Sitting Pressure Measurements in Wheelchair Users-Can the Effects of Daily Relief Activities Be Depicted?

Seat pressure measurements in wheelchair users have been available for some time; however, repeated measurements from a commercially available pressure mat over 90 min did not differ in the pressure-loaded measurement area or the coordinates of the center of pressure, even in participants who were able to reposition themselves in the wheelchair. The question therefore arises: to what extent are there other parameters that reflect the activity of wheelchair users with the pressure mat? To investigate this, a commercial pressure mat (BodiTrak®) was used to perform the measurements of pressure of 33 adult wheelchair-dependent people with spinal cord injury after 30 and 90 min sitting on the cushion. In addition to the standard output of the pressure mat, graph-based surface analyses (calculation of the area of maximum pressure, calculation of the pressure-loaded measurement area, and pressure-area ratio) was performed retrospectively using Python 3.7. The analysis of the measurements after 30 and 90 min was performed by distinguishing the participants between those who could actively change their position (N = 24) and those who could not (N = 9). The parameters of the pressure mat and the graph-based analyses remained unchanged for active participants. In participants who were unable to actively change their position, the area of maximum pressure and the pressure-area ratio (ratio of maximum pressure area and total pressure-loaded area) increased. Significant differences between minutes 30 and 90 are only found for the pressure-area ratio. Thus, when measuring the seat pressure of wheelchair users, the pressure-area ratio should be taken into account as it reflects the daily relief activities of wheelchair users.

Virtual pivot point: Always experimentally observed in human walking?

A main challenge in human walking is maintaining stability. One strategy to balance the whole body dynamically is to direct the ground reaction forces toward a point above the center of mass, called virtual pivot point (VPP). This strategy could be observed in various experimental studies for human and animal gait. A VPP was also observed when VPP input variables like center of mass or ground reaction forces were perturbed. In this study, the kinetic and kinematic consequences of a center of pressure manipulation and the influence on the VPP are investigated. Thus, eleven participants walked with manipulated center of pressure (i.e. barefoot, backwards, with a rigid sole, with stilts, and in handstand compared to shoe walking). In all conditions a VPP could be observed, only one participant showed no VPP in handstand walking. The vertical VPP position only differs between shoe walking and rigid sole walking, there are no significant differences between the conditions in the horizontal VPP position and the spread around the VPP. However, it is conceivable that for more severe gait changes, walking without VPP could be observed. To further analyze this issue, the authors provide a VPP calculation tool for testing data regarding the existence of the VPP.

How Do Everyday Life Activities Affect Seating Pressure Measurements?

BACKGROUND: Pressure measurements to detect risks for pressure injuries in wheelchair users with spinal cord injury (SCI) have been available for quite some time. Unfortunately, knowledge of how postural changes during longer periods of daily life activity affecting the results is still limited. In the present study, the authors expected to note shifts in the center of pressure as well as in the pressure distribution, especially in patients who were able to change their position actively. METHODS: A seat pressure mat (BodiTrak2) was used to perform measurements of 34 SCI wheelchair users after initial transfer into the wheelchair as well as 30 and 90 minutes later. Mean pressure, maximum pressure, pressure-loaded measuring surface, and the coordinates of the center of pressure were analyzed, and findings were statistically analyzed using the t test and Intraclass Correlation Coefficient. To quantify the drift of the measurement system, recordings with a reference weight were performed. RESULTS: The analyzed parameters from the initial measurement differed significantly from the later measurements at 30 and 90 minutes, whereas the parameters were stable after 30 minutes. The measurements with the reference weight showed the same measurement course. CONCLUSIONS: The measurements after 30 and 90 minutes were consistent, contrary to expectations. The activity of the participant between measurements did not appear to be of much importance. CLINICAL RELEVANCE: Relief maneuvers appear to provide relief only while the maneuver is being performed. Thus, relief activities must be frequent enough and long enough to allow adequate blood flow to the tissues.

Walking like a robot: do the ground reaction forces still intersect near one point when humans imitate a humanoid robot?

Bipedal walking while keeping the upper body upright is a complex task. One strategy to cope with this task is to direct the ground reaction forces toward a point above the centre of mass of the whole body, called virtual pivot point (VPP). This behaviour could be observed in various experimental studies for human and animal walking, but not for the humanoid robot LOLA. The question arose whether humans still show a VPP when walking like LOLA. For this purpose, ten participants imitated LOLA in speed, posture, and mass distribution (LOLA-like walking). It could be found that humans do not differ from LOLA in spatio-temporal parameters for the LOLA-like walking, in contrast to upright walking with preferred speed. Eight of the participants show a VPP in all conditions (R2 > 0.90 ± 0.09), while two participants had no VPP for LOLA-like walking (R2 < 0.52). In the latter case, the horizontal ground reaction forces are not balanced around zero in the single support phase, which is presumably the key variable for the absence of the VPP.

Validity of an inertial sensor-based system for the assessment of spatio-temporal parameters in people with multiple sclerosis.

Background: Gait variability in people with multiple sclerosis (PwMS) reflects disease progression or may be used to evaluate treatment response. To date, marker-based camera systems are considered as gold standard to analyze gait impairment in PwMS. These systems might provide reliable data but are limited to a restricted laboratory setting and require knowledge, time, and cost to correctly interpret gait parameters. Inertial mobile sensors might be a user-friendly, environment- and examiner-independent alternative. The purpose of this study was to evaluate the validity of an inertial sensor-based gait analysis system in PwMS compared to a marker-based camera system. Methods: A sample N = 39 PwMS and N = 19 healthy participants were requested to repeatedly walk a defined distance at three different self-selected walking speeds (normal, fast, slow). To measure spatio-temporal gait parameters (i.e., walking speed, stride time, stride length, the duration of the stance and swing phase as well as max toe clearance), an inertial sensor system as well as a marker-based camera system were used simultaneously. Results: All gait parameters highly correlated between both systems (r > 0.84) with low errors. No bias was detected for stride time. Stance time was marginally overestimated (bias = -0.02 ± 0.03 s) and gait speed (bias = 0.03 ± 0.05 m/s), swing time (bias = 0.02 ± 0.02 s), stride length (0.04 ± 0.06 m), and max toe clearance (bias = 1.88 ± 2.35 cm) were slightly underestimated by the inertial sensors. Discussion: The inertial sensor-based system captured appropriately all examined gait parameters in comparison to a gold standard marker-based camera system. Stride time presented an excellent agreement. Furthermore, stride length and velocity presented also low errors. Whereas for stance and swing time, marginally worse results were observed.

'Virtual pivot point' in human walking: Always experimentally observed but simulations suggest it may not be necessary for stability.

The intersection of ground reaction forces near a point above the center of mass has been observed in computer simulation models and human walking experiments. Observed so ubiquitously, the intersection point (IP) is commonly assumed to provide postural stability for bipedal walking. In this study, we challenge this assumption by questioning if walking without an IP is possible. Deriving gaits with a neuromuscular reflex model through multi-stage optimization, we found stable walking patterns that show no signs of the IP-typical intersection of ground reaction forces. The non-IP gaits found are stable and successfully rejected step-down perturbations, which indicates that an IP is not necessary for locomotion robustness or postural stability. A collision-based analysis shows that non-IP gaits feature center of mass (CoM) dynamics with vectors of the CoM velocity and ground reaction force increasingly opposing each other, indicating an increased mechanical cost of transport. Although our computer simulation results have yet to be confirmed through experimental studies, they already indicate that the role of the IP in postural stability should be further investigated. Moreover, our observations on the CoM dynamics and gait efficiency suggest that the IP may have an alternative or additional function that should be considered.

Stability of sensor-based gait parameters reassessed after a period of one year in people with multiple sclerosis.

BACKGROUND: Currently, there are several studies showing that wearable inertial sensors are highly sensitive in the detection of gait disturbances in people with multiple sclerosis (PwMS), showing excellent reliability within one or 7-14 days. However, it is not known how stable these gait parameters remain over a longer period of time. This is surprising, because many treatments last longer than two weeks. Thus, the purpose of the current study was to examine gait parameters obtained by means of wearable inertial sensors during a 6-min walk and to reassess these parameters after a period of one year. METHODS: Fifty PwMS (without a relapse or a recent change in the Expanded Disability Status Scale (EDSS) or treatment) and 20 healthy participants were examined at two assessment points (interval between assessments: 14.4 ± 6.6 months). At each assessment point, all participants had to complete a 6-min walking test, an observer-rater test (Berg Balance Scale, BBS) and a Timed-up and Go Test (TUG). To measure mean gait parameters (i.e. walking speed, stride length, stride time, the duration of the stance and swing phase and minimum toe-to-floor distance), as well as the intraindividual standard deviation of each mean gait parameter, wearable inertial sensors were utilized. RESULTS: We found that even after one year all mean gait parameters showed excellent Intraclass Correlation Coefficients (ICC between 0.75 and 0.95) in PwMS. Looking at MS subgroups, the ICCs were slightly higher in MS subgroup 2 (EDSS 2.0-5.0) than those in MS subgroup 1 (EDSS 0.0-1.5) and healthy controls. Compared to the mean gait parameters, parameters of gait variability showed only good-to-fair ICC values in PwMS. Concerning BBS and TUG, the ICC values after one year were close to the ICC values of the measured mean gait parameters. CONCLUSIONS: Due to the excellent stability of mean gait parameters after one year, these sensor-based gait parameters can be identified as clinically relevant markers to evaluate treatment effects over a longer (several months) period of time in MS.

Evaluating anticipatory control strategies for their capability to cope with step-down perturbations in computer simulations of human walking.

Previous simulation studies investigated the role of reflexes and central pattern generators to explain the kinematic and dynamic adaptations in reaction to step-down perturbations. However, experiments also show preparatory adaptations in humans based on visual anticipation of a perturbation. In this study, we propose a high-level anticipatory strategy augmenting a low-level muscle-reflex control. This strategy directly changes the gain of the reflex control exclusively during the last contact prior to a drop in ground level. Our simulations show that especially the anticipatory reduction of soleus activity and the increase of hamstrings activity result in higher robustness. The best results were obtained when the change in stimulation of the soleus muscle occurred 300 ms after the heel strike of the contralateral leg. This enabled the model to descend perturbation heights up to - 0.21 m and the resulting kinematic and dynamic adaptations are similar to the experimental observations. This proves that the anticipatory strategy observed in experiments has the purpose of increasing robustness. Furthermore, this strategy outperforms other reactive strategies, e.g., pure feedback control or combined feedback and feed-forward control, with maximum perturbation heights of - 0.03 and - 0.07 m, respectively.

Mindfulness training during brief periods of hospitalization in multiple sclerosis (MS): beneficial alterations in fatigue and the mediating role of depression.

OBJECTIVES: Persons with MS (PwMS) are frequently affected by fatigue and depression. Mindfulness-based interventions may reduce these symptoms in PwMS and consequently their application has been extended to various settings. Only few efforts have been made to explore effects of short-term mindfulness training during brief periods of hospitalization. In the current study, the feasibility and potential effects of short-term mindfulness training on depression, fatigue, rumination and cognition were explored in PwMS in an acute-care hospital setting. Based on previous work, it was further examined whether the relation between trait mindfulness and fatigue prior to and following the intervention was mediated by depression and whether a mediation effect was also observable throughout the intervention. METHODS: A short-term mindfulness training protocol was developed, tailored to the requirements of the acute-care setting. Subsequently, 30 PwMS were recruited sequentially and received mindfulness training during the routine clinical process (median duration in hospital: eight days, number of sessions: four). Participants completed relevant self-report measures (depression, fatigue, rumination) and a neuropsychological assessment before and after training. RESULTS: Participants reported significantly increased trait mindfulness and decreased depression and fatigue following the intervention. Respective change scores were highly correlated so that increased trait mindfulness was associated with decreased symptoms. In the rumination domain, patients reported a tendency for an increased adaptive ability to engage in distractive behavior during arising negative mood. Other measures of trait rumination and cognition remained relatively stable. Results of the mediation analyses indicated that depression mediated the negative relationship between trait mindfulness and fatigue symptoms at pre and post assessments. With regards to the change scores, an association between mindfulness and cognitive fatigue ceased to be significant when depression was controlled, albeit in this case, the mediation effect did not reach significance. CONCLUSION: Results of the current study indicate that short-term mindfulness training during brief periods of hospitalization may be beneficial for PwMS. They further complement previous work by identifying depression as a potential mediator of the antagonistic relationship between mindfulness and fatigue. Based on the current exploratory study, future trials are warranted to address this mechanism of mindfulness training in more detail.

Wearable inertial sensors are highly sensitive in the detection of gait disturbances and fatigue at early stages of multiple sclerosis.

BACKGROUND: The aim of the current study was to examine multiple gait parameters obtained by wearable inertial sensors and their sensitivity to clinical status in early multiple sclerosis (MS). Further, a potential correlation between gait parameters and subjective fatigue was explored. METHODS: Automated gait analyses were carried out on 88 MS patients and 31 healthy participants. To measure gait parameters (i.e. walking speed, stride length, stride duration, duration of stance and swing phase, minimal toe-to-floor distance), wearable inertial sensors were utilized throughout a 6-min 25-ft walk. Additionally, self-reported subjective fatigue was assessed. RESULTS: Mean gait parameters consistently revealed significant differences between healthy participants and MS patients from as early as an Expanded Disability Status Scale (EDSS) value of 1.5 onwards. Further, MS patients showed a significant linear trend in all parameters, reflecting continuously deteriorating gait performance throughout the test. This linear deterioration trend showed significant correlations with fatigue. CONCLUSIONS: Wearable inertial sensors are highly sensitive in the detection of gait disturbances, even in early MS, where global scales such as the EDSS do not provide any clinical information about deviations in gait behavior. Moreover, these measures provide a linear trend parameter of gait deterioration that may serve as a surrogate marker of fatigue. In sum, these results suggest that classic timed walking tests in routine clinical practice should be replaced by readily and automatically applicable gait assessments, as provided by inertial sensors.

Measuring Gait Stability in People with Multiple Sclerosis Using Different Sensor Locations and Time Scales.

The evaluation of local divergence exponent (LDE) has been proposed as a common gait stability measure in people with multiple sclerosis (PwMS). However, differences in methods of determining LDE may lead to different results. Therefore, the purpose of the current study was to determine the effect of different sensor locations and LDE measures on the sensitivity to discriminate PwMS. To accomplish this, 86 PwMS and 30 healthy participants were instructed to complete a six-minute walk wearing inertial sensors attached to the foot, trunk and lumbar spine. Due to possible fatigue effects, the LDE short (~50% of stride) and very short (~5% of stride) were calculated for the remaining first, middle and last 30 strides. The effect of group (PwMS vs. healthy participants) and time (begin, mid, end) and the effect of Expanded Disability Status Scale (EDSS) and time were assessed with linear random intercepts models. We found that perturbations seem to be better compensated in healthy participants on a longer time scale based on trunk movements and on a shorter time scale (almost instantaneously) according to the foot kinematics. Therefore, we suggest to consider both sensor location and time scale of LDE when calculating local gait stability in PwMS.

Ground reaction forces intersect above the center of mass in single support, but not in double support of human walking.

There are various simplifying models that describe balance strategies of human walking. In one model it is assumed that ground reaction forces are directed to a point (virtual pivot point) above the center of mass during the whole stride. This was observed in several experimental investigations, but only for the single support phase. It has not yet been concretely considered whether humans use the same stabilization strategy during the double support phase. For analyzing this, nine volunteers walked at self-selected speed while kinetic and kinematic data were measured. We found that in contrast to the single support phase, where the virtual pivot point was significantly above the center of mass, in the double support phase of human walking the ground reaction forces point around the center of mass with a small spread (R2=92.5%). The different heights of the virtual pivot point in the different support phases could be caused by the vertical movement of the center of mass, which has a lower amplitude in the double support phase. This is also reflected in the ground reaction forces, whereby the ratio of the horizontal and vertical ground reaction forces can explain the height of the virtual pivot point. In the double support phase the ratio is shifted in favor of the horizontal component compared to the single support phase, because of a shorter contact time and a delayed braking impulse. Thus, the whole body seems to rotate around the center of mass, which presumably minimizes required energy.

Trait mindfulness is primarily associated with depression and not with fatigue in multiple sclerosis (MS): implications for mindfulness-based interventions.

OBJECTIVES: Persons with MS (PwMS) often display symptoms of depression and fatigue. Mindfulness-based interventions are known to counteract these symptoms. However, to-date the exact relations between trait mindfulness, depression and fatigue remain to be examined. Fatigue is generally regarded as a symptom immanent to the disease and as a direct neurobiological consequence of increased cytokine levels and cortical atrophy. In depression on the other hand, psychosocial factors in the context of adaptation difficulties are probably of higher relevance. Hence, one may argue that mindfulness, as a trait that promotes successful adaption, may show a strong negative association with depression and a relatively minor negative association with fatigue in PwMS. METHODS: In the current study, the association between self-reported trait mindfulness, fatigue and depression was examined in a sample of 69 PwMS. RESULTS: Trait mindfulness showed highly significant negative correlations with both, depression and fatigue. Mediation analyses however, revealed that depression mediated the relation between mindfulness and fatigue. CONCLUSION: It may be concluded that in PwMS, trait mindfulness shows a genuine negative association with depression, but that it is only secondarily associated with fatigue. Implications for mindfulness-based interventions in MS are discussed. Based on the results of the current study, it may be feasible to promote the acceptance of default fatigue symptoms, instead of an actual reduction of fatigue symptoms.

Negotiating ground level perturbations in walking: Visual perception and expectation of curb height modulate muscle activity.

To negotiate visible and unpredictable changes in ground level, humans use different control strategies depending on the visibility. In case of fully visible perturbations, humans can anticipate the occurrence and the magnitude of the perturbation. In case of a camouflaged perturbation, they can anticipate the occurrence based on the camouflage cover but need to predict the magnitude from experience, as it is not visible. The purpose of this study was to investigate the anticipatory muscular control strategy humans employ when walking down curbs of different height and to investigate how this strategy differs if the step down is fully visible or camouflaged. The activity of five bilateral lower limb muscles (M. gastrocnemius medialis, M. soleus, M. tibialis anterior, M. biceps femoris and M. vastus medialis) of eight healthy subjects was recorded during walking down visible (0, -10 and -20 cm) and camouflaged curbs (0 and -10 cm). The results reveal that the M. gastrocnemius shows a clear anticipatory adaptation to visible curbs in the contralateral and partly also the ipsilateral leg, which further depends on the curb height. Furthermore, in case of a camouflaged perturbation, M. gastrocnemius activity of the contralateral leg shows an adaptation that indicates an average prediction of the curb height, presumably based on previous experience.

Postural stability in human running with step-down perturbations: an experimental and numerical study.

Postural stability is one of the most crucial elements in bipedal locomotion. Bipeds are dynamically unstable and need to maintain their trunk upright against the rotations induced by the ground reaction forces (GRFs), especially when running. Gait studies report that the GRF vectors focus around a virtual point above the centre of mass (VPA), while the trunk moves forward in pitch axis during the stance phase of human running. However, a recent simulation study suggests that a virtual point below the centre of mass (VPB) might be present in human running, because a VPA yields backward trunk rotation during the stance phase. In this work, we perform a gait analysis to investigate the existence and location of the VP in human running at 5 m s-1, and support our findings numerically using the spring-loaded inverted pendulum model with a trunk. We extend our analysis to include perturbations in terrain height (visible and camouflaged), and investigate the response of the VP mechanism to step-down perturbations both experimentally and numerically. Our experimental results show that the human running gait displays a VPB of ≈-30 cm and a forward trunk motion during the stance phase. The camouflaged step-down perturbations affect the location of the VPB. Our simulation results suggest that the VPB is able to encounter the step-down perturbations and bring the system back to its initial equilibrium state.

Prosthetic push-off power in trans-tibial amputee level ground walking: A systematic review.

OBJECTIVE: Unilateral trans-tibial amputation signifies a challenge to locomotion. Prosthetic ankle-foot units are developed to mimic the missing biological system which adapts push-off power to walking speed in some new prosthetic ankle-foot designs. The first systematic review including the two factors aims to investigate push-off power differences among Solid Ankle Cushion Heel (SACH), Energy Storage And Return (ESAR) and Powered ankle-foot units (PWR) and their relation to walking speed. DATA SOURCES: A literature search was undertaken in the Web of Science, PubMed, IEEE xplore, and Google Scholar databases. The search term included: ampu* AND prosth* AND ankle-power AND push-off AND walking. STUDY APPRAISAL AND SYNTHESIS METHODS: Studies were included if they met the following criteria: unilateral trans-tibial amputees, lower limb prosthesis, reported analysis of ankle power during walking. Data extracted from the included studies were clinical population, type of the prosthetic ankle-foot units (SACH, ESAR, PWR), walking speed, and peak ankle power. Linear regression was used to determine whether the push-off power of different prosthetic ankle-foot units varied regarding walking speed. Push-off power of the different prosthetic ankle-foot units were compared using one-way between subjects' ANOVAs with post hoc analysis, separately for slower and faster walking speeds. RESULTS: 474 publications were retrieved, 28 of which were eligible for inclusion. Correlations between walking speed and peak push-off power were found for ESAR (r = 0.568, p = 0.006) and PWR (r = 0.820, p = 0.000) but not for SACH (r = 0.267, p = 0.522). ESAR and PWR demonstrated significant differences in push-off power for slower and faster walking speeds (ESAR (p = 0.01) and PWR (p = 0.02)). CONCLUSION: Push-off power can be used as a selection criterion to differentiate ankle-foot units for prosthetic users and their bandwidth of walking speeds.

Reactive gait and postural adjustments following the first exposures to (un)expected stepdown.

This study evaluated the reactive biomechanical strategies associated with both upper- and lower-body (lead and trail limbs) following the first exposures to (un)expected stepdown at comfortable (1.22 ±â€¯0.08 m/s) and fast (1.71 ±â€¯0.11 m/s) walking velocities. Eleven healthy adults completed 34 trails per walking velocity over an 8-m, custom-built track with two forceplates embedded in its center. For the expected stepdown, the track was lowered by 0-, -10- and -20-cm from the site of the second forceplate, whereas the unexpected stepdown was created by camouflaging the second forceplate (-10-cm). Two-way repeated-measurement ANOVAs detected no velocity-related effects of stepdown on kinematic and kinetic parameters during lead limb stance-phase, and on the trail limb stepping kinematics. However, analyses of significant interactions revealed greater peak flexion angles across the trunk and the trail limb joints (hip, knee and ankle) in unexpected versus expected stepdown conditions at a faster walking velocity. The -10-cm unexpected stepdown (main effect) had a greater influence on locomotor behavior compared to expected conditions due mainly to the absence of predictive adjustments, reflected by a significant decrease in peak knee flexion, contact time and vertical impulse during stance-phase. Walking faster (main effect) was associated with an increase in hip peak flexion and net anteroposterior impulse, and a decrease in contact time and vertical impulse during stepdown. The trail limb, in response, swung forward faster, generating a larger and faster recovery step. However, such reactive stepping following unexpected stepdown was yet a sparse compensation for an unstable body configuration, assessed by significantly smaller step width and anteroposterior margin-of-stability at foot-contact in the first-recovery-step compared with expected conditions. These findings depict the impact of the expectedness of stepdown onset on modulation of global dynamic postural control for a successful accommodation of (un)expected surface elevation changes in young, healthy adults.

Ground reaction forces intersect above the center of mass even when walking down visible and camouflaged curbs.

A main objective in bipedal walking is controlling the whole body to stay upright. One strategy that promotes this objective is to direct the ground reaction forces (GRFs) to a point above the center of mass (COM). In humans, such force patterns can be observed for unperturbed walking, but it is not known whether the same strategy is used for a walkway that changes in height. In this study, 11 volunteers stepped down off a visible (0, 10 and 20 cm) and a camouflaged (0 or 10 cm) curb while walking at two different speeds (1.2±0.1 and 1.7±0.1 m s-1). The results showed that in all conditions the GRFs pointed predominantly above the COM. Vectors directed from the center of pressure (COP) to the intersection point (IP) closely fitted the measured GRF direction not only in visible conditions (R2>97.5%) but also in camouflaged curb negotiation (R2>89.8%). Additional analysis of variables included in the calculation of the IP location showed considerable differences for the camouflaged curb negotiation: compared with level walking, the COP shifted posterior relative to the COM and the vertical GRFs were higher in the beginning and lower in later parts of the stance phase of the perturbed contact. The results suggest that IP behavior can be observed for both visible and camouflaged curb negotiation. For further regulation of the whole-body angle, the asymmetrical vertical GRFs could counteract the effect of a posterior shifted step.

Dynamic postural control during (in)visible curb descent at fast versus comfortable walking velocity.

BACKGROUND: The unexpectedness of ground-contact onset in stepping down due, e.g., to a camouflaged curb during ongoing gait may impose potential postural control challenges, which might be deteriorated when walking faster. RESEARCH QUESTION: Does traversing camouflaged versus visible curbs, at a fast walking velocity, induce more unstable body configurations, assessed by a smaller anteroposterior "margin of stability" (MoS)? METHODS: For twelve healthy participants, we investigated MoS at foot touchdown in descent and in the first recovery step from 0- and 10-cm visible and camouflaged curbs at comfortable (1.22 ± 0.08 m/s) and fast (1.71 ± 0.11 m/s) walking velocities. Three-way (velocity, elevation, visibility) and two-way (velocity, visibility) repeated-measurement ANOVAs were performed to determine their interactions on MoS, and its determining parameters, during curb negotiation and recovery step, respectively. RESULTS: No greater postural instability when traversing a camouflaged versus visible curb at a faster walking velocity during curb descent, indicated by no three-way interaction effects on MoS. However, an elevation-by-visibility interaction showed a dramatic decrease of MoS when descending a 10-cm camouflaged versus visible curb. This was because of a farther anterior displacement of center-of-mass with a larger velocity. Furthermore, the walking velocity was independently associated with a smaller MoS and a more anteriorly-shifted center-of-mass with a higher velocity. In the recovery step, participants demonstrated a reduced stability of the body configuration when walking faster or recovering from a camouflaged than from a visible curb. The mentioned result implies that the potential to increase the base-of-support to compensate for an increased center-of-mass velocity, induced by an increased walking velocity, is limited. SIGNIFICANCE: Despite a significant independent main effect of walking velocity, a more unstable postural control observed during traversing of camouflaged versus visible curbs was found not to be walking velocity-related in young individuals. Further research, including elderly may shed more light on these results.

Humans adjust the height of their center of mass within one step when running across camouflaged changes in ground level.

In running, humans use different control strategies that are most likely influenced by environmental conditions. For example, when human runners face a change in ground level, they adapt the height of their center of mass (CoM) in preparation. In a situation in which a drop might occur but without visual cues regarding its actual height, such a preparation is not possible. We here used camouflaged drops (which occurred by chance) as mechanical disturbances and analyzed the adaptations in the vertical oscillation of the runners CoM. We found that humans lowered their CoM by about 25% of the possible drop height in preparation for the camouflaged contact, regardless of whether a drop occurred or not. In flight phase following the disturbance, the CoM was lowered by about 90% of the drop height in the case of the camouflaged drop and remained almost unaffected (+5%) in the case of level ground. Thus, runners resort to a CoM-control strategy with a fixed desired trajectory height in the flight phase following the camouflaged ground contact. In contrast to previously reported results which show that visible ground level changes were compensated within several steps, this strategy compensates ground level disturbances instantly within a single step.