Where to Step? Contributions of Stance Leg Muscle Spindle Afference to Planning of Mediolateral Foot Placement for Balance Control in Young and Old Adults

Stride width and postural stability in frontal gait disorders and Parkinson's disease

Older adults, as well as those with certain neurological disorders, may compensate for poor neural control of postural stability by widening their base of foot support while walking. However, the extent to which this wide-based gait improves postural stability or affects postural control strategies has not been explored. People with idiopathic Parkinson's disease (iPD, n = 72), frontal gait disorders (FGD, n = 16), and healthy older adults (n = 32) performed walking trials at their preferred speed over an 8-m-long, instrumented walkway. People with iPD were tested in their OFF medication state. Analyses of covariance were performed to determine the associations between stride width and measures of lateral stability control. People with FGD exhibited a wide-based gait compared to both healthy older adults and iPD. An increased stride width was associated with an increase in lateral margin of stability in FGD. Unlike healthy older adults or iPD, people with FGD did not externally rotate their feet (toe-out angle) or shift their center of pressure laterally to aid lateral dynamic stability during walking but slowed their gait instead to increase stability. By adopting a slow, wide-based gait, people with FGD take advantage of the passive, pendular mechanics of walking.

Force-field perturbations and muscle vibration strengthen stability-related foot placement responses during steady-state gait in healthy adults

Mediolateral gait stability can be maintained by coordinating our foot placement with respect to the center-of-mass (CoM) kinematic state. Neurological impairments can reduce the degree of foot placement control. For individuals with such impairments, interventions that could improve foot placement control could thus contribute to improved gait stability. In this study we aimed to better understand two potential interventions, by investigating their effect in neurologically intact individuals. The degree of foot placement control can be quantified based on a foot placement model, in which the CoM position and velocity during swing predict subsequent foot placement. Previously, perturbing foot placement with a force-field resulted in an enhanced degree of foot placement control as an after-effect. Moreover, timed muscle vibration enhanced the degree of foot placement control whilst the vibration was applied. Here, we replicated these two findings and further investigated whether Q1) timed muscle vibration leads to an after-effect and Q2) whether combining timed muscle vibration with force-field perturbations leads to a larger after-effect, as compared to force-field perturbations only. In addition, we evaluated several potential contributors to the degree of foot placement control, by considering foot placement errors, CoM variability and the CoM position gain (βpos) of the foot placement model, next to the R2 measure as the degree of foot placement control. Timed muscle vibration led to a higher degree of foot placement control as an after-effect (Q1). However, combining timed muscle vibration and force-field perturbations did not lead to a larger after-effect, as compared to following force-field perturbations only (Q2). Furthermore, we showed that the improved degree of foot placement control following force-field perturbations and during/following muscle vibration, did not reflect diminished foot placement errors. Rather, participants demonstrated a stronger active response (higher βpos) as well as higher CoM variability.

Generalizability of foot placement control strategies during unperturbed and perturbed gait

Control of foot placement is an essential strategy for maintaining balance during walking. During unperturbed, steady-state walking, foot placement can be accurately described as a linear function of the body's centre of mass (CoM) state at midstance. However, it is uncertain if this mapping from CoM state to foot placement generalizes to larger perturbations that could potentially cause falls. Recovery from these perturbations may require reactive control strategies not observed during unperturbed walking. Here, we used unpredictable changes in treadmill belt speed to assess the generalizability of foot placement mappings identified during unperturbed walking. We found that foot placement mappings generalized poorly from unperturbed to perturbed walking and differed for forward perturbation versus backward perturbation. We also used the singular value decomposition of the mapping matrix to reveal that people were more sensitive to backward versus forward perturbations. Together, these results indicate that a single linear mapping cannot describe the foot placement control during both forward and backward losses of balance induced by treadmill belt speed perturbations. Better characterization of human balance control strategies could improve our understanding of why different neuromotor disorders result in heightened fall risk and inform the design of controllers for balance-assisting devices.

Dynamics of brain-muscle networks reveal effects of age and somatosensory function on gait

Walking is a complex motor activity that requires coordinated interactions between the sensory and motor systems. We used mobile EEG and EMG to investigate the brain-muscle networks involved in gait control during overground walking in young people, older people, and individuals with Parkinson's disease. Dynamic interactions between the sensorimotor cortices and eight leg muscles within a gait cycle were assessed using multivariate analysis. We identified three distinct brain-muscle networks during a gait cycle. These networks include a bilateral network, a left-lateralized network activated during the left swing phase, and a right-lateralized network active during the right swing. The trajectories of these networks are contracted in older adults, indicating a reduction in neuromuscular connectivity with age. Individuals with the impaired tactile sensitivity of the foot showed a selective enhancement of the bilateral network, possibly reflecting a compensation strategy to maintain gait stability. These findings provide a parsimonious description of interindividual differences in neuromuscular connectivity during gait.

The condition for dynamic stability in humans walking with feedback control

The walking human body is mechanically unstable. Loss of stability and falling is more likely in certain groups of people, such as older adults or people with neuromotor impairments, as well as in certain situations, such as when experiencing conflicting or distracting sensory inputs. Stability during walking is often characterized biomechanically, by measures based on body dynamics and the base of support. Neural control of upright stability, on the other hand, does not factor into commonly used stability measures. Here we analyze stability of human walking accounting for both biomechanics and neural control, using a modeling approach. We define a walking system as a combination of biomechanics, using the well known inverted pendulum model, and neural control, using a proportional-derivative controller for foot placement based on the state of the center of mass at midstance. We analyze this system formally and show that for any choice of system parameters there is always one periodic orbit. We then determine when this periodic orbit is stable, i.e. how the neural control gain values have to be chosen for stable walking. Following the formal analysis, we use this model to make predictions about neural control gains and compare these predictions with the literature and existing experimental data. The model predicts that control gains should increase with decreasing cadence. This finding appears in agreement with literature showing stronger effects of visual or vestibular manipulations at different walking speeds.

Altered foot placement modulation with somatosensory stimulation in people with chronic stroke

Many individuals who experience a stroke exhibit reduced modulation of their mediolateral foot placement, an important gait stabilization strategy. One factor that may contribute to this deficit is altered somatosensory processing, which can be probed by applying vibration to the involved muscles (e.g., the hip abductors). The purpose of this study was to investigate whether appropriately controlled hip abductor vibration can increase foot placement modulation among people with chronic stroke. 40 people with chronic stroke performed a series of treadmill walking trials without vibration and with vibration of either the hip abductors or lateral trunk (a control condition) that scaled with their real-time mediolateral motion. To assess participants' vibration sensitivity, we also measured vibration detection threshold and lateral sway evoked by abductor vibration during quiet standing. As a group, foot placement modulation increased significantly with either hip or trunk vibration, compared to without vibration. However, these changes were quite variable across participants, and were not predicted by either vibration detection threshold or the lateral sway evoked by hip vibration during standing. Overall, we found that somatosensory stimulation had small, positive effects on post-stroke foot placement modulation. Unexpectedly, these effects were observed with both hip abductor and lateral trunk vibration, perhaps indicating that the trunk can also provide useful somatosensory feedback during walking. Future work is needed to determine whether repeated application of such somatosensory stimulation can produce sustained effects on this important gait stabilization strategy.

Vibration perception among children and adolescents with Charcot-Marie-tooth disease and implications for foot posture

BACKGROUND

Alterations in vibration perception among children and adolescents with Charcot-Marie-Tooth disease might explain observed changes in foot posture. Therefore, this cross-sectional study compared the vibration perception of the lower limbs in youths with and without Charcot-Marie-Tooth disease and verified the cut-off value of the distal vibration perception for the Charcot-Marie-Tooth group. In addition, associations between dynamic plantar pressure, vibration perception and isometric muscle strength were investigated.

METHODS

Participants aged 9-18 (Charcot-Marie-Tooth group n = 32; Typical group n = 32) had vibration perception measured by a 128-Hz graduated tuning fork. The static and dynamic foot posture were evaluated by the Foot Posture Index and pressure distribution measuring system, respectively. For the Charcot-Marie-Tooth group, a hand-held dynamometer evaluated the isometric muscle strength of the lower limbs.

FINDINGS

Children with Charcot-Marie-Tooth disease presented impaired vibration perception at the distal phalanx of the hallux and head of the first metatarsal compared to their typically developing peers, while adolescents with Charcot-Marie-Tooth disease showed impairment in all the tested regions compared to their typically developing peers. The cut-off value for vibration perception for participants with Charcot-Marie-Tooth disease was 5.7, considering the original grade of the tuning-fork 128 Hz. Among the associations established for the Charcot-Marie-Tooth group, a greater vibration perception at the distal phalanx of the hallux was associated with a longer rearfoot contact time (β = 31.02, p = 0.04).

INTERPRETATION

These new findings may guide the clinical evaluation and rehabilitation treatment for children and adolescents with Charcot-Marie-Tooth disease.

Mediolateral foot placement control can be trained: Older adults learn to walk more stable, when ankle moments are constrained

Falls are a problem, especially for older adults. Placing our feet accurately relative to the center-of-mass helps us to prevent falling during gait. The degree of foot placement control with respect to the center-of mass kinematic state is decreased in older as compared to young adults. Here, we attempted to train mediolateral foot placement control in healthy older adults. Ten older adults trained by walking on shoes with a narrow ridge underneath (LesSchuh), restricting mediolateral center-of-pressure shifts. As a training effect, we expected improved foot placement control during normal walking. A training session consisted of a normal walking condition, followed by a training condition on LesSchuh and finally an after-effect condition. Participants performed six of such training sessions, spread across three weeks. As a control, before the first training session, we included two similar sessions, but on normal shoes only. We evaluated whether a training effect was observed across sessions and weeks in a repeated-measures design. Whilst walking with LesSchuh, the magnitude of foot placement error reduced half-a-millimeter between sessions within a week (cohen's d = 0.394). As a training effect in normal walking, the magnitude of foot placement errors was significantly lower compared to the control week, by one millimeter in weeks 2 (cohen's d = 0.686) and 3 (cohen's d = 0.780) and by two millimeters in week 4 (cohen's d = 0.875). Local dynamic stability of normal walking also improved significantly. More precise foot placement may thus have led to improved stability. It remains to be determined whether the training effects were the result of walking on LesSchuh or from repeated treadmill walking itself. Moreover, enhancement of mechanisms beyond the scope of our outcome measures may have improved stability. At the retention test, gait stability returned to similar levels as in the control week. Yet, a reduction in foot placement error persisted.

Does ankle push-off correct for errors in anterior-posterior foot placement relative to center-of-mass states?

Understanding the mechanisms humans use to stabilize walking is vital for predicting falls in elderly. Modeling studies identified two potential mechanisms to stabilize gait in the anterior-posterior direction: foot placement control and ankle push-off control: foot placement depends on position and velocity of the center-of-mass (CoM) and push-off covaries with deviations between actual and predicted CoM trajectories. While both control mechanisms have been reported in humans, it is unknown whether especially the latter one is employed in unperturbed steady-state walking. Based on the finding of Wang and Srinivasan that foot placement deviates in the same direction as the CoM states in the preceding swing phase, and assuming that this covariance serves the role of stabilizing gait, the covariance between the CoM states and foot placement can be seen as a measure of foot placement accuracy. We subsequently interpreted the residual variance in foot placement from a linear regression model as "errors" that must be compensated, and investigated whether these foot placement errors were correlated to push-off kinetic time series of the subsequent double stance phase. We found ankle push-off torque to be correlated to the foot placement errors in 30 participants when walking at normal and slow speeds, with peak correlations over the double stance phase up to 0.39. Our study suggests that humans use a push-off strategy for correcting foot placement errors in steady-state walking.

The Effect of Human Settlement Pedestrian Environment on Gait of Older People: An Umbrella Review

Older people are limited by the pedestrian environment in human settlements and are prone to travel difficulties, falls, and stumbles. Furthermore, we still lack systematic knowledge of the pedestrian environment affecting the gait of older people. The purpose of this review is to synthesize current evidence of effective human settlement pedestrian environments interfering with gait in older people. The systematic effects of the human settlement pedestrian environment on gait in older people are discussed. Databases such as Web of Science, Medline (via PubMed), Scopus, and Embase were searched for relevant studies up to June 2022. The literature was screened to extract relevant evidence from the included literature, assess the quality of the evidence, and analyze the systematic effects of the pedestrian environment on gait in older people. From the 4297 studies identified in the initial search, 11 systematic reviews or meta-analysis studies were screened, from which 18 environmental factors and 60 gait changes were extracted. After removing duplicate elements and merging synonymous features, a total of 53 relationships between environmental factors and gait change in older people were extracted: the main human settlement pedestrian environmental factors affecting gait change in older people in existing studies were indoor and outdoor stairs/steps, uneven and irregular ground, obstacles, walking path turns, vibration interventions, mechanical perturbation during gait, and auditory sound cues. Under the influence of these factors, older people may experience changes in the degree of cautiousness and conservatism of gait and stability, and their body posture performance and control, and muscle activation may also be affected. Factors such as ground texture or material, mechanical perturbations during gait, and vibration interventions stimulate older people's understanding and perception of their environment, but there is controversy over the results of specific gait parameters. The results support that human settlements' pedestrian environment affects the gait changes of older people in a positive or negative way. This review may likely contribute evidence-based information to aid communication among practitioners in public health, healthcare, and environmental construction. The above findings are expected to provide useful preference for associated interdisciplinary researchers to understand the interactions among pedestrian environments, human behavior, and physiological characteristics.

Leveraging a virtual alley with continuously varying width modulates step width variability during self-paced treadmill walking

Increased fall risk in older adults and clinical populations is linked with increased amount and altered temporal structure of step width variability. One approach to rehabilitation seeks to reduce fall risk in older adults by reducing the amount of step width variability and restoring the temporal structure characteristic of healthy young adults. The success of such a program depends on our ability to modulate step width variability effectively. To this end, we investigated how manipulation of the visual walking space in a virtual environment could modulate the amount and temporal structure of step width variability. Nine healthy adults performed self-paced treadmill walking in a virtual alley in a fixed-width Control condition (1.91 m) and two conditions in which the alley's width oscillated sinusoidally at 0.03 Hz: between 0.38 and 1.14 m and 0.38-2.67 m in Narrow and Wide conditions, respectively. The step width time series from each condition was evaluated using: (i) the standard deviation to identify changes in the amount of variability and (ii) the fractal scaling exponent estimated using detrended fluctuation analysis (DFA) to identify changes in the temporal structure of variability in terms of persistence in fluctuations. The Wide condition neither affected the standard deviation nor the fractal scaling exponent of step width time series. The Narrow condition did not affect the standard deviation of step width time series compared to the Control condition but significantly increased its fractal scaling exponent compared to the Control and Wide conditions, suggestive of more persistent fluctuations characteristic of a healthy gait. These results show that virtual reality based rehabilitative intervention can modulate step width variability to potentially reduce fall risk in older adults and clinical populations.

Asymmetry measures for quantification of mechanisms contributing to dynamic stability during stepping-in-place gait

The goal of this study is to introduce and to motivate the use of new quantitative methods to improve our understanding of mechanisms that contribute to the control of dynamic balance during gait. Dynamic balance refers to the ability to maintain a continuous, oscillating center-of-mass (CoM) motion of the body during gait even though the CoM frequently moves outside of the base of support. We focus on dynamic balance control in the frontal plane or medial-lateral (ML) direction because it is known that active, neurally-mediated control mechanisms are necessary to maintain ML stability. Mechanisms that regulate foot placement on each step and that generate corrective ankle torque during the stance phase of gait are both known to contribute to the generation of corrective actions that contribute to ML stability. Less appreciated is the potential role played by adjustments in step timing when the duration of the stance and/or swing phases of gait can be shortened or lengthened to allow torque due to gravity to act on the body CoM over a shorter or longer time to generate corrective actions. We introduce and define four asymmetry measures that provide normalized indications of the contribution of these different mechanisms to gait stability. These measures are 'step width asymmetry', 'ankle torque asymmetry', 'stance duration asymmetry', and 'swing duration asymmetry'. Asymmetry values are calculated by comparing corresponding biomechanical or temporal gait parameters from adjacent steps. A time of occurrence is assigned to each asymmetry value. An indication that a mechanism is contributing to ML control is obtained by comparing asymmetry values to the ML body motion (CoM angular position and velocity) at the time points associated with the asymmetry measures. Example results are demonstrated with measures obtained during a stepping-in-place (SiP) gait performed on a stance surface that either remained fixed and level or was pseudorandomly tilted to disturb balance in the ML direction. We also demonstrate that the variability of asymmetry measures obtained from 40 individuals during unperturbed, self-paced SiP were highly correlated with corresponding coefficient of variation measures that have previously been shown to be associated with poor balance and fall risk.

Individuals with cerebral palsy show altered responses to visual perturbations during walking

Individuals with cerebral palsy (CP) have deficits in processing of somatosensory and proprioceptive information. To compensate for these deficits, they tend to rely on vision over proprioception in single plane upper and lower limb movements and in standing. It is not known whether this also applies to walking, an activity where the threat to balance is higher. Through this study, we used visual perturbations to understand how individuals with and without CP integrate visual input for walking balance control. Additionally, we probed the balance mechanisms driving the responses to the visual perturbations. More specifically, we investigated differences in the use of ankle roll response i.e., the use of ankle inversion, and the foot placement response, i.e., stepping in the direction of perceived fall. Thirty-four participants (17 CP, 17 age-and sex-matched typically developing controls or TD) were recruited. Participants walked on a self-paced treadmill in a virtual reality environment. Intermittently, the virtual scene was rotated in the frontal plane to induce the sensation of a sideways fall. Our results showed that compared to their TD peers, the overall body sway in response to the visual perturbations was magnified and delayed in CP group, implying that they were more affected by changes in visual cues and relied more so on visual information for walking balance control. Also, the CP group showed a lack of ankle response, through a significantly reduced ankle inversion on the affected side compared to the TD group. The CP group showed a higher foot placement response compared to the TD group immediately following the visual perturbations. Thus, individuals with CP showed a dominant proximal foot placement strategy and diminished ankle roll response, suggestive of a reliance on proximal over distal control of walking balance in individuals with CP.

The effect of external lateral stabilization on ankle moment control during steady-state walking

External lateral stabilization can help identify stability control mechanisms during steady-state walking. The degree of step-by-step foot placement control and step width are known to decrease when walking with external lateral stabilization. Here, we investigated the effect of external lateral stabilization on ankle moment control in healthy participants. Ankle moment control complements foot placement, by allowing a corrective center-of-pressure shift once the foot has been placed. This is reflected by a model predicting this center-of-pressure shift based on the preceding foot placement error. Here, the absolute explained variance accounted for by this model decreased when walking with external lateral stabilization. In other words, we found a reduction in the contribution of step-by-step ankle moment control to mediolateral gait stability when externally stabilized. Concurrently, foot placement error and the average center-of-pressure shift remained unchanged.

Perturbations during Gait: A Systematic Review of Methodologies and Outcomes

BACKGROUND

Despite extensive literature regarding laboratory-based balance perturbations, there is no up-to-date systematic review of methods. This systematic review aimed to assess current perturbation methods and outcome variables used to report participant biomechanical responses during walking.

METHODS

Web of Science, CINAHL, and PubMed online databases were searched, for records from 2015, the last search was on 30th of May 2022. Studies were included where participants were 18+ years, with or without clinical conditions, conducted in non-hospital settings. Reviews were excluded. Participant descriptive, perturbation method, outcome variables and results were extracted and summarised. Bias was assessed using the Appraisal tool for Cross-sectional Studies risk of bias assessment tool. Qualitative analysis was performed as the review aimed to investigate methods used to apply perturbations.

RESULTS

644 records were identified and 33 studies were included, totaling 779 participants. The most frequent method of balance perturbation during gait was by means of a treadmill translation. The most frequent outcome variable collected was participant step width, closely followed by step length. Most studies reported at least one spatiotemporal outcome variable. All included studies showed some risk of bias, generally related to reporting of sampling approaches. Large variations in perturbation type, duration and intensity and outcome variables were reported.

CONCLUSIONS

This review shows the wide variety of published laboratory perturbation methods. Moreover, it demonstrates the significant impact on outcome measures of a study based on the type of perturbation used.

REGISTRATION

PROSPERO ID: CRD42020211876.

Uncontrolled manifold analysis of gait kinematic synergy during normal and narrow path walking in individuals with knee osteoarthritis compared to asymptomatic individuals

Knee osteoarthritis (KOA) is a common musculoskeletal disorder resulting in altered gait patterns. Uncontrolled manifold (UCM) analysis has been demonstrated as a useful approach for quantitative analysis of motor variability and synergies. The present study aimed to investigate the changes in the kinematic synergy, controlling the center of mass (COM) position while walking on normal and narrow paths in people with KOA compared to asymptomatic participants. In this cross-sectional study, twenty people with mild to moderate KOA and twenty asymptomatic individuals walked at their comfortable preferred speed across normal and narrow paths on a treadmill. The UCM analysis was performed separately using the lower limb segmental angles as elemental variables and the COM displacement as a performance variable during the stance phase of gait for the frontal and sagittal planes. The results revealed that KOA and asymptomatic individuals could exploit kinematic synergy to control the COM displacement regardless of walking conditions (p < 0.05). Furthermore, the variance within the UCM and synergy index were significantly higher on the narrow path than the normal walking in the mediolateral direction in the KOA group (p < 0.05). The findings of this study suggest that individuals with KOA modify their gait kinematic variability to ensure a stronger kinematic synergy when walking on a challenging narrow path.

The effects of challenging walking conditions on kinematic synergy and stability of gait in people with knee osteoarthritis: A study protocol

Background:

Knee osteoarthritis (KOA) may considerably change the gait parameters, including the gait variability patterns. Uncontrolled manifold (UCM) analysis has been used to evaluate the relationship between motor control and gait variability as a useful index for assessing the multi-segmental movements’ coordination during walking. To our knowledge, no research has evaluated the alterations in the gait kinematic parameters during normal and narrow path walking in individuals with KOA as compared to asymptomatic people.

Materials and Methods:

In this cross-sectional study, individuals diagnosed with mild to moderate medial KOA and asymptomatic people will walk at their comfortable preferred speed on a treadmill. A motion capture system will be used to record at least 50 successful gait cycles. The kinematic variability of joints during gait will be analyzed using UCM, with the center of mass (COM) displacement considered as the performance variable. The primary outcome measure will be the lower limb synergy index. Variability of the COM displacement and changes in angles and angular velocities of lower extremity joints will be assessed as the secondary outcomes.

Results:

The results of this protocol study provide information on the lower limb kinematic synergy during gait on normal and narrow paths for individuals with KOA and asymptomatic controls.

Conclusion:

This information will help the researchers and clinicians understand KOA patients’ gait variability characteristics more deeply. Moreover, it may lead to an enhanced evidence-based approach for clinical decision-making concerning improving gait stability and decreasing the falling risk in these people.

Augmented Hip Proprioception Influences Mediolateral Foot Placement During Walking

Hip abductor proprioception contributes to the control of mediolateral foot placement, which varies with step-by-step fluctuations in pelvis dynamics. Prior work has used hip abductor vibration as a sensory probe to investigate the link between vibration within a single step and subsequent foot placement. Here, we extended prior findings by applying time and location varying vibration in every step, seeking to predictably manipulate the continuous, step-by-step relationship between pelvis dynamics and foot placement. We compared participants' (n = 32; divided into two groups of 16 with slightly different vibration control) gait behavior across four treadmill walking conditions: 1) No feedback; 2) Random feedback, with vibration unrelated to pelvis motion; 3) Augmented feedback, with vibration designed to evoke proprioceptive feedback paralleling the actual pelvis motion; 4) Disrupted feedback, with vibration designed to evoke proprioceptive feedback inversely related to pelvis motion. We hypothesized that the relationship between pelvis dynamics and foot placement would be strengthened by Augmented feedback but weakened by Disrupted feedback. For both participant groups, the strength of the relationship between pelvis dynamics at the start of a step and foot placement at the end of a step was significantly (p ≤ 0.0002) influenced by the feedback condition. The link between pelvis dynamics and foot placement was strongest with Augmented feedback, but not significantly weakened with Disrupted feedback, partially supporting our hypotheses. Our approach to augmenting proprioceptive feedback during gait may have implications for clinical populations with a weakened relationship between pelvis motion and foot placement.

The immediate effect of foot orthoses on gluteal and lower limb muscle activity during overground walking in healthy young adults

BACKGROUND

Although foot orthoses are often used in the management of lower limb musculoskeletal conditions, their effects on muscle activation is unclear, especially in more proximal segments of the lower limb.

RESEARCH QUESTION

Primary aim: Is there an immediate effect of foot orthoses on gluteal muscle activity during overground walking in healthy young adults? Secondary aim: Is there an immediate effect of foot orthoses on the activity of hamstring, quadriceps and calf muscles?

METHODS

In eighteen healthy young adults, muscle activity was recorded using fine wire electrodes for gluteus minimus (GMin; anterior, posterior) and gluteus medius (GMed; anterior, middle, posterior); and surface electrodes for gluteus maximus (GMax), hamstring, quadriceps and calf muscles. Participants completed six walking trials for two conditions; shoe and shoe with prefabricated foot orthoses. Muscle activity was normalised to the peak activity of the shoe condition and analysed using one-dimensional statistical non-parametric mapping to identify differences across the gait cycle.

RESULTS

Activity of GMed (anterior, middle, posterior) and GMin (posterior) was reduced in early stance phase when the orthosis was worn in the shoe (p < 0.05). GMin (anterior) activity was significantly reduced during swing (p < 0.05). Muscle activity was also significantly reduced during the orthoses condition for the lateral hamstrings and calf muscles (p < 0.05).

SIGNIFICANCE

Using foot orthoses may provide a strategy to reduce demand on GMin, GMed, lateral hamstring and calf muscles while walking.

Neuromechanical response of the upper body to unexpected perturbations during gait initiation in young and older adults

BACKGROUND

Control of upper body motion deteriorates with ageing leading to impaired ability to preserve balance during gait, but little is known on the contribution of the upper body to preserve balance in response to unexpected perturbations during locomotor transitions, such as gait initiation.

AIM

To investigate differences between young and older adults in the ability to modify the trunk kinematics and muscle activity following unexpected waist lateral perturbations during gait initiation.

METHODS

Ten young (25 ± 2 years) and ten older adults (73 ± 5 years) initiated locomotion from stance while a lateral pull was randomly applied to the pelvis. Two force plates were used to define the feet centre-of-pressure displacement. Angular displacement of the trunk in the frontal plane was obtained through motion analysis. Surface electromyography of cervical and thoracic erector spinae muscles was recorded bilaterally.

RESULTS

A lower trunk lateral bending towards the stance leg side in the preparatory phase of gait initiation was observed in older participants following perturbation. Right thoracic muscle activity was increased in response to the perturbation during the initial phase of gait initiation in young (+ 68%) but not in older participants (+ 7%).

CONCLUSIONS

The age-related reduction in trunk movement could indicate a more rigid behaviour of the upper body employed by older compared to young individuals in response to unexpected perturbations preceding the initiation of stepping. Older adults' delayed activation of thoracic muscles could suggest impaired reactive mechanisms that may potentially lead to a fall in the early stages of the gait initiation.

How does external lateral stabilization constrain normal gait, apart from improving medio-lateral gait stability?

BACKGROUND

The effect of external lateral stabilization on medio-lateral gait stability has been investigated previously. However, existing lateral stabilization devices not only constrain lateral motions but also transverse and frontal pelvis rotations. This study aimed to investigate the effect of external lateral stabilization with and without constrained transverse pelvis rotation on mechanical and metabolic gait features.

METHODS

We undertook two experiments with 11 and 10 young adult subjects, respectively. Kinematic, kinetic and breath-by-breath oxygen consumption data were recorded during three walking conditions (normal walking (Normal), lateral stabilization with (Free) and without transverse pelvis rotation (Restricted)) and at three speeds (0.83, 1.25 and 1.66 m s-1) for each condition. In the second experiment, we reduced the weight of the frame, and allowed for longer habituation time to the stabilized conditions.

RESULTS

External lateral stabilization significantly reduced the amplitudes of the transverse and frontal pelvis rotations, in addition to medio-lateral, anterior-posterior, and vertical pelvis displacements, transverse thorax rotation, arm swing, step length and step width. The amplitudes of free vertical moment, anterior-posterior drift over a trial, and energy cost were not significantly influenced by external lateral stabilization. The removal of pelvic rotation restrictions by our experimental set-ups resulted in normal frontal pelvis rotation in Experiment 1 and significantly higher transverse pelvis rotation in Experiment 2, although transverse pelvis rotation still remained significantly less than in the Normal condition. Step length increased with the increased transverse pelvis rotation.

CONCLUSION

Existing lateral stabilization set-ups not only constrain medio-lateral motions (i.e. medio-lateral pelvis displacement) but also constrain other movements such as transverse and frontal pelvis rotations, which leads to several other gait changes such as reduced transverse thorax rotation, and arm swing. Our new set-ups allowed for normal frontal pelvis rotation and more transverse pelvis rotation (yet less than normal). However, this did not result in more normal thorax rotation and arm swing. Hence, to provide medio-lateral support without constraining other gait variables, more elaborate set-ups are needed.

Active foot placement control ensures stable gait: Effect of constraints on foot placement and ankle moments

Step-by-step foot placement control, relative to the center of mass (CoM) kinematic state, is generally considered a dominant mechanism for maintenance of gait stability. By adequate (mediolateral) positioning of the center of pressure with respect to the CoM, the ground reaction force generates a moment that prevents falling. In healthy individuals, foot placement is complemented mainly by ankle moment control ensuring stability. To evaluate possible compensatory relationships between step-by-step foot placement and complementary ankle moments, we investigated the degree of (active) foot placement control during steady-state walking, and under either foot placement-, or ankle moment constraints. Thirty healthy participants walked on a treadmill, while full-body kinematics, ground reaction forces and EMG activities were recorded. As a replication of earlier findings, we first showed step-by-step foot placement is associated with preceding CoM state and hip ab-/adductor activity during steady-state walking. Tight control of foot placement appears to be important at normal walking speed because there was a limited change in the degree of foot placement control despite the presence of a foot placement constraint. At slow speed, the degree of foot placement control decreased substantially, suggesting that tight control of foot placement is less essential when walking slowly. Step-by-step foot placement control was not tightened to compensate for constrained ankle moments. Instead compensation was achieved through increases in step width and stride frequency.

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.

Older but not younger adults rely on multijoint coordination to stabilize the swinging limb when performing a novel cued walking task

Motor flexibility, the ability to employ multiple motor strategies to meet task demands, may facilitate ambulation in complex environments that constrain movements; loss of motor flexibility may impair mobility. The purpose of this study was to determine the effects of obesity (a specific model of mobility impairment) and advanced age on motor flexibility during a task that constrained foot placement while walking. Twenty-one community-dwelling obese (OB) and 25 normal weight (NW) older adults (46 total older adults-OA) and 10 younger adults (YA) walked normally on a treadmill (baseline) then walked while stepping on lighted cues projected onto the treadmill at locations corresponding to average foot placement during normal walking (cued). The uncontrolled manifold (UCM) analysis was used to partition total variance in a set of seven lower-limb segment angles into components that did ("bad" variance) and did not ("good" variance) affect step-to-step variance in the trajectory of the swing foot. Motor flexibility was operationalized as an increase (baseline to cued) in total variance with an increase in good variance that exceeded the change in bad variance. There was no significant group × walking task interaction for total and good variance for OB vs NW, but there was a strong and significant interaction effect for OA vs YA (p < 0.01; Cohen's d > 1.0). Whereas YA reduced both good and bad variance, OA increased good variance beyond the change in bad variance. In OA, these changes were associated with several functional measures of mobility. Cued walking may place greater demands on OA requiring greater reliance on motor flexibility, although otherwise healthy older obese adults may be able to compensate for functional and cognitive declines associated with obesity by increasing motor flexibility under such tasks. The extent to which motor flexibility is employed during novel or constrained tasks may be a biomarker of healthy aging and a target for (re)habilitation.

The effect of external lateral stabilization on the use of foot placement to control mediolateral stability in walking and running

It is still unclear how humans control mediolateral (ML) stability in walking and even more so for running. Here, foot placement strategy as a main mechanism to control ML stability was compared between walking and running. Moreover, to verify the role of foot placement as a means to control ML stability in both modes of locomotion, this study investigated the effect of external lateral stabilization on foot placement control. Ten young adults participated in this study. Kinematic data of the trunk (T₆) and feet were recorded during walking and running on a treadmill in normal and stabilized conditions. Correlation between ML trunk CoM state and subsequent ML foot placement, step width, and step width variability were assessed. Paired t-tests (either SPM1d or normal) were used to compare aforementioned parameters between normal walking and running. Two-way repeated measures ANOVAs (either SPM1d or normal) were used to test for effects of walking vs. running and of normal vs. stabilized condition. We found a stronger correlation between ML trunk CoM state and ML foot placement and significantly higher step width variability in walking than in running. The correlation between ML trunk CoM state and ML foot placement, step width, and step width variability were significantly decreased by external lateral stabilization in walking and running, and this reduction was stronger in walking than in running. We conclude that ML foot placement is coordinated to ML trunk CoM state to stabilize both walking and running and this coordination is stronger in walking than in running.

Revealing the optimal thresholds for movement performance: A systematic review and meta-analysis to benchmark pathological walking behaviour

In order to address whether increased levels of movement output variability indicate pathological performance, we systematically reviewed and synthesized meta-analysis data on healthy and pathological motor behavior. After screening up to 24'000 reports from four databases, 85 studies were included containing 2409 patients and 2523 healthy asymptomatic controls. The optimal thresholds of variability with uncertainty boundaries (in % Coefficient of Variation ± Standard Error) were estimated in 7 parameters: stride time (2.34 ± 0.21), stride length (2.99 ± 0.37), step length (3.34 ± 0.84), swing time (2.94 ± 0.60), step time (3.35 ± 0.23), step width (15.87 ± 1.86), and dual-limb support time (6.08 ± 2.83). All spatio-temporal parameters exhibited a positive effect size (pathology led to increased variability) except step width variability (Effect Size = -0.21). By objectively benchmarking thresholds for pathological motor variability also presented through a case-study, this review provides access to movement signatures to understand neurological changes in an individual that are apparent in movement variability. The comprehensive evidence presented now qualifies stride time variability as a movement biomarker, endorsing its applicability as a viable outcome measure in clinical trials.

Correlations of pelvis state to foot placement do not imply within-step active control

Experimental studies of human walking have shown that within an individual step, variations in the center of mass (CoM) state can predict corresponding variations in the next foot placement. This has been interpreted by some to indicate the existence of active control in which the nervous system uses the CoM state at or near mid-stance to regulate subsequent foot placement. However, the passive dynamics of the moving body and/or moving limbs also contribute (perhaps strongly) to foot placement, and thus to its variation. The extent to which correlations of CoM state to foot placement reflect the effects of within-step active control, those of passive dynamics, or some combination of both, remains an important and still open question. Here, we used an open-loop-stable 2D walking model to show that this predictive ability cannot by itself be taken as evidence of within-step active control. In our simulations, we too find high correlations between the CoM state and subsequent foot placement, but these correlations are entirely due to passive dynamics as our system has no active control, either within a step or between steps. This demonstrates that any inferences made from such correlations about within-step active control require additional supporting evidence beyond the correlations themselves. Thus, these within-step predictive correlations leave unresolved the relative importance of within-step active control as compared to passive dynamics, meaning that such methods should be used to characterize control in human walking only with caution.

Muscle contributions to mediolateral and anteroposterior foot placement during walking

Foot placement is critical to balance control during walking and is primarily controlled by muscle force generation. Although gluteus medius activity has been associated with mediolateral foot placement, how other muscles contribute to foot placement is not clear. Furthermore, although dynamic walking models have suggested that anteroposterior foot placement can be passively controlled, the extent to which muscles actively contribute to anteroposterior foot placement has not been determined. The objective of this study was to identify individual muscle contributions to mediolateral and anteroposterior foot placement during walking in healthy adults. Dynamic simulations of walking were developed for six older adults and a segmental power analysis was performed to determine the individual muscle contributions to the mediolateral and anteroposterior power delivered to the foot segment. The simulations revealed the ipsilateral swing limb gluteus medius, iliopsoas, rectus femoris and hamstrings and the contralateral stance limb gluteus medius and ankle plantarflexors were primary contributors to both mediolateral and anteroposterior foot placement. Muscle contributions to foot placement were found to be highly influenced by their contributions to pelvis power, which was dominated by those muscles crossing the hip joint. Thus, impaired balance control may be improved by focusing rehabilitation interventions on optimizing the coordination of those muscles crossing the hip joint and the ankle plantarflexors.

Load Carriage Device for Studying Medial-Lateral Stability of Walking: Design and Performance Evaluation

When walking, the trunk not only oscillates in the vertical direction, but also in the medial-lateral direction. We developed a novel backpack that uses the medial-lateral oscillations of the trunk as input motion to drive medial-lateral oscillations of weight carried in a modified backpack. We use a combination of spring and damping elements to control mass motion, resulting in the ability to prescribe a variety of mass oscillation amplitudes and phase angles. We propose the device as a platform that can be used to study medial-lateral stability during walking. In particular, if the body's ability to predict medial-lateral centre-of-mass state is affected by an oscillating external mass. In this paper, we present the design, model, and model evaluation of our novel load carriage device. During testing, our model was able to predict the oscillation dynamics of the carried mass while walking: demonstrating its capability to create a variety of load carriage scenarios for the user.

Reconstructing the First Metatarsophalangeal Joint of Homo naledi

The aim of the present study was to develop a new method to reconstruct damaged metatarsophalangeal joint (MTPJ) of Homo naledi's fossil and to deepen the understanding of the first metatarsal head (FMH) morphological adaptation in different gait patterns. To this purpose three methods were introduced. The first served to compare the anthropometric linear and volumetric measurements of Homo naledi's MTPJ to that of 10 various athletes. The second was employed to measure curvature diameter in FMH's medial and lateral grooves for sesamoid bones. The third was used to determine the parallelism between medial and lateral FMH grooves. The anthropometric measurements of middle-distance runner to the greatest extent mimicked that of Homo naledi. Thus, it was used to successfully reconstruct the damaged Homo naledi's MTPJ. The highest curvature diameter of medial FMH groove was found in Homo naledi, while in lateral FMH groove it was the highest in volleyball player, suggesting their increased bear loading. The parallelism of medial and lateral FMH grooves was observed only in Homo naledi, while in investigated athletes it was dis-parallel. Athletes' dis-paralleled structures make first MTPJ simple flexion movement a complicated one: not rotating about one axis, but about many, which may result in bringing a negative effect on running. In conclusion, the presented method for the reconstruction of the damaged foot bone paves the way for morphological and structural analysis of modern population and fossil hominins' gait pattern.

Locomotor patterns change over time during walking on an uneven surface

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

Humans use multi-objective control to regulate lateral foot placement when walking

A fundamental question in human motor neuroscience is to determine how the nervous system generates goal-directed movements despite inherent physiological noise and redundancy. Walking exhibits considerable variability and equifinality of task solutions. Existing models of bipedal walking do not yet achieve both continuous dynamic balance control and the equifinality of foot placement humans exhibit. Appropriate computational models are critical to disambiguate the numerous possibilities of how to regulate stepping movements to achieve different walking goals. Here, we extend a theoretical and computational Goal Equivalent Manifold (GEM) framework to generate predictive models, each posing a different experimentally testable hypothesis. These models regulate stepping movements to achieve any of three hypothesized goals, either alone or in combination: maintain lateral position, maintain lateral speed or “heading”, and/or maintain step width. We compared model predictions against human experimental data. Uni-objective control models demonstrated clear redundancy between stepping variables, but could not replicate human stepping dynamics. Most multi-objective control models that balanced maintaining two of the three hypothesized goals also failed to replicate human stepping dynamics. However, multi-objective models that strongly prioritized regulating step width over lateral position did successfully replicate all of the relevant step-to-step dynamics observed in humans. Independent analyses confirmed this control was consistent with linear error correction and replicated step-to-step dynamics of individual foot placements. Thus, the regulation of lateral stepping movements is inherently multi-objective and balances task-specific trade-offs between competing task goals. To determine how people walk in their environment requires understanding both walking biomechanics and how the nervous system regulates movements from step-to-step. Analogous to mechanical “templates” of locomotor biomechanics, our models serve as “control templates” for how humans regulate stepping movements from each step to the next. These control templates are symbiotic with well-established mechanical templates, providing complimentary insights into walking regulation.

When we walk, we walk in real-world contexts and with specific goal to achieve. Side-to-side movements are paramount because walking bipeds (humans, animals, robots, etc.) are inherently more unstable laterally. This is particularly important in older adults as sideways falls greatly increase hip fracture risk. Additionally, we normally walk on paths that limit (more or less) our lateral movements. Appropriately regulating lateral stepping movements is thus critical to achieving successful locomotion in any such context. Here, we use appropriate models to test competing hypotheses about how humans regulate lateral stepping movements from each step to the next to identify what task goals they try to achieve. Our work both bridges and unifies perspectives from dynamic walking and computational motor control to provide a coherent theoretical and computational framework from which to study motor regulation in the context of goal-directedness across a wide range of walking tasks and/or conditions.