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Introduction: Asymmetrical sensorimotor function after stroke creates unique challenges for bipedal tasks such as walking or perturbation-induced reactive stepping. Preference for initiating steps with the less-involved (preferred) leg after a perturbation has been reported with limited information on the stepping response of the more-involved (non-preferred) leg. Understanding the capacity of both legs to respond to a perturbation would enhance the design of future treatment approaches. This pilot study investigated the difference in perturbation-induced stepping between legs in stroke participant and non-impaired controls. We hypothesized that stepping performance will be different between groups as well as between legs for post-stroke participants. Methods: Thirty-six participants (20 persons post-stroke, 16 age matched controls) were given an anterior perturbation from three stance positions: symmetrical (SS), preferred asymmetrical (PAS-70% body weight on the preferred leg), and non-preferred asymmetrical (N-PAS-70% body weight on the non-preferred leg). Kinematic and kinetic data were collected to measure anticipatory postural adjustment (APA), characteristics of the first step (onset, length, height, duration), number of steps, and velocity of the body at heel strike. Group differences were tested using the Mann-Whitney U-test and differences between legs tested using the Wilcoxon signed-rank test with an alpha level of 0.05. Results: Stepping with the more-involved leg increased from 11.5% of trials in SS and N-PAS up to 46% in PAS stance position for participants post-stroke. Post-stroke participants had an earlier APA and always took more steps than controls to regain balance. However, differences between post-stroke and control participants were mainly found when stance position was modified. Compare to controls, steps with the preferred leg (N-PAS) were earlier and shorter (in time and length), whereas steps with the non-preferred leg (PAS) were also shorter but took longer. For post-stroke participants, step duration was longer and utilized more steps when stepping with the more-involved leg compared to the less-involved leg. Conclusions: Stepping with the more-involved leg can be facilitated by unweighting the leg. The differences between groups, and legs in post-stroke participants illustrate the simultaneous bipedal role (support and stepping) both legs have in reactive stepping and should be considered for reactive balance training.

Physiological and degenerative changes affecting human standing balance are major contributors to falls with ageing. During imbalance, stepping is a powerful protective action for preserving balance that may be voluntarily initiated in recognition of a balance threat, or be induced by an externally imposed mechanical or sensory perturbation. Paradoxically, with ageing and falls, initiation slowing of voluntary stepping is observed together with perturbation-induced steps that are triggered as fast as or faster than for younger adults. While age-associated changes in sensorimotor conduction, central neuronal processing and cognitive functions are linked to delayed voluntary stepping, alterations in the coupling of posture and locomotion may also prolong step triggering. It is less clear, however, how these factors may explain the accelerated triggering of induced stepping. We present a conceptual model that addresses this issue. For voluntary stepping, a disruption in the normal coupling between posture and locomotion may underlie step-triggering delays through suppression of the locomotion network based on an estimation of the evolving mechanical state conditions for stability. During induced stepping, accelerated step initiation may represent an event-triggering process whereby stepping is released according to the occurrence of a perturbation rather than to the specific sensorimotor information reflecting the evolving instability. In this case, errors in the parametric control of induced stepping and its effectiveness in stabilizing balance would be likely to occur. We further suggest that there is a residual adaptive capacity with ageing that could be exploited to improve paradoxical triggering and other changes in protective stepping to impact fall risk.

Impairments of balance and gait leading to loss of mobility, falls, and disability are common occurrences in many neurologic conditions and with older age. Much of our current understanding about posture and balance control and its impairments has come from investigations of how healthy individuals and those with neurologic disorders respond to situations that perturb standing balance during instructed voluntary tasks or in reaction to externally imposed challenges to stability. Knowledge obtained from these investigations has come from documenting the physical and physiologic characteristics of the perturbations together with the body's electrophysiologic, structural, kinetic, kinematic, and behavioral responses. From these findings, basic mechanisms, diagnostic and pathologic criteria, and targets for clinical care have been identified while continued gaps in understanding have been exposed. In this chapter, we synthesize and discuss current concepts and understanding concerning the sensorimotor control of posture and balance while standing. We draw insights gained from perturbation studies investigating these functions in healthy adults, and those with neurologic pathologies.

This study investigated the stepping boundary – the force that can be resisted without stepping – for force-controlled perturbations of different durations. Twenty-two healthy young adults (19–37 years old) were instructed to try not to step in response to 86 different force/time combinations of forward waist-pulls. The forces at which 50% of subjects stepped (F50) were identified for each tested perturbation durations. Results showed that F50 decreased hyperbolically when the perturbation’s duration increased and converged toward a constant value (about 10% BW) for longer perturbations (over 1500 ms). The effect of perturbation duration was critical for the shortest perturbations (less than 1 s). In parallel, a simple function was proposed to estimate this stepping boundary. Considering the dynamics of a linear inverted pendulum + foot model and simple balance recovery reactions, we could express the maximum pulling force that can be withstood without stepping as a simple function of the perturbation duration. When used with values of the main model parameters determined experimentally, this function replicated adequately the experimental results. This study demonstrates for the first time that perturbation duration has a major influence on the outcomes of compliant perturbations such as force-controlled pulls. The stepping boundary corresponds to a constant perturbation force-duration product and is largely explained by only two parameters: the reaction time and the displacement of the center of pressure within the functional base of support. Future work should investigate pathological populations and additional parameters characterizing the perturbation time-profile such as the time derivative of the perturbation.

Falls and their consequences are among the major health care problems affecting functional mobility and quality of life among older people. Even for people living independently in the community, falls are common occurrences (1,2). Although the causes of falls are varied and complex, a critical requirement needed for fall prevention is the ability to respond effectively to loss of balance. Protective stepping, an automatic reaction to instability, is a common strategy for maintaining balance in the everyday environment (3). Stepping is not a strategy of last resort but is often initiated well before the body approaches the limits of stability provided by the base of support (4) especially for older people (5,6). To effectively sustain standing balance, steps must be appropriately directed, timed, and executed to arrest the motion of the body center of mass by altering the base of support. Mounting evidence indicates that age-associated impairments in protective stepping contribute importantly to loss of balance and falls (3,6–8). An impaired ability to maintain lateral balance stability is especially relevant to falls among older people. For example, measures of mediolateral stability are associated with future falls (9) and past falls (10), and impaired control of lateral balance through stepping is particularly apparent for older adults (6,11,12). Furthermore, age-related falls often involve lateral body motion and associated hip fractures (13,14). Therefore, these observations underscore the specific challenges to maintaining balance and preventing falls that are linked with deficits in controlling lateral balance stability. Although there is accumulating support for the existence of a directional vulnerability to lateral instability and falls with aging, comprehensive and systematic investigations of this postulation are currently lacking. The aims of this study were (i) to determine the protective stepping response patterns evoked by different directions of externally applied waist-pull perturbations of standing balance and (ii) to identify the changes in stepping characteristics attributable to aging and in relation to the risk of falling. We hypothesized that the laterally oriented disturbances to balance would exacerbate the changes with age in protective stepping responses due to the increased demands for controlling lateral stability, and that these changes would be greater for those individuals who subsequently experienced falls.

International audience; During the initiation of stepping, anticipatory postural adjustments (APAs) for lateral weight transfer and propulsion normally precede the onset of locomotion. In Parkinson's disease (PD), impaired step initiation typically involves altered APA ground force production with delayed step onset and deficits in stepping performance. If, as in stance and gait, sensory information about lower limb load is important for the control of stepping, then perturbations influencing loading conditions could affect the step initiation process. This study investigated the influence of changes in lower limb loading during step initiation in patients with PD and healthy control subjects. Participants performed rapid self-triggered step initiation with the impending single stance limb positioned over a pneumatically actuated platform. In perturbation trials, the stance limb ground support surface was either moved vertically downward (DROP) or upward (ELEVATE) by 1.5 cm shortly after the onset of the APA phase. Overall, PD patients demonstrated a longer APA duration, longer time to first step onset, and slower step speed than controls. In both groups, the DROP perturbation reinforced the intended APA kinetic changes for lateral weight transfer and resulted in a significant reduction in APA duration, increase in peak amplitude, and earlier time to first step onset compared with other conditions. During ELEVATE trials that opposed the intended weight transfer forces both groups rapidly adapted their stepping to preserve standing stability by decreasing step length and duration, and increasing step height and foot placement laterally. The findings suggested that sensory information associated with limb load and/or foot pressure modulates the spatial and temporal parameters of posture and locomotion components of step initiation in interaction with a centrally generated feedforward mode of neural control. Moreover, impaired step initiation in PD may at least acutely be enhanced by augmenting the coupling between posture and locomotion.

International audience; Aim: Voluntary apnoea induces several physiological adaptations, including bradycardia, arterial hypertension and redistribution of regional blood flows. Elite breath-hold divers (BHDs) are able to maintain very long apnoea, inducing severe hypoxaemia without brain injury or black-out. It has thus been hypothesized that they develop protection mechanisms against hypoxia, as well as a decrease in overall oxygen uptake. Methods: To test this hypothesis, the apnoea response was studied in BHDs and non-divers (NDs) during static and dynamic apnoeas (SA, DA). Heart arterial oxygen saturation (SaO(2)), and popliteal artery blood flow were rate, recorded to investigate the oxygen-conserving effect of apnoea response, and the internal carotid artery blood flow was used to examine the mechanisms of cerebral protection. Results: The bradycardia and peripheral vasoconstriction were accentuated in BHDs compared with NDs (P < 0.01), in association with a smaller SaO(2) decrease (-2.7% vs. -4.9% during SA, P < 0.01 and -6% vs. -11.3% during DA, P < 0.01). Greater increase in carotid artery blood flow was also measured during apnoea in BHDs than in controls. Conclusion: These results confirm that elite divers present a potentiation of the well-known apnoea response in both SA and DA conditions. This response is associated with higher brain perfusion which may partly explain the high levels of world apnoea records.

Hilliard MJ, Martinez KM, Janssen I, Edwards B, Mille M-L, Zhang Y, Rogers MW. Lateral balance factors predict future falls in community-living older adults. Objective To prospectively determine the capacity of measures of mediolateral (ML) protective stepping performance, maximum hip abduction torque, and trunk mobility, in order to predict the risk of falls among community-living older people. Design Cross-sectional study. Setting A balance and falls research laboratory. Participants Medically screened and functionally independent community-living older adult volunteers (N=51). Interventions Not applicable. Main Outcome Measures Measures included: (1) protective stepping responses: percentage of trials with multiple balance recovery steps and sidestep/crossover step recovery patterns, and first step length following motor-driven waist-pull perturbations of ML standing balance; (2) hip abduction strength and axial mobility: (3) peak isokinetic hip abduction joint torque and trunk functional axial rotation (FAR) range of motion; and (4) fall incidence: monthly mail-in reporting of fall occurrences with follow-up contact for 1 year post-testing. One- and 2-variable logistic regression analysis models determined which single and combined measures optimally predicted fall status. Results The single variable model with the strongest predictive value for falls was the use of multiple steps in all trials (100% multiple steps) (odds ratio, 6.2; P =.005). Two-variable models, including 100% multiple steps and either hip abduction torque or FAR variables, significantly improved fall prediction over 100% multiple steps alone. The hip abduction and FAR logistic regression optimally predicted fall status. Conclusions The findings identify new predictor variables for risk of falling that underscore the importance of dynamic balance recovery performance through ML stepping in relation to neuromusculoskeletal factors contributing to lateral balance stability. The results also highlight focused risk factors for falling that are amenable to clinical interventions for enhancing lateral balance function and preventing falls.

Anticipatory postural adjustments (APAs) for lateral weight transfer and stability precede and accompany voluntary stepping. Patients with Parkinson's disease (PD) show delays in step initiation with altered APA characteristics that may reflect impaired interactions between posture and locomotion. The purpose of this study was to examine the influence of a lateral postural assist on step initiation in patients with early stage PD while off medication and healthy controls. Subjects performed self-paced rapid forward steps. In one condition (ASSIST), the APA was assisted at onset with a lateral pull applied to the pelvis by a motor-driven robotic system. Ground reaction forces and whole body kinematics were recorded to characterize the APA and step characteristics. Overall, PD subjects had a longer APA duration (P < 0.01) and longer first step duration (P < 0.027) than Control subjects. With the ASSIST, the APA duration for both groups was shorter (P < 0.001), the step onset time was earlier (P < 0.001), and the speed of the first step became faster for PD subjects. Postural assistance affecting the interaction between posture and locomotion may have therapeutic potential for improving movement function in patients with PD.

Physiological and degenerative changes affecting human standing balance are major contributors to falls with ageing. During imbalance, stepping is a powerful protective action for preserving balance that may be voluntarily initiated in recognition of a balance threat, or be induced by an externally imposed mechanical or sensory perturbation. Paradoxically, with ageing and falls, initiation slowing of voluntary stepping is observed together with perturbation‐induced steps that are triggered as fast as or faster than for younger adults. While age‐associated changes in sensorimotor conduction, central neuronal processing and cognitive functions are linked to delayed voluntary stepping, alterations in the coupling of posture and locomotion may also prolong step triggering. It is less clear, however, how these factors may explain the accelerated triggering of induced stepping. We present a conceptual model that addresses this issue. For voluntary stepping, a disruption in the normal coupling between posture and locomotion may underlie step‐triggering delays through suppression of the locomotion network based on an estimation of the evolving mechanical state conditions for stability. During induced stepping, accelerated step initiation may represent an event‐triggering process whereby stepping is released according to the occurrence of a perturbation rather than to the specific sensorimotor information reflecting the evolving instability. In this case, errors in the parametric control of induced stepping and its effectiveness in stabilizing balance would be likely to occur. We further suggest that there is a residual adaptive capacity with ageing that could be exploited to improve paradoxical triggering and other changes in protective stepping to impact fall risk.

International audience; The initiation of human walking involves postural motor actions for body orientation and balance stabilization that must be effectively integrated with locomotion to allow safe and efficient transport. Our ability to coordinately adapt these functions to environmental or bodily changes through error-based motor learning is essential to effective performance. Predictive compensations for postural perturbations through anticipatory postural adjustments (APAs) that stabilize mediolateral (ML) standing balance normally precede and accompany stepping. The temporal sequencing between these events may involve neural processes that suppress stepping until the expected stability conditions are achieved. If so, then an unexpected perturbation that disrupts the ML APAs should delay locomotion. This study investigated how the central nervous system (CNS) adapts posture and locomotion to perturbations of ML standing balance. Healthy human adults initiated locomotion while a resistance force was applied at the pelvis to perturb posture. In experiment 1, using random perturbations, step onset timing was delayed relative to the APA onset indicating that locomotion was withheld until expected stability conditions occurred. Furthermore, stepping parameters were adapted with the APAs indicating that motor prediction of the consequences of the postural changes likely modified the step motor command. In experiment 2, repetitive postural perturbations induced sustained locomotor aftereffects in some parameters (i.e., step height), immediate but rapidly readapted aftereffects in others, or had no aftereffects. These results indicated both rapid but transient reactive adaptations in the posture and gait assembly and more durable practice-dependent changes suggesting feedforward adaptation of locomotion in response to the prevailing postural conditions.

Background. Aging appears to present particular problems for lateral balance stability related to falls. Protective stepping is a common strategy for maintaining balance that may be impaired with aging due to changes in neuromusculoskeletal factors. This study assessed the response patterns, kinematics, and single support hip abduction torque during lateral protective stepping for balance recovery in healthy young and elderly adults. Methods. Ten healthy elderly and 10 younger adults received stepper-motor driven waist-pulls of bipedal stance applied pseudorandomly to either side. Stepping response strategies were quantified with force platforms and motion analysis. Findings. The young responded primarily using a single lateral sidestep with the limb that was initially loaded passively by the waist-pull, while older subjects favored crossover stepping using multiple steps with more inter-limb collisions. When the elderly did use loaded side steps, the steps were longer, slower, and higher and included greater and prolonged lateral trunk motion than in the young. Overall, older subjects produced greater and less rapid stabilizing hip abduction torque during the single support phase. Interpretation. Age-associated differences in lateral balance control through stepping included using a riskier recovery strategy with increased collisions between the limbs, multiple steps, altered first step characteristics and lateral trunk motion during direct sidestepping, and a generally greater support hip torque. The difficulties with lateral balance control in aging may reflect factors such as impaired hip abduction torque-time capacity and lateral trunk mobility/control. Our findings contribute additional knowledge pertaining to the problem of balance dysfunction and falls among the elderly. 2005 Elsevier Ltd. All rights reserved.

Standing subjects were unexpectedly pulled forward to identify a threshold boundary that evokes stepping in terms of the size of the pull relative to the base of support (BoS). Performances in a range of sensorimotor tests were correlated with the threshold boundary parameters. Younger and older subjects were studied to identify age-related changes in stepping and the threshold boundaries. The threshold boundary had a forward limit (TL) that, when crossed, always made subjects step no matter how slowly they were pulled. As velocity increased, the threshold position that produced a step shifted nearer to the ankles. Eventually a pull velocity was reached above which velocity had no further effect and a position threshold (TH) was identified behind which subjects never stepped. Thus the position threshold boundary for stepping is a posterior-going sigmoidal function of perturbation velocity. Older subjects stepped more than the young (69% vs. 40% of trials). For the older subjects, TL (91% vs. 107% BoS) and TH (59% vs. 72% BoS) were closer to the ankles, and the transition between TL and TH occurred at lower velocities (96% vs. 121% BoS.s–1). Across the entire study population many sensorimotor factors were associated with TL and TH. However, these associations were not present when age was removed as a factor. Thus, although the older subjects use protective stepping more often, this cannot be attributed directly to the sensorimotor factors tested here. It can be explained by stepping as a triggered response to the perturbation event rather than later sensory input about body movement.

Human stepping is a commonly executed control strategy for maintaining standing balance in the natural environment. Aging changes in the initiation triggering of both voluntary (longer latency) and perturbation-induced (shorter latency) stepping are associated with falling, and are a complex function of altered sensorimotor, neuromuscular, and cognitive system factors. The aim of this study was to determine the effect of contextual uncertainty about balance stability on the triggering of protective stepping in young and older individuals. Subjects initiated forward stepping during simple reaction time and waist-pull perturbation conditions with and without contextual uncertainty about balance stability. The results showed that, regardless of age, the initiation timing for triggering both voluntary and induced stepping was delayed substantially (100-300 ms) by the presence of balance uncertainty, and that age-associated timing differences were exacerbated with contextual uncertainty. The initiation timing of the first step liftoff for perturbation-induced stepping did not reflect entirely an immediate necessity or last resort strategy to balance instability determined directly by specific sensory input, but rather a decision to step. Moreover, the time to liftoff onset for perturbation-induced stepping was similar for the old and young with contextual certainty, and occurred 130 ms earlier for the old than for the young when balance stability was uncertain. Overall, we concluded that older individuals can retain a residual capacity to sustain stationary standing stability as a function of the prevailing task conditions, and that the reduced timing threshold with age may involve a pre-selected strategy triggered earlier by non-specific event-related sensory input rather than specific movement-related information.

International audience; Objectives: To examine the stepping performance during voluntary and waist-pull perturbation-induced step initiation in people with chronic stroke. Design: Repeated-measures single-case design. Setting: University-based research laboratory. Participants: Community-dwelling stroke survivors (N=10). Interventions: Not applicable. Main Outcome Measures: Ground reaction forces and kinematic data were recorded to assess anticipatory postural adjustments (APAs) and step characteristics for both voluntary and induced stepping conditions. Results: Induced stepping was performed with both the paretic (35% trials) and nonparetic legs (65% trials). Induced first steps occurred earlier and were executed faster than rapid voluntary steps. Compared with voluntary stepping, induced first step APAs were shorter in duration. Step height was higher with the nonparetic leg for both stepping conditions. Use of the paretic leg increased (52%) during the diagonal perturbations that passively unloaded the stepping limb compared with the use of the paretic leg (33%) for forward perturbations. Conclusions: The results indicated differences in executing voluntary and induced stepping, and between the paretic and nonparetic limbs in individuals with chronic stroke. The findings suggested guidelines for using stepping as a component of neurorehabilitation programs for enhancing balance and mobility. Additional larger-scale studies remain to be undertaken to further investigate these issues. Archives of Physical Medicine and Rehabilitation 2013;94:2425-32 (C) 2013 by the American Congress of Rehabilitation Medicine

This study was aimed at identifying changes in equilibrium and movement control strategies in trans-tibial amputees (TTA) related to both the biomechanical changes and the loss of afferent inflow. The coordinations between equilibrium and movement were studied in traumatical TTA and in controls during transition from bipedal to monopodal stance. TTA failed to perform the task in a high percentage of trials both when the sound and the prosthetic limb were supporting. Significant differences were also found between TTA and controls in the duration of the weight transfer phase, in the length of the initial centre of pressure (CP) displacement and in the electromyographic (EMG) patterns. Despite adaptive posturomotor control strategies, transition from bipedal to monopodal stance remains a difficult task to perform for TTA, both when the supporting limb is the affected one and when the sound one is. The results of this study are discussed with respect to the rehabilitation programme and the prosthesis design for transtibial amputees.

During a lateral leg raising task, the position of the center of gravity (CG) in the horizontal plane shifts towards the supporting leg prior to the movement onset. The aim of this study was to explore whether the anticipatory postural adjustments were calibrated as a function of the initial horizontal location of the CG. Experiments were performed on 8 healthy subjects, with three initial positions of the CG (close to the supporting leg, between the two legs, close to the moving leg). Simultaneous kinematic, kinetic and electromyographic (EMG) data were recorded with the ELITE. system. The results show that the duration of the kinetic variables and EMG pattern are scaled as a function of the distance covered by the CG and constitute the means of modulating the CG shift. They suggest that the evaluation of the support conditions is necessary to calibrate the CG shift, this is done during the early phase of the postural adjustments.

Background Prior studies demonstrated that hesitation-prone persons with Parkinson’s disease (PDs) acutely improve step initiation using a novel self-triggered stimulus that enhances lateral weight shift prior to step onset. PDs showed reduced anticipatory postural adjustment (APA) durations, earlier step onsets, and faster 1st step speed immediately following stimulus exposure. Objective This study investigated the effects of long-term stimulus exposure. Methods Two groups of hesitation-prone subjects with Parkinson’s disease (PD) participated in a 6-week step-initiation training program involving one of two stimulus conditions: 1) Drop. The stance-side support surface was lowered quickly (1.5 cm); 2) Vibration. A short vibration (100 ms) was applied beneath the stance-side support surface. Stimuli were self-triggered by a 5% reduction in vertical force under the stance foot during the APA. Testing was at baseline, immediately post-training, and 6 weeks post-training. Measurements included timing and magnitude of ground reaction forces, and step speed and length. Results Both groups improved their APA force modulation after training. Contrary to previous results, neither group showed reduced APA durations or earlier step onset times. The vibration group showed 55% increase in step speed and a 39% increase in step length which were retained 6 weeks post-training. The drop group showed no stepping-performance improvements. Conclusions The acute sensitivity to the quickness-enhancing effects of stimulus exposure demonstrated in previous studies was supplanted by improved force modulation following prolonged stimulus exposure. The results suggest a potential approach to reduce the severity of start hesitation in PDs, but further study is needed to understand the relationship between short- and long-term effects of stimulus exposure.

Disorders of posture, balance, and gait are debilitating motor manifestations of advancing Parkinson's disease requiring rehabilitation intervention. These problems often reflect difficulties with coupling or sequencing posture and locomotion during complex whole body movements linked with falls. Considerable progress has been made with demonstrating the effectiveness of exercise interventions for individuals with Parkinson's disease. However, gaps remain in the evidence base for specific interventions and the optimal content of exercise interventions. Using a conceptual theoretical framework and experimental findings, this perspective and review advances the viewpoint that rehabilitation interventions focused on separate or isolated components of posture, balance, or gait may limit the effectiveness of current clinical practices. It is argued that treatment effectiveness may be improved by directly targeting posture and locomotion coupling problems as causal factors contributing to balance and gait dysfunction. This approach may help advance current clinical practice and improve outcomes in rehabilitation for persons with Parkinson's disease. “. . .postural activity should be regarded as a function in its own right and not merely as a component of movement. . .” James Purdon Martin