belts move at different speeds, reveals a mechanism through which energy can be extracted from the environment. When a person walks with positive step length asymmetry on a split-belt treadmill, the treadmill can perform net positive work on the person. Here we use a split-belt rimless wheel model to explore how people could take advantage of the treadmill. We show that a split-belt rimless wheel can passively walk steadily by capturing energy from the treadmill to overcome collision losses, whereas it loses energy on each step with no way to recover the losses when walking on tied belts. Our simulated split-belt rimless wheel can walk steadily for a variety of leg angle and belt speed combinations, tolerating both speed disturbances and ground height variability. The wheel can even capture enough energy to walk uphill. We also built a physical split-belt rimless wheel robot and demonstrated that it can walk continuously without additional energy input. In comparing the wheel solutions to human split-belt gait, we found that humans do not maximize positive work performed by the treadmill. Other aspects of walking, such as costs associated with swing, balance, and free vertical moments, likely limit people’s ability to benefit from the treadmill. This study uses a simple walking model to characterize the mechanics and energetics of split-belt walking, demonstrating that energy capture through intermittent contact with two belts is possible and providing a simple model framework for understanding human adaptation during split-belt walking.

Ergonomics Design and Assistance Strategy of A-Suit

Concerning the biomechanics and energy consumption of the lower limbs, a soft exoskeleton for the powered plantar flexion of the ankle, named A-Suit, was developed to improve walking endurance in the lower limbs and reduce metabolic consumption. The method of ergonomics design was used based on the biological structures of the lower limbs. A profile of auxiliary forces was constructed according to the biological force of the Achilles tendon, and an iterative learning control was applied to shadow this auxiliary profile by iteratively modifying the traction displacements of drive units. During the evaluation of the performance experiments, four subjects wore the A-Suit and walked on a treadmill at different speeds and over different inclines. Average heart rate was taken as the evaluation index of metabolic consumption. When subjects walked at a moderate speed of 1.25 m/s, the average heart rate H_av under the Power-ON condition was 7.25 ± 1.32% (mean ± SEM) and 14.40 ± 2.63% less than the condition of No-suit and Power-OFF. Meanwhile, the additional mass of A-Suit led to a maximum H_av increase of 7.83 ± 1.44%. The overall reduction in H_av with Power-ON over the different inclines was 6.93 ± 1.84% and 13.4 ± 1.93% compared with that of the No-Suit and Power-OFF condition. This analysis offers interesting insights into the viability of using this technology for human augmentation and assistance for medical and other purposes.

Foot contact forces can be used to personalize a wearable robot during human walking

Individuals with below-knee amputation (BKA) experience increased physical effort when walking, and the use of a robotic ankle-foot prosthesis (AFP) can reduce such effort. The walking effort could be further reduced if the robot is personalized to the wearer using human-in-the-loop (HIL) optimization of wearable robot parameters. The conventional physiological measurement, however, requires a long estimation time, hampering real-time optimization due to the limited experimental time budget. This study hypothesized that a function of foot contact force, the symmetric foot force-time integral (FFTI), could be used as a cost function for HIL optimization to rapidly estimate the physical effort of walking. We found that the new cost function presents a reasonable correlation with measured metabolic cost. When we employed the new cost function in HIL ankle-foot prosthesis stiffness parameter optimization, 8 individuals with simulated amputation reduced their metabolic cost of walking, greater than 15% (p < 0.02), compared to the weight-based and control-off conditions. The symmetry cost using the FFTI percentage was lower for the optimal condition, compared to all other conditions (p < 0.05). This study suggests that foot force-time integral symmetry using foot pressure sensors can be used as a cost function when optimizing a wearable robot parameter.

Which lower limb joints compensate for destabilizing energy during walking in humans?

Current approaches to investigating stabilizing responses during locomotion lack measures that both directly relate to perturbation demands and are shared across different levels of description (i.e. joints and legs). Here, we investigated whether mechanical energy could serve as a 'common currency' during treadmill walking with transient unilateral belt accelerations. We hypothesized that by delivering perturbations in either early or late stance, we could elicit net negative or positive work, respectively, from the perturbed leg at the leg/treadmill interface, which would dictate the net demand at the overall leg level. We further hypothesized that of the lower limb joints, the ankle would best reflect changes in overall leg work. On average across all seven participants and 222 perturbations, we found early stance perturbations elicited no change in net work performed by the perturbed leg on the treadmill, but net positive work by the overall leg, which did not support our hypotheses. Conversely, late stance perturbations partially supported our hypotheses by eliciting positive work at the leg/treadmill interface, but no change in net work by the overall leg. In support of our final hypothesis, changes in perturbed ankle work, in addition to contralateral knee work, best reflected changes in overall leg work.

General variability leads to specific adaptation toward optimal movement policies

Our nervous systems can learn optimal control policies in response to changes to our bodies, tasks, and movement contexts. For example, humans can learn to adapt their control policy in walking contexts where the energy-optimal policy is shifted along variables such as step frequency or step width. However, it is unclear how the nervous system determines which ways to adapt its control policy. Here, we asked how human participants explore through variations in their control policy to identify more optimal policies in new contexts. We created new contexts using exoskeletons that apply assistive torques to each ankle at each walking step. We analyzed four variables that spanned the levels of the whole movement, the joint, and the muscle: step frequency, ankle angle range, total soleus activity, and total medial gastrocnemius activity. We found that, across all of these analyzed variables, variability increased upon initial exposure to new contexts and then decreased with experience. This led to adaptive changes in the magnitude of specific variables, and these changes were correlated with reduced energetic cost. The timescales by which adaptive changes progressed and variability decreased were faster for some variables than others, suggesting a reduced search space within which the nervous system continues to optimize its policy. These collective findings support the principle that exploration through general variability leads to specific adaptation toward optimal movement policies.

A passive exoskeleton can assist split-belt adaptation

An exoskeletal device can assist walking in those with gait deficits. A passive exoskeleton can be a favorable choice for local or home rehabilitation settings because it is affordable, light weight, and less complex to utilize. While there is research that investigates the effects of exoskeleton on gait research examining the effects of such devices on gait adaptation, is rare. This is important because in diseases like stroke, the ability to flexibly adapt is affected, such that functional recovery becomes difficult. The purpose of this study was to characterize gait adaptation patterns that result from exoskeleton usage during a split-belt adaptation task. Healthy young participants were randomly assigned to a unilateral exoskeleton or a no-exoskeleton group. Each participant performed the specific split-belt adaptation tasks on the treadmill, where the speed of each belt could be controlled independently. Symmetry indices of spatiotemporal variables were calculated to quantify gait adaptation. To analyze the adaptation, trials were divided into early and late adaptation. We also analyzed degree of adaptation, and transfer effects. We also measured the symmetry of the positive power generated by the individual legs during the split-belt task to determine if using exoskeleton assistance reduced power in the exoskeleton group versus the no-exoskeleton group. Use of a passive exoskeleton device altered gait adaptation during a split-belt treadmill task in comparison to the control group. Such adaptation was found to be largely restricted to the temporal domain. Changes in the gait coordination patterns consisted of both early and late adaptive changes, especially in intra-limb patterns like stance time rather than inter-limb patterns like step time. Although the symmetry of the positive power generated during the split-belt task was found to be reduced for the exoskeleton-assistance group, it was shown that this was primarily the result of increased positive power generated by the side not receiving exoskeletal assistance. An unpowered assistive device can provide a unique solution for coordinating the lower limbs during different gait tasks. Such a solution could reduce the neural burden of adaptation consequently resulting in a reduction of the mechanical burden of walking during the bilateral gait coordination task. This may be useful for accelerating gait rehabilitation in different patient populations. However, balance control is important to consider during unilateral exoskeletal assistance.

The energy cost of split-belt walking for a variety of belt speed combinations

Walking on a split-belt treadmill is often compared to walking on tied belts at the average speed, but the relationship between the metabolic energy costs of split- and tied-belt walking remains largely unexplored. Recent simulation work has suggested that people could take advantage of a belt speed difference and lower their energy costs, but this effect has not yet been observed experimentally. To relate metabolic energy costs across a range of belt speed combinations, we had 10 participants each complete 14 tied-belt and 39 split-belt walking trials, with early split-belt trials incorporating additional time for training. The average speeds ranged from 0.6 to 1.8 m/s with belt speed differences up to 1.4 m/s. We used ANOVA to determine differences in energy cost due to average speed and belt speed difference. We fit a linear model to estimate the tied-belt speed with the same energy cost as a given pair of split belt speeds. The cost of split-belt walking was on average just 0.13 ± 0.32 W/kg more expensive than the cost of tied-belt walking at the average speed. Contrary to predictions from simple dynamical models, increased belt speed difference resulted in increased energy cost, and the energetically equivalent tied-belt speed could be estimated as v_eq=v_avg+0.065⋅Δv. Clinicians designing rehabilitation protocols can balance the therapeutic benefits of higher belt speed difference with increased energetic demands. Open questions remain about why people cannot fully take advantage of mechanical work performed by a split-belt treadmill.

Age differences in adaptation of medial-lateral gait parameters during split-belt treadmill walking

The split-belt treadmill has been used to examine the adaptation of spatial and temporal gait parameters. Historically, similar studies have focused on anterior-posterior (AP) spatiotemporal gait parameters because this paradigm is primarily a perturbation in the AP direction, but it is important to understand whether and how medial-lateral (ML) control adapts in this scenario. The ML control of balance must be actively controlled and adapted in different walking environments. Furthermore, it is well established that older adults have balance difficulties. Therefore, we seek to determine whether ML balance adaptation differs in older age. We analyzed split belt induced changes in gait parameters including variables which inform us about ML balance control in younger and older adults. Our primary finding is that younger adults showed sustained asymmetric changes in these ML balance parameters during the split condition. Specifically, younger adults sustained a greater displacement between their fast stance foot and their upper body, relative to the slow stance foot, in the ML direction. This finding suggests that younger adults may be exploiting passive dynamics in the ML direction, which may be more metabolically efficient. Older adults did not display the same degree of asymmetry, suggesting older adults may be more concerned about maintaining a stable gait.

How adaptation, training, and customization contribute to benefits from exoskeleton assistance

Exoskeletons can enhance human mobility, but we still know little about why they are effective. For example, we do not know the relative importance of training, how much is required, or what type is most effective; how people adapt with the device; or the relative benefits of customizing assistance. We conducted experiments in which naïve users learned to walk with ankle exoskeletons under one of three training regimens characterized by different levels of variation in device behavior. Assistance was also customized for one group. After moderate-variation training, the benefits of customized assistance were large; metabolic rate was reduced by 39% compared with walking with the exoskeleton turned off. Training contributed about half of this benefit and customization about one-quarter; a generic controller reduced energy cost by 10% before training and 31% afterward. Training required much more exposure than typical of exoskeleton studies, about 109 minutes of assisted walking. Type of training also had a strong effect; the low-variation group required twice as long as the moderate-variation group to become expert, and the high-variation group never acquired this level of expertise. Curiously, all users adapted in a way that resulted in less mechanical power from the exoskeleton as they gained expertise. Customizing assistance required less time than training for all parameters except peak torque magnitude, which grew slowly over the study, suggesting a longer time scale adaptation in the person. These results underscore the importance of training to the benefits of exoskeleton assistance and suggest the topic deserves more attention.

Forward and backward walking share the same motor modules and locomotor adaptation strategies

Forward and backward walking are remarkably similar motor behaviors to the extent that backward walking has been described as a time-reversed version of forward walking. However, because they display different muscle activity patterns, it has been questioned if forward and backward walking share common control strategies. To investigate this point, we used a split-belt treadmill experimental paradigm designed to elicit healthy individuals' motor adaptation by changing the speed of one of the treadmill belts, while keeping the speed of the other belt constant. We applied this experimental paradigm to both forward and backward walking. We analyzed several adaptation parameters including step symmetry, stability, and energy expenditure as well as the characteristics of the synergies of lower-limb muscles. We found that forward and backward walking share the same muscle synergy modules. We showed that these modules are marked by similar patterns of adaptation driven by stability and energy consumption minimization criteria, both relying on modulating the temporal activation of the muscle synergies. Our results provide evidence that forward and backward walking are governed by the same control and adaptation mechanisms.

Deep reinforcement learning for modeling human locomotion control in neuromechanical simulation

Modeling human motor control and predicting how humans will move in novel environments is a grand scientific challenge. Researchers in the fields of biomechanics and motor control have proposed and evaluated motor control models via neuromechanical simulations, which produce physically correct motions of a musculoskeletal model. Typically, researchers have developed control models that encode physiologically plausible motor control hypotheses and compared the resulting simulation behaviors to measurable human motion data. While such plausible control models were able to simulate and explain many basic locomotion behaviors (e.g. walking, running, and climbing stairs), modeling higher layer controls (e.g. processing environment cues, planning long-term motion strategies, and coordinating basic motor skills to navigate in dynamic and complex environments) remains a challenge. Recent advances in deep reinforcement learning lay a foundation for modeling these complex control processes and controlling a diverse repertoire of human movement; however, reinforcement learning has been rarely applied in neuromechanical simulation to model human control. In this paper, we review the current state of neuromechanical simulations, along with the fundamentals of reinforcement learning, as it applies to human locomotion. We also present a scientific competition and accompanying software platform, which we have organized to accelerate the use of reinforcement learning in neuromechanical simulations. This “Learn to Move” competition was an official competition at the NeurIPS conference from 2017 to 2019 and attracted over 1300 teams from around the world. Top teams adapted state-of-the-art deep reinforcement learning techniques and produced motions, such as quick turning and walk-to-stand transitions, that have not been demonstrated before in neuromechanical simulations without utilizing reference motion data. We close with a discussion of future opportunities at the intersection of human movement simulation and reinforcement learning and our plans to extend the Learn to Move competition to further facilitate interdisciplinary collaboration in modeling human motor control for biomechanics and rehabilitation research

Evaluating the energetics of entrainment in a human-machine coupled oscillator system

During locomotion, humans sometimes entrain (i.e. synchronize) their steps to external oscillations: e.g. swaying bridges, tandem walking, bouncy harnesses, vibrating treadmills, exoskeletons. Previous studies have discussed the role of nonlinear oscillators (e.g. central pattern generators) in facilitating entrainment. However, the energetics of such interactions are unknown. Given substantial evidence that humans prioritize economy during locomotion, we tested whether reduced metabolic expenditure is associated with human entrainment to vertical force oscillations, where frequency and amplitude were prescribed via a custom mechatronics system during walking. Although metabolic cost was not significantly reduced during entrainment, individuals expended less energy when the oscillation forces did net positive work on the body and roughly selected phase relationships that maximize positive work. It is possible that individuals use mechanical cues to infer energy cost and inform effective gait strategies. If so, an accurate prediction may rely on the relative stability of interactions with the environment. Our results suggest that entrainment occurs over a wide range of oscillation parameters, though not as a direct priority for minimizing metabolic cost. Instead, entrainment may act to stabilize interactions with the environment, thus increasing predictability for the effective implementation of internal models that guide energy minimization.

Velocity of the Body Center of Mass During Walking on Split-Belt Treadmill

ABSTRACT

Walking on split-belt treadmills (each belt rotating at a different velocity) has inspired a growing number of researchers to study gait adaptation and rehabilitation. An overlooked peculiarity of this artificial form of gait is that the mean velocity adopted by the participant, considered as a whole system represented by the body Center of Mass, can be different from the mean velocity of the two belts. Twelve healthy adults (21-34 yrs) were requested to walk for 15 mins on a treadmill with belts rotating at 0.4 and 1.2 m sec-1, respectively (mean = 0.8 m sec-1). Each belt was supported by four 3-dimensional force sensors. For each participant, six strides were analyzed during the 1st and the 15th minute of the trial. The mean Center of Mass velocity was computed as the sum of the velocities of each belt weighted by the percentage of time during which the resulting forces, underlying the accelerations of the Center of Mass, originated from each belt. Across early and late observations, the median Center of Mass velocities were 0.72 and 0.67 m sec-1, respectively (P < 0.05). Therefore, the real velocity of the Center of Mass and its time course should be individually assessed when studying walking on split-belt treadmills.

Reduced joint motion supersedes asymmetry in explaining increased metabolic demand during walking with mechanical restriction

Recent research has highlighted the complex interactions among chronic injury- or disease-induced joint limitations, walking asymmetry, and increased metabolic cost. Determining the specific metabolic impacts of asymmetry or joint impairment in clinical populations is difficult because of concurrent neurological and physiological changes. This work investigates the metabolic impact of gait asymmetry and joint restriction by unilaterally (asymmetric) and bilaterally (symmetric) restricting ankle, knee, and combined ankle and knee ranges of motion in unimpaired individuals. We calculated propulsive asymmetry, temporal asymmetry, and step-length asymmetry for an average gait cycle; metabolic rate; average positive center of mass power using the individual limbs method; and muscle effort using lower limb electromyography measurements weighted by corresponding physiological cross-sectional areas. Unilateral restriction caused propulsive and temporal asymmetry but less metabolically expensive gait than bilateral restriction. Changes in asymmetry did not correlate with changes in metabolic cost. Interestingly, bilateral restriction increased average positive center of mass power compared to unilateral restriction. Further, increased average positive center of mass power correlated with increased energy costs, suggesting asymmetric step-to-step transitions did not drive metabolic changes. The number of restricted joints reduces available degrees of freedom and may have a larger metabolic impact than gait asymmetry, as this correlated significantly with increases in metabolic rate for 7/9 participants. These results emphasize symmetry is not by definition metabolically optimal, indicate that the mechanics underlying symmetry are meaningful, and suggest that available degrees of freedom should be considered in designing future interventions.

Different Biomechanical Variables Explain Within-Subjects Versus Between-Subjects Variance in Step Length Asymmetry Post-Stroke

Step length asymmetry (SLA) is common in most stroke survivors. Several studies have shown that factors such as paretic propulsion can explain between-subjects differences in SLA. However, whether the factors that account for between-subjects variance in SLA are consistent with those that account for within-subjects, stride-by-stride variance in SLA has not been determined. SLA direction is heterogeneous, and different impairments likely contribute to differences in SLA direction. Here, we identified common predictors between-subjects that explain within-subjects variance in SLA using sparse partial least squares regression (sPLSR). We determined whether the SLA predictors differ based on SLA direction and whether predictors obtained from within-subjects analyses were the same as those obtained from between-subjects analyses. We found that for participants who walked with longer paretic steps paretic double support time, braking impulse, peak vertical ground reaction force, and peak plantarflexion moment explained 59% of the within-subjects variance in SLA. However the within-subjects variance accounted for by each individual predictor was less than 10%. Peak paretic plantarflexion moment accounted for 4% of the within-subjects variance and 42% of the between-subjects variance in SLA. In participants who walked with shorter paretic steps, paretic and non-paretic braking impulse explained 18% of the within-subjects variance in SLA. Conversely, paretic braking impulse explained 68% of the between-subjects variance in SLA, but the association between SLA and paretic braking impulse was in the opposite direction for within-subjects vs. between-subjects analyses. Thus, the relationships that explain between-subjects variance might not account for within-subjects stride-by-stride variance in SLA.

Assessing the effects of gait asymmetry: Using a split-belt treadmill walking protocol to change step length and peak knee joint contact force symmetry

Asymmetrical gait may affect important outcomes such as knee joint contact force (KJCF). A split-belt treadmill (SBTM) can be used to provoke changes in step length symmetry (SLsym) and may produce a similar response in KJCF symmetry (KJCFsym) between limbs. The purpose of this study was to explore the utility of employing a SBTM walking paradigm to alter KJCF and KJCFsym and to determine if changes in SLsym coincided with changes in KJCFsym. Twenty healthy individuals performed a standardized SBTM protocol, where baseline and post-adaptation conditions had tied belt speeds of 0.5 m/s and the split-adaptation condition used a 3:1 belt speed ratio. OpenSim techniques were used to produce normalized, averaged stance phase peak KJCF during baseline walking, early- and late-adaptation, and post-adaptation. SLsym and KJCFsym values were determined. Comparisons were made for symmetry values between early- and late-adaptation and between baseline and post-adaptation. SLsym and KJCFsym did not respond in the same manner during the walking conditions. While step lengths (SL) were asymmetric during early adaptation but become more symmetric by late adaptation (p < 0.01), KJCF was symmetric throughout adaptation. Conversely, SL and KJCF exhibited similar responses during the baseline and post-adaptation conditions, with symmetry at baseline and asymmetry during post-adaptation (p < 0.01). In the post-adaptation condition, higher peak forces were demonstrated on the limb taking a shorter step. Results suggest a SBTM program may alter KJCF and KJCFsym between limbs. Furthermore, a comparison between baseline and post-adaptation may be more appropriate for evaluating the relationship between SL and KJCF.

Dynamic Asymmetries Do Not Match Spatiotemporal Step Asymmetries during Split-Belt Walking

While walking on split-belt treadmills (two belts running at different speeds), the slower limb shows longer anterior steps than the limb dragged by the faster belt. After returning to basal conditions, the step length asymmetry is transiently reversed (after-effect). The lower limb joint dynamics, however, were not thoroughly investigated. In this study, 12 healthy adults walked on a force-sensorised split-belt treadmill for 15 min. Belts rotated at 0.4 m s−1 on both sides, or 0.4 and 1.2 m s−1 under the non-dominant and dominant legs, respectively. Spatiotemporal step parameters, ankle power and work, and the actual mean velocity of the body’s centre of mass (CoM) were computed. On the faster side, ankle power and work increased, while step length and stance time decreased. The mean velocity of the CoM slightly decreased. As an after-effect, modest converse asymmetries developed, fading within 2–5 min. These results may help to decide which belt should be assigned to the paretic and the unaffected lower limb when split-belt walking is applied for rehabilitation research in hemiparesis.

Disentangling the energetic costs of step time asymmetry and step length asymmetry in human walking

The metabolic cost of walking in healthy individuals increases with spatiotemporal gait asymmetries. Pathological gait, such as post-stroke, often has asymmetry in step length and step time which may contribute to an increased energy cost. But paradoxically, enforcing step length symmetry does not reduce metabolic cost of post-stroke walking. The isolated and interacting costs of asymmetry in step time and step length remain unclear, because previous studies did not simultaneously enforce spatial and temporal gait asymmetries. Here, we delineate the isolated costs of asymmetry in step time and step length in healthy human walking. We first show that the cost of step length asymmetry is predicted by the cost of taking two non-preferred step lengths (one short and one long), but that step time asymmetry adds an extra cost beyond the cost of non-preferred step times. The metabolic power of step time asymmetry is about 2.5 times greater than the cost of step length asymmetry. Furthermore, the costs are not additive when walking with asymmetric step time and asymmetric step length: the metabolic power of concurrent asymmetry in step length and step time is driven by the cost of step time asymmetry alone. The metabolic power of asymmetry is explained by positive mechanical power produced during single support phases to compensate for a net loss of center of mass power incurred during double support phases. These data may explain why metabolic cost remains invariant to step length asymmetry in post-stroke walking and suggest how effects of asymmetry on energy cost can be attenuated.

Using a simple rope-pulley system that mechanically couples the arms, legs, and treadmill reduces the metabolic cost of walking

BACKGROUND

Emphasizing the active use of the arms and coordinating them with the stepping motion of the legs may promote walking recovery in patients with impaired lower limb function. Yet, most approaches use seated devices to allow coupled arm and leg movements. To provide an option during treadmill walking, we designed a rope-pulley system that physically links the arms and legs. This arm-leg pulley system was grounded to the floor and made of commercially available slotted square tubing, solid strut channels, and low-friction pulleys that allowed us to use a rope to connect the subject's wrist to the ipsilateral foot. This set-up was based on our idea that during walking the arm could generate an assistive force during arm swing retraction and, therefore, aid in leg swing.

METHODS

To test this idea, we compared the mechanical, muscular, and metabolic effects between normal walking and walking with the arm-leg pulley system. We measured rope and ground reaction forces, electromyographic signals of key arm and leg muscles, and rates of metabolic energy consumption while healthy, young subjects walked at 1.25 m/s on a dual-belt instrumented treadmill (n = 8).

RESULTS

With our arm-leg pulley system, we found that an assistive force could be generated, reaching peak values of 7% body weight on average. Contrary to our expectation, the force mainly coincided with the propulsive phase of walking and not leg swing. Our findings suggest that subjects actively used their arms to harness the energy from the moving treadmill belt, which helped to propel the whole body via the arm-leg rope linkage. This effectively decreased the muscular and mechanical demands placed on the legs, reducing the propulsive impulse by 43% (p < 0.001), which led to a 17% net reduction in the metabolic power required for walking (p = 0.001).

CONCLUSIONS

These findings provide the biomechanical and energetic basis for how we might reimagine the use of the arms in gait rehabilitation, opening the opportunity to explore if such a method could help patients regain their walking ability.

TRIAL REGISTRATION

 Study registered on 09/29/2018 in ClinicalTrials.gov (ID-NCT03689647).

Speed-Related Energy Flow and Joint Function Change During Human Walking

During human walking, mechanical energy transfers between segments via joints. Joint mechanics of the human body are coordinated with each other to adapt to speed change. The aim of this study is to analyze the functional behaviors of major joints during walking, and how joints and segments alter walking speed during different periods (collision, rebound, preload, and push-off) of stance phase. In this study, gait experiment was performed with three different self-selected speeds. Mechanical works of joints and segments were determined with collected data. Joint function indices were calculated based on net joint work. The results show that the primary functional behaviors of joints would not change with altering walking speed, but the function indices might be changed slightly (e.g., strut functions decrease with increasing walking speed). Waist acts as strut during stance phase and contributes to keep stability during collision when walking faster. Knee of stance leg does not contribute to altering walking speed. Hip and ankle absorb more mechanical energy to buffer the strike during collision with increasing walking speed. What is more, hip and ankle generate more energy during push-off with greater motion to push distal segments forward with increasing walking speed. Ankle also produces more mechanical energy during push-off to compensate the increased heel-strike collision of contralateral leg during faster walking. Thus, human may utilize the cooperation of hip and ankle during collision and push-off to alter walking speed. These findings indicate that speed change in walking leads to fundamental changes to joint mechanics.

Cognitive and Motor Perseveration Are Associated in Older Adults

Aging causes perseveration (difficulty to switch between actions) in motor and cognitive tasks, suggesting that the same neural processes could govern these abilities in older adults. To test this, we evaluated the relation between independently measured motor and cognitive perseveration in young (21.4 ± 3.7 y/o) and older participants (76.5 ± 2.9 y/o). Motor perseveration was measured with a locomotor task in which participants had to transition between distinct walking patterns. Cognitive perseveration was measured with a card matching task in which participants had to switch between distinct matching rules. We found that perseveration in the cognitive and motor domains were positively related in older, but not younger individuals, such that participants exhibiting greater perseveration in the motor task also perseverated more in the cognitive task. Additionally, exposure reduces motor perseveration: older adults who had practiced the motor task could transition between walking patterns as proficiently as naïve, young individuals. Our results suggest an overlap in neural processes governing cognitive and motor perseveration with aging and that exposure can counteract the age-related motor perseveration.

Adapting gait with asymmetric visual feedback affects deadaptation but not adaptation in healthy young adults

Split-belt treadmill walking allows researchers to understand how new gait patterns are acquired. Initially, the belts move at two different speeds, inducing asymmetric step lengths. As people adapt their gait on a split-belt treadmill, left and right step lengths become more symmetric over time. Upon returning to normal walking, step lengths become asymmetric in the opposite direction, indicating deadaptation. Then, upon re-exposure to the split belts, step length asymmetry is less than the asymmetry at the start of the initial exposure, indicating readaptation. Changes in step length symmetry are driven by changes in step timing and step position asymmetry. It is critical to understand what factors can promote step timing and position adaptation and therefore influence step length asymmetry. There is limited research regarding the role of visual feedback to improve gait adaptation. Using visual feedback to promote the adaptation of step timing or position may be useful of understanding temporal or spatial gait impairments. We measured gait adaptation, deadaptation, and readaptation in twenty-nine healthy young adults while they walked on a split-belt treadmill. One group received no feedback while adapting; one group received asymmetric real-time feedback about step timing while adapting; and the last group received asymmetric real-time feedback about step position while adapting. We measured step length difference (non-normalized asymmetry), step timing asymmetry, and step position asymmetry during adaptation, deadaptation, and readaptation on a split-belt treadmill. Regardless of feedback, participants adapted step length difference, indicating that walking with temporal or spatial visual feedback does not interfere with gait adaptation. Compared to the group that received no feedback, the group that received temporal feedback exhibited smaller early deadaptation step position asymmetry (p = 0.005). There was no effect of temporal or spatial feedback on step timing. The feedback groups adapted step timing and position similarly to walking without feedback. Future work should investigate whether asymmetric visual feedback also results in typical gait adaptation in populations with altered step timing or position control.

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.

Cerebellar Transcranial Direct Current Stimulation for Motor Learning in People with Chronic Stroke: A Pilot Randomized Controlled Trial

Cerebellar transcranial direct current stimulation (ctDCS) is a non-invasive brain stimulation technique that alters neural plasticity through weak, continuous, direct currents delivered to the cerebellum. This study aimed to evaluate the feasibility of conducting a randomized controlled trial (RCT) delivering three consecutive days of ctDCS during split-belt treadmill training (SBTT) in people with chronic stroke. Using a double-blinded, parallel-group RCT design, eligible participants were randomly allocated to receive either active anodal ctDCS or sham ctDCS combined with SBTT on three consecutive days. Outcomes were assessed at one-week follow-up, using step length symmetry as a measure of motor learning and comfortable over-ground walking speed as a measure of walking capacity. The feasibility of the RCT protocol was evaluated based on recruitment, retention, protocol deviations and data completeness. The feasibility of the intervention was assessed based on safety, adherence and intervention fidelity. Of the 26 potential participants identified over four months, only four were enrolled in the study (active anodal ctDCS n = 1, sham ctDCS n = 3). Both the inclusion criteria and the fidelity of the SBTT relied upon the accurate estimation of step length asymmetry. The method used to determine the side of the step length asymmetry was unreliable and led to deviations in the protocol. The ctDCS intervention was well adhered to, safe, and delivered as per the planned protocol. Motor learning outcomes for individual participants revealed that treadmill step length symmetry remained unchanged for three participants but improved for one participant (sham ctDCS). Comfortable over-ground walking speed improved for two participants (sham ctDCS). The feasibility of the planned protocol and intervention was limited by intra-individual variability in the magnitude and side of the step length asymmetry. This limited the sample and compromised the fidelity of the SBTT intervention. To feasibly conduct a full RCT investigating the effect of ctDCS on locomotor adaptation, a reliable method of identifying and defining step length asymmetry in people with stroke is required. Future ctDCS research should either optimize the methods for SBTT delivery or utilize an alternative motor adaptation task.

The human preference for symmetric walking often disappears when one leg is constrained

KEY POINTS

We hypothesized that minimization of metabolic power could drive people to walk asymmetrically when one leg is constrained We studied healthy young adults and independently constrained one or both step lengths to be markedly shorter or longer than preferred using visual feedback When one leg was constrained to take a shorter or longer step than preferred, asymmetric walking patterns were less metabolically costly than symmetric walking patterns When one leg was constrained to take a shorter or longer step than preferred and the other leg was allowed to move freely, most participants naturally adopted an asymmetric gait People may prefer to walk asymmetrically to minimize metabolic power when the function of one leg is constrained during fixed-speed treadmill walking ABSTRACT: The bilateral symmetry inherent in healthy human walking is often disrupted in clinical conditions that primarily affect one leg (e.g. stroke). This seems intuitive: with one leg constrained, gait becomes asymmetric. However, the emergence of asymmetry is not inevitable. Consider that symmetric walking could be preserved by matching the movement of the unconstrained leg to that of the constrained leg. While this is theoretically possible, it is rarely observed in clinical populations. Here, we hypothesized that minimization of metabolic power could drive people to walk asymmetrically when one leg is constrained, even when symmetric walking remains possible. We tested this hypothesis by performing two experiments in healthy adults. In Experiment 1, we constrained one step to be markedly shorter or longer than preferred. We observed that participants could significantly reduce metabolic power by adopting an asymmetric gait (one short/long step, one preferred step) rather than maintaining a symmetric gait (bilateral short/long steps). Indeed, when allowed to walk freely in this situation, participants naturally adopted a less effortful asymmetric gait. In Experiment 2, we applied a milder constraint that more closely approximated magnitudes of step length asymmetry that are observed in clinical populations. Responses in this experiment were more heterogeneous, though most participants adopted an asymmetric gait. These findings support two central conclusions: (1) symmetry is not necessarily energetically optimal in constrained human walking, and (2) people may prefer to walk asymmetrically to minimize metabolic power when one leg is constrained during fixed-speed treadmill walking, especially when the constraint is large.

Unilateral step training can drive faster learning of novel gait patterns

Humans are capable of learning many new walking patterns. People have learned to snowshoe up mountains, racewalk marathons, and march in precise synchrony. But what is required to learn a new walking pattern? Here, we demonstrate that people can learn new walking patterns without actually walking. Through a series of experiments, we observe that stepping with only one leg can facilitate learning of an entirely new walking pattern (i.e., split-belt treadmill walking). We find that the nervous system learns from the relative speed difference between the legs-whether or not both legs are moving-and can transfer this learning to novel gaits. We also show that locomotor learning requires active movement: observing another person adapt their gait did not result in significantly faster learning. These findings reveal that people can learn new walking patterns without bilateral gait training, as stepping with one leg can facilitate adaptive learning that transfers to novel gait patterns.

Altered active control of step width in response to mediolateral leg perturbations while walking

During human walking, step width is predicted by mediolateral motion of the pelvis, a relationship that can be attributed to a combination of passive body dynamics and active sensorimotor control. The purpose of the present study was to investigate whether humans modulate the active control of step width in response to a novel mechanical environment. Participants were repeatedly exposed to a force-field that either assisted or perturbed the normal relationship between pelvis motion and step width, separated by washout periods to detect the presence of potential after-effects. As intended, force-field assistance directly strengthened the relationship between pelvis displacement and step width. This relationship remained strengthened with repeated exposure to assistance, and returned to baseline afterward, providing minimal evidence for assistance-driven changes in active control. In contrast, force-field perturbations directly weakened the relationship between pelvis motion and step width. Repeated exposure to perturbations diminished this negative direct effect, and produced larger positive after-effects once the perturbations ceased. These results demonstrate that targeted perturbations can cause humans to adjust the active control that contributes to fluctuations in step width.

Augmenting propulsion demands during split-belt walking increases locomotor adaptation of asymmetric step lengths

Background

Promising studies have shown that the gait symmetry of individuals with hemiparesis due to brain lesions, such as stroke, can improve through motor adaptation protocols forcing patients to use their affected limb more. However, little is known about how to facilitate this process. Here we asked if increasing propulsion demands during split-belt walking (i.e., legs moving at different speeds) leads to more motor adaptation and more symmetric gait in survivors of a stroke, as we previously observed in subjects without neurological disorders.

Methods

We investigated the effect of propulsion forces on locomotor adaptation during and after split-belt walking in the asymmetric motor system post-stroke. To test this, 12 subjects in the chronic phase post-stroke experienced a split-belt protocol in a flat and incline session so as to contrast the effects of two different propulsion demands. Step length asymmetry and propulsion forces were used to compare the motor behavior between the two sessions because these are clinically relevant measures that are altered by split-belt walking.

Results

The incline session resulted in more symmetric step lengths during late split-belt walking and larger after-effects following split-belt walking. In both testing sessions, subjects who have had a stroke adapted to regain speed and slope-specific leg orientations similarly to young, intact adults. Importantly, leg orientations, which were set by kinetic demands, during baseline walking were predictive of those achieved during split-belt walking, which in turn predicted each individual’s post-adaptation behavior. These results are relevant because they provide evidence that survivors of a stroke can generate the leg-specific forces to walk more symmetrically, but also because we provide insight into factors underlying the therapeutic effect of split-belt walking.

Conclusions

Individuals post-stroke at a chronic stage can adapt more during split-belt walking and have greater after-effects when propulsion demands are augmented by inclining the treadmill surface. Our results are promising since they suggest that increasing propulsion demands during paradigms that force patients to use their paretic side more could correct gait asymmetries post-stroke more effectively.

Handrail Holding During Treadmill Walking Reduces Locomotor Learning in Able-Bodied Persons

Treadmills used for gait training in clinical rehabilitation and experimental settings are commonly fitted with handrails to assist or support persons in locomotor tasks. However, the effects of balance support through handrail holding on locomotor learning are unknown. Locomotor learning can be studied on split-belt treadmills, where participants walk on two parallel belts with asymmetric left and right belt speeds, to which they adapt their stepping pattern within a few minutes. The aim of this study was to determine how handrail holding affects the walking pattern during split-belt adaptation and after-effects in able-bodied persons. Fifty healthy young participants in five experimental groups were instructed to hold handrails, swing arms freely throughout the experiment or hold handrails during adaptation and swing arms freely during after-effects. Step length asymmetry and double support asymmetry were measured to assess the spatiotemporal walking pattern. The results showed that holding handrails during split-belt adaptation reduces magnitude of initial perturbation of step length asymmetry and reduces after-effects in step length asymmetry upon return to symmetric belt speeds. The findings of this study imply that balance support during gait training reduces locomotor learning, which should be considered in daily clinical gait practice and future research on locomotor learning.