The broken escalator phenomenon. Aftereffect of walking onto a moving platform

Dissociated cerebellar contributions to feedforward gait adaptation

The cerebellum is important for motor adaptation. Lesions to the vestibulo-cerebellum selectively cause gait ataxia. Here we investigate how such damage affects locomotor adaptation when performing the 'broken escalator' paradigm. Following an auditory cue, participants were required to step from the fixed surface onto a moving platform (akin to an airport travellator). The experiment included three conditions: 10 stationary (BEFORE), 15 moving (MOVING) and 10 stationary (AFTER) trials. We assessed both behavioural (gait approach velocity and trunk sway after stepping onto the moving platform) and neuromuscular outcomes (lower leg muscle activity, EMG). Unlike controls, cerebellar patients showed reduced after-effects (AFTER trials) with respect to gait approach velocity and leg EMG activity. However, patients with cerebellar damage maintain the ability to learn the trunk movement required to maximise stability after stepping onto the moving platform (i.e., reactive postural behaviours). Importantly, our findings reveal that these patients could even initiate these behaviours in a feedforward manner, leading to an after-effect. These findings reveal that the cerebellum is crucial for feedforward locomotor control, but that adaptive locomotor behaviours learned via feedback (i.e., reactive) mechanisms may be preserved following cerebellum damage.

The influence of postural threat-induced anxiety on locomotor learning and updating

The ability to adapt our locomotion in a feedforward (i.e., "predictive") manner is crucial for safe and efficient walking behavior. Equally important is the ability to quickly deadapt and update behavior that is no longer appropriate for the given context. It has been suggested that anxiety induced via postural threat may play a fundamental role in disrupting such deadaptation. We tested this hypothesis, using the "broken escalator" phenomenon: Fifty-six healthy young adults walked onto a stationary walkway ("BEFORE" condition, 5 trials), then onto a moving walkway akin to an airport travelator ("MOVING" condition, 10 trials), and then again onto the stationary walkway ("AFTER" condition, 5 trials). Participants completed all trials while wearing a virtual reality headset, which was used to induce postural threat-related anxiety (raised clifflike drop at the end of the walkway) during different phases of the paradigm. We found that performing the locomotor adaptation phase in a state of increased threat disrupted subsequent deadaptation during AFTER trials: These participants displayed anticipatory muscular activity as if expecting the platform to move and exhibited inappropriate anticipatory forward trunk movement that persisted during multiple AFTER trials. In contrast, postural threat induced during AFTER trials did not affect behavioral or neurophysiological outcomes. These findings highlight that actions learned in the presence of postural threat-induced anxiety are strengthened, leading to difficulties in deadapting these behaviors when no longer appropriate. Given the associations between anxiety and persistent maladaptive gait behaviors (e.g., "overly cautious" gait, functional gait disorders), the findings have implications for the understanding of such conditions.NEW & NOTEWORTHY Safe and efficient locomotion frequently requires movements to be adapted in a feedforward (i.e., "predictive") manner. These adaptations are not always correct, and thus inappropriate behavior must be quickly updated. Here we showed that increased threat disrupts this process. We found that locomotor actions learned in the presence of postural threat-induced anxiety are strengthened, subsequently impairing one's ability to update (or "deadapt") these actions when they are no longer appropriate for the current context.

Clinical neurophysiology of functional motor disorders: IFCN Handbook Chapter

Functional Motor Disorders are common and disabling. Clinical diagnosis has moved from one of exclusion of other causes for symptoms to one where positive clinical features on history and examination are used to make a "rule in" diagnosis wherever possible. Clinical neurophysiological assessments have developed increasing importance in assisting with this positive diagnosis, not being used simply to demonstrate normal sensory-motor pathways, but instead to demonstrate specific abnormalities that help to positively diagnose these disorders. Here we provide a practical review of these techniques, their application, interpretation and pitfalls. We also highlight particular areas where such tests are currently lacking in sensitivity and specificity, for example in people with functional dystonia and functional tic-like movements.

Priming overconfidence in belief systems reveals negative return on postural control mechanisms

BACKGROUND

Modulation of postural control strategies and heightened perceptual ratings of instability when exposed to postural threats, illustrates the association between anxiety and postural control.

RESEARCH QUESTION

Here we test whether modulating prior expectations can engender postural-related anxiety which, in turn, may impair postural control and dissociate the well-established relationship between sway and subjective instability.

METHODS

We modulated expectations of the difficulty posed by an upcoming postural task via priming. In the visual priming condition, participants watched a video of an actor performing the task with either a stable or unstable performance, before themselves proceeding with the postural task. In the verbal priming paradigm, participants were given erroneous verbal information regarding the amplitude of the forthcoming platform movement, or no prior information.

RESULTS

Following the visual priming, the normal relationship between trunk sway and subjective instability was preserved only in those individuals that viewed the stable but not the unstable actor. In the verbal priming experiment we observed an increase in subjective instability and anxiety during task performance in individuals who were erroneously primed that sled amplitude would increase, when in fact it did not.

SIGNIFICANCE

Our findings show that people's subjective experiences of instability and anxiety during a balancing task are powerfully modulated by priming. The contextual provision of erroneous cognitive priors dissociates the normally 'hard wired' relationship between objective measures and subjective ratings of sway. Our findings have potential clinical significance for the development of enhanced cognitive retraining in patients with balance disorders, e.g. via modifying expectations.

Effects of Augmented Reality Object and Texture Presentation on Walking Behavior

Wearable devices that display visual augmented reality (AR) are now on the market, and we are becoming able to see AR displays on a daily basis. By being able to use AR displays in everyday environments, we can benefit from the ability to display AR objects in places where it has been difficult to place signs, to change the content of the display according to the user or time of day, and to display video. However, there has not been sufficient research on AR displays’ effect on users in everyday environments. In this study, we investigate how users are affected by AR displays. In this paper, we report our research results on the AR displays’ effect on the user’s walking behavior. We conducted two types of experiments—one on the effects of displaying AR objects on the user’s walking path, and the other on the effects of changing the floor texture by AR on the user’s walking behavior. As a result of the experiments, we found that the AR objects/textures affected the user’s walking behavior.

Dissociated motor learning and de-adaptation in patients with functional gait disorders

Walking onto a stationary platform that had been previously experienced as moving generates a locomotor after-effect—the so-called ‘broken escalator’ phenomenon. The motor responses that occur during locomotor after-effects have been mapped theoretically using a hierarchal Bayesian model of brain function that takes into account current sensory information that is weighted according to prior contextually-relevant experiences; these in turn inform automatic motor responses. Here, we use the broken escalator phenomenon to explore motor learning in patients with functional gait disorders and probe whether abnormal postural mechanisms override ascending sensory information and conscious intention, leading to maladaptive and disabling gait abnormalities. Fourteen patients with functional gait disorders and 17 healthy control subjects walked onto a stationary sled (‘Before’ condition, five trials), then onto a moving sled (‘Moving’ condition, 10 trials) and then again onto the stationary sled (‘After’ condition, five trials). Subjects were warned of the change in conditions. Kinematic gait measures (trunk displacement, step timing, gait velocity), EMG responses, and subjective measures of state anxiety/instability were recorded per trial. Patients had slower gait velocities in the Before trials (P < 0.05) but were able to increase this to accommodate the moving sled, with similar learning curves to control subjects (P = 0.87). Although trunk and gait velocity locomotor after-effects were present in both groups, there was a persistence of the locomotor after-effect only in patients (P < 0.05). We observed an increase in gait velocity during After trials towards normal values in the patient group. Instability and state anxiety were greater in patients than controls (P < 0.05) only during explicit phases (Before/After) of the task. Mean ‘final’ gait termination EMG activity (right gastrocnemius) was greater in the patient group than controls. Despite a dysfunctional locomotor system, patients show normal adaptive learning. The process of de-adaptation, however, is prolonged in patients indicating a tendency to perpetuate learned motor programmes. The trend to normalization of gait velocity following a period of implicit motor learning has implications for gait rehabilitation potential in patients with functional gait disorders and related disorders (e.g. fear of falling).

Which Are the Key Kinematic and Kinetic Components to Distinguish Recovery Strategies for Overground Slips Among Community-Dwelling Older Adults?

Slip outcomes are categorized as either a backward loss of balance (LOB) or a no loss of balance (no-LOB) in which an individual does not take a backward step to regain their stability. LOB includes falls and nonfalls, while no-LOB includes skate overs and walkovers. Researchers are uncertain about which factors determine slip outcomes and at which critical instants they do so. The purpose of the study was to investigate factors affecting slip outcomes in proactive and early reactive phases by analyzing 136 slip trials from 68 participants (age: 72.2 [5.3] y, female: 22). Segment angles and average joint moments in the sagittal plane of the slipping limb were compared for different slip outcomes. The results showed that knee flexor, hip extensor, and plantar flexor moments were significantly larger for no-LOB than for LOB in the midproactive phase, leading to smaller shank-ground and foot-ground angles at the slip onset, based on forward dynamics. In the early reactive phase, the hip extensor and plantar flexor moments were larger for no-LOB than for LOB, and all segment angles were smaller for no-LOB. Our findings indicate that the shank angle and knee moment were the major determinants of slip outcomes in both proactive and reactive phases.

The "broken escalator" phenomenon: Vestibular dizziness interferes with locomotor adaptation

BACKGROUND

Although vestibular lesions degrade postural control we do not know the relative contributions of the magnitude of the vestibular loss and subjective vestibular symptoms to locomotor adaptation.

OBJECTIVE

To study how dizzy symptoms interfere with adaptive locomotor learning.

METHODS

We examined patients with contrasting peripheral vestibular deficits, vestibular neuritis in the chronic stable phase (n = 20) and strongly symptomatic unilateral Meniere's disease (n = 15), compared to age-matched healthy controls (n = 15). We measured locomotor adaptive learning using the "broken escalator" aftereffect, simulated on a motorised moving sled.

RESULTS

Patients with Meniere's disease had an enhanced "broken escalator" postural aftereffect. More generally, the size of the locomotor aftereffect was related to how symptomatic patients were across both groups. Contrastingly, the degree of peripheral vestibular loss was not correlated with symptom load or locomotor aftereffect size. During the MOVING trials, both patient groups had larger levels of instability (trunk sway) and reduced adaptation than normal controls.

CONCLUSION

Dizziness symptoms influence locomotor adaptation and its subsequent expression through motor aftereffects. Given that the unsteadiness experienced during the "broken escalator" paradigm is internally driven, the enhanced aftereffect found represents a new type of self-generated postural challenge for vestibular/unsteady patients.

Motor Control: No Constant but Change

We rely on predictions to rapidly select our walking gaits. New research suggests that the formation of these predictions is driven by the difference between the walk we expect and the walk we get.

Theoretical framework for "unexplained" dizziness in the elderly: The role of small vessel disease

In this paper we postulate that disruption of connectivity in the human brain can lead to dizziness, a symptom normally associated with focal disease of the vestibular system. The specific case that we will examine is the development of "unexplained" dizziness in the elderly-an extremely common clinical problem. Magnetic resonance imaging of the brain in the elderly usually show variable degrees of multifocal micro-angiopathy (small vessel white matter disease, SVD); thus, we review the literature, present a conceptual model and report preliminary quantitative EEG data in support of the hypothesis that such hemispheric SVD leads to central nervous system disconnection that elderly patients report as dizziness. Loss of connectivity by age-related build-up of SVD could lead to dizzy feelings through one or more of the following mechanisms: disconnection of cortical vestibular centers, disconnection between frontal gait centers and the basal ganglia, and disconnection between intended motor action (efference copy) and sensory re-afference. Finally, we propose that SVD-mediated dysregulation of cerebral blood pressure is linked to dizziness during standing and walking in elderly patients with "unexplained" dizziness.

Subjective stability perception is related to postural anxiety in older subjects

BACKGROUND

Under static conditions, the objective and subjective measures of postural stability correlate well. However, age-related changes in postural control and task-related anxiety may modify the relationship between these subjective and objective measures. Ultimately, patients' symptoms represent subjective reports, thus understanding this relationship has clinical implications.

AIMS

This study investigates the relationship between subjective-objective measures of postural stability in dynamic conditions and whether this relationship is influenced by age or task-related anxiety.

METHODS

50 healthy participants (aged 18-83 years) stood on a platform oscillating at variable amplitudes, with-without a fall-preventing harness to modulate task-related anxiety. Trunk sway path, hip velocity and foot lifts (objective measures) and subjective scores of instability and task-related anxiety were recorded.

RESULTS

The subjective perception of stability accurately matched objective body sway, following a logarithmic function profile (r² = 0.72, p < 0.001). This function did not change significantly with age, harness or task presentation order. A strong relationship was observed between subjective measures of stability and task-related anxiety for all subjects (r = 0.81, p < 0.001). Task repetition reduced anxiety in the young, uncoupling anxiety changes from subjective instability, but not in the elderly who retained higher anxiety levels in line with subjective unsteadiness.

DISCUSSION

Subjects accurately rate their own instability during dynamic postural challenges, irrespective of age and actual fall risk. However, anxiety may selectively modulate the perception of instability in older subjects. The perception of stability relies upon the integration of sensory afferents but also recruits emotional-cognitive processes, particularly in older individuals. The use of a safety harness has no influence on subjective or objective postural stability.

Walking Speed Influences the Effects of Implicit Visual Feedback Distortion on Modulation of Gait Symmetry

The use of visual feedback in gait rehabilitation has been suggested to promote recovery of locomotor function by incorporating interactive visual components. Our prior work demonstrated that visual feedback distortion of changes in step length symmetry entails an implicit or unconscious adaptive process in the subjects’ spatial gait patterns. We investigated whether the effect of the implicit visual feedback distortion would persist at three different walking speeds (slow, self-preferred and fast speeds) and how different walking speeds would affect the amount of adaption. In the visual feedback distortion paradigm, visual vertical bars portraying subjects’ step lengths were distorted so that subjects perceived their step lengths to be asymmetric during testing. Measuring the adjustments in step length during the experiment showed that healthy subjects made spontaneous modulations away from actual symmetry in response to the implicit visual distortion, no matter the walking speed. In all walking scenarios, the effects of implicit distortion became more significant at higher distortion levels. In addition, the amount of adaptation induced by the visual distortion was significantly greater during walking at preferred or slow speed than at the fast speed. These findings indicate that although a link exists between supraspinal function through visual system and human locomotion, sensory feedback control for locomotion is speed-dependent. Ultimately, our results support the concept that implicit visual feedback can act as a dominant form of feedback in gait modulation, regardless of speed.

Adaptation and post-adaptation effects of haptic forces on locomotion in healthy young adults

BACKGROUND

Developing rehabilitation strategies to improve functional walking and postural control in patients is a priority for rehabilitation clinicians and researchers alike. One possible strategy is the use of sensory modalities to elicit adaptive locomotor gait patterns. This study aimed to explore to what extent haptic inputs, in the form of forward-leading tensile forces delivered to the hand, compared to no force, may lead to adaptation and post-adaptation effects on gait parameters, during and after the haptic exposure, respectively.

METHODS

Thirteen healthy young individuals were recruited for this study. We developed an innovative system combining virtual reality and haptic tensile forces in the direction of locomotion to simulate walking with a dog. A robotic arm generated forces via an adapted leash to the participant's hand while they walked on a self-paced treadmill immersed in a virtual environment with scene progression synchronized to the treadmill.

RESULTS

All participants showed significant increases in instantaneous gait velocity and stride length, with accompanying decreases in double-limb support time (p < 0.05) when walking with a haptic tensile force of either 10 or 20 N, relative to pre-force epoch levels, indicating an adaptation effect. When the 10 or 20 N force was removed, gait measures generally remained changed relative to baseline pre-force levels (p < 0.05), providing evidence of a post-adaptation effect.

CONCLUSIONS

Changes in spatiotemporal outcomes provide evidence that both adaptation and post-adaptation effects were present in response to the application and removal of a haptic force. Future studies will investigate whether similar changes in elderly and post-stroke populations can be actualized during steady-state walking.

Disruptive technology disorder

Based upon an analysis of 6 major historical technological advances over the last 150 years, a new syndrome, disruptive technology disorder (DTD), is introduced. DTD describes the human health ailments that accompany the implementation of disruptive technologies. Elevator sickness, railway spine, and bicycle face are representative examples. Though the underlying causative disruptive technologies may differ, many neurologic symptoms (headache, dizziness, weakness) are common to multiple DTDs. Born of technology-driven societal change, DTDs manifest as a complex interplay between biological and psychological symptoms.

Robot-induced perturbations of human walking reveal a selective generation of motor adaptation

The processes underlying the generation of motor adaptation in response to mechanical perturbations during human walking have been subject to debate. We used a robotic system to apply mechanical perturbations to step length and step height over consecutive gait cycles. Specifically, we studied perturbations affecting only step length, only step height, and step length and height in combination. Both step-length and step-height perturbations disrupt normal walking patterns, but step-length perturbations have a far greater impact on locomotor stability. We found a selective process of motor adaptation in that participants failed to adapt to step-height perturbations but strongly adapted to step-length perturbations, even when these adaptations increased metabolic cost. These results indicate that motor adaptation during human walking is primarily driven by locomotor stability, and only secondarily by energy expenditure and walking pattern preservation. These findings have substantial implications for the design of protocols for robot-assisted gait rehabilitation.

Sick of sitting

Sitting too much kills. Epidemiological, physiological and molecular data suggest that sedentary lifestyle can explain, in part, how modernity is associated with obesity, more than 30 chronic diseases and conditions and high healthcare costs. Excessive sitting--sitting disease--is not innate to the human condition. People were designed to be bipedal and, before the industrial revolution, people moved substantially more throughout the day than they do presently. It is encouraging that solutions exist to reverse sitting disease. Work environments, schools, communities and cities can be re-imagined and re-invented as walking spaces, and people thereby offered more active, happier, healthier and more productive lives.

Locomotor adaptation is modulated by observing the actions of others

Observing the motor actions of another person could facilitate compensatory motor behavior in the passive observer. Here we explored whether action observation alone can induce automatic locomotor adaptation in humans. To explore this possibility, we used the "broken escalator" paradigm. Conventionally this involves stepping upon a stationary sled after having previously experienced it actually moving (Moving trials). This history of motion produces a locomotor aftereffect when subsequently stepping onto a stationary sled. We found that viewing an actor perform the Moving trials was sufficient to generate a locomotor aftereffect in the observer, the size of which was significantly correlated with the size of the movement (postural sway) observed. Crucially, the effect is specific to watching the task being performed, as no motor adaptation occurs after simply viewing the sled move in isolation. These findings demonstrate that locomotor adaptation in humans can be driven purely by action observation, with the brain adapting motor plans in response to the size of the observed individual's motion. This mechanism may be mediated by a mirror neuron system that automatically adapts behavior to minimize movement errors and improve motor skills through social cues, although further neurophysiological studies are required to support this theory. These data suggest that merely observing the gait of another person in a challenging environment is sufficient to generate appropriate postural countermeasures, implying the existence of an automatic mechanism for adapting locomotor behavior.

Treatments for Neurological Gait and Balance Disturbance: The Use of Noninvasive Electrical Brain Stimulation

Neurological gait disorders are a common cause of falls, morbidity, and mortality, particularly amongst the elderly. Neurological gait and balance impairment has, however, proved notoriously difficult to treat. The following review discusses some of the first experiments to modulate gait and balance in healthy adults using anodal transcranial direct current stimulation (tDCS) by stimulating both cerebral hemispheres simultaneously. We review and discuss published data using this novel tDCS approach, in combination with physical therapy, to treat locomotor and balance disorders in patients with small vessel disease (leukoaraiosis) and Parkinson’s disease. Finally, we review the use of bihemispheric anodal tDCS to treat gait impairment in patients with stroke in the subacute phase. The findings of these studies suggest that noninvasive electrical stimulation techniques may be a useful adjunct to physical therapy in patients with neurological gait disorders, but further mutlicentre randomized sham-controlled studies are needed to evaluate whether experimental tDCS use can translate into mainstream clinical practice for the treatment of neurological gait disorders.

Anticipatory adjustments to abrupt changes of opposing forces

Anticipatory adjustments to abrupt load changes are based on task-specific predictive information. The authors asked whether anticipatory adjustments to abrupt offsets of horizontal forces are related to expectancy. In two experiments participants held a position against an opposing force or moved against it. At force offset they had to stop rapidly. Duration of the opposing force or distance moved against it varied between blocks of trials and was constant within each block, or it varied from trial to trial. These two variations resulted in opposite changes of the expectancy of force offset with the passage of time or distance. With constant force durations or distances in each block of trials, anticipatory adjustments tended to be poorest with the longest duration or distance, but with variable force durations or distances they tended to be best with the longest duration or distance. Thus anticipatory adjustments were related to expectancy rather than time or distance per se. Anticipatory adjustments resulted in shorter peak amplitudes of the involuntary movements, accompanied by longer movement times in Experiment 1 and faster movement times in Experiment 2. Thus, for different states of the limb at abrupt dynamic changes anticipatory adjustments involve different mechanisms that modulate different mechanical characteristics.

Fast and slow processes underlie the selection of both step frequency and walking speed

People prefer gaits that minimize their energetic cost. Research focused on step frequency selection suggests that a fast predictive process and a slower optimization process underlie this energy optimization. Our purpose in this study was to test whether the mechanisms controlling step frequency selection are used more generally to select one of the most relevant characteristics of walking - preferred speed. To accomplish this, we contrasted the dynamic adjustments in speed following perturbations to step frequency against the dynamic adjustments in step frequency following perturbations to speed. Despite the use of different perturbations and contexts, we found that the responses were very similar. In both experiments, subjects responded to perturbations by first rapidly changing their speed or step frequency towards their preferred pattern, and then slowly adjusting their gait to converge onto their preferred pattern. We measured similar response times for both the fast processes (1.4±0.3 versus 2.7±0.6 s) and the slow processes (74.2±25.4 versus 79.7±20.2 s). We also found that the fast process, although quite variable in amplitude, dominated the adjustments in both speed and step frequency. These distinct but complementary experiments demonstrate that people appear to rely heavily on prediction to rapidly select the most relevant aspects of their preferred gait and then gradually fine-tune that selection, perhaps using direct optimization of energetic cost.

Attention modulates adaptive motor learning in the 'broken escalator' paradigm

The physical stumble caused by stepping onto a stationary (broken) escalator represents a locomotor aftereffect (LAE) that attests to a process of adaptive motor learning. Whether such learning is primarily explicit (requiring attention resources) or implicit (independent of attention) is unknown. To address this question, we diverted attention in the adaptation (MOVING) and aftereffect (AFTER) phases of the LAE by loading these phases with a secondary cognitive task (sequential naming of a vegetable, fruit and a colour). Thirty-six healthy adults were randomly assigned to 3 equally sized groups. They performed 5 trials stepping onto a stationary sled (BEFORE), 5 with the sled moving (MOVING) and 5 with the sled stationary again (AFTER). A 'Dual-Task-MOVING (DTM)' group performed the dual-task in the MOVING phase and the 'Dual-Task-AFTEREFFECT (DTAE)' group in the AFTER phase. The 'control' group performed no dual task. We recorded trunk displacement, gait velocity and gastrocnemius muscle EMG of the left (leading) leg. The DTM, but not the DTAE group, had larger trunk displacement during the MOVING phase, and a smaller trunk displacement aftereffect compared with controls. Gait velocity was unaffected by the secondary cognitive task in either group. Thus, adaptive locomotor learning involves explicit learning, whereas the expression of the aftereffect is automatic (implicit). During rehabilitation, patients should be actively encouraged to maintain maximal attention when learning new or challenging locomotor tasks.

Lack of motor prediction, rather than perceptual conflict, evokes an odd sensation upon stepping onto a stopped escalator

When stepping onto a stopped escalator, we often perceive an "odd sensation" that is never felt when stepping onto stairs. The sight of an escalator provides a strong contextual cue that, in expectation of the backward acceleration when stepping on, triggers an anticipatory forward postural adjustment driven by a habitual and implicit motor process. Here we contrast two theories about why this postural change leads to an odd sensation. The first theory links the odd sensation to a lack of sensorimotor prediction from all low-level implicit motor processes. The second theory links the odd sensation to the high-level conflict between the conscious awareness that the escalator is stopped and the implicit perception that evokes an endogenous motor program specific to a moving escalator. We show very similar postural changes can also arise from reflexive responses to visual stimuli, such as contracting/expanding optic flow fields, and that these reflexive responses produce similar odd sensations to the stopped escalator. We conclude that the high-level conflict is not necessary for such sensations. In contrast, the implicitly driven behavioral change itself essentially leads to the odd sensation in motor perception since the unintentional change may be less attributable to self-generated action because of a lack of motor predictions.

A locomotor adaptation including explicit knowledge and removal of postadaptation errors induces complete 24-hour retention

Locomotor patterns are generally very consistent but also contain a high degree of adaptability. Motor adaptation is a short-term type of learning that utilizes this plasticity to alter locomotor behaviors quickly and transiently. In this study, we used a variation of an adaptation paradigm in order to test whether explicit information as well as the removal of the visual error signal after adaptation could improve retention of a newly learned walking pattern 24 h later. On two consecutive days of testing, participants walked on a treadmill while viewing a visual display that showed erroneous feedback of swing times for each leg. Participants were instructed to use this feedback to monitor and adjust swing times so they appeared symmetric within the display. This was achieved by producing a novel interlimb asymmetry between legs. For both legs, we measured adaptation magnitudes and rates and immediate and 24-h retention magnitudes. Participants showed similar adaptation on both days but a faster rate of readaptation on day 2. There was complete retention of adapted swing times on the increasing leg (i.e., no evidence of performance decay over 24 h). Overall, these findings suggest that the inclusion of explicit information and the removal of the visual error signal are effective in inducing full retention of adapted increases in swing time over a moderate (24 h) interval of time.

Effects of perturbations to balance on neuromechanics of fast changes in direction during locomotion

This study investigated whether the modular control of changes in direction while running is influenced by perturbations to balance. Twenty-two healthy men performed 90° side-step unperturbed cutting manoeuvres while running (UPT) as well as manoeuvres perturbed at initial contact (PTB, 10 cm translation of a moveable force platform). Surface EMG activity from 16 muscles of the supporting limb and trunk, kinematics, and ground reaction forces were recorded. Motor modules composed by muscle weightings and their respective activation signals were extracted from the EMG signals by non-negative matrix factorization. Knee joint moments, co-contraction ratios and co-contraction indexes (hamstrings/quadriceps) and motor modules were compared between UPT and PTB. Five motor modules were enough to reconstruct UPT and PTB EMG activity (variance accounted for UPT  = 92±5%, PTB = 90±6%). Moreover, higher similarities between muscle weightings from UPT and PTB (similarity = 0.83±0.08) were observed in comparison to the similarities between the activation signals that drive the temporal properties of the motor modules (similarity = 0.71±0.18). In addition, the reconstruction of PTB EMG from fixed muscle weightings from UPT resulted in higher reconstruction quality (82±6%) when compared to reconstruction of PTB EMG from fixed activation signals from UPT (59±11%). Perturbations at initial contact reduced knee abduction moments (7%), as well as co-contraction ratio (11%) and co-contraction index (12%) shortly after the perturbation onset. These changes in co-contraction ratio and co-contraction index were caused by a reduced activation of hamstrings that was also verified in the activation signals of the specific motor module related to initial contact. Our results suggested that perturbations to balance influence modular control of cutting manoeuvres, especially the temporal properties of muscle recruitment, due to altered afferent inputs to the motor patterns. Furthermore, reduced knee stability during perturbed events may be related to overall control of lower limb muscles.

Balance

This chapter addresses the important and undertreated problem of balance disorders. The chapter has a simplified summary of the physiology of balance problems in order to set the scene. The issue of assessment is next addressed with discussion of important tests including the Berg Balance Scale and the Get Up and Go Test, and others. Posturography is discussed as well as assessment of the gravitional vertical. The assessment of vestibular function is of key importance and discussed in some detail. The focus of the chapter is on balance rehabilitation. Re-training of postural alignment and of sensory strategies are key but adaptation of the environment and re-training of cognitive strategies are also helpful in individual cases. Vestibular exercises can also be used. The chapter then critically analyses the efficacy of these treatments in specific balance disorders such as in stroke, Parkinson disease, polyneuropathies, multiple sclerosis, and vestibular disorders. Overall, there is a growing body of evidence that balance rehabilitation improves symptoms, function, and quality of life for those troubled by these disabling problems.

Motion controlled gait enhancing mobile shoe for rehabilitation

Walking on a split-belt treadmill, which has two belts that can be run at different speeds, has been shown to improve walking patterns post-stroke. However, these improvements are only temporarily retained once individuals transition to walking over ground. We hypothesize that longer-lasting effects would be observed if the training occurred during natural walking over ground, as opposed to on a treadmill. In order to study such long-term effects, we have developed a mobile and portable device which can simulate the same gait altering movements experienced on a split-belt treadmill. The new motion controlled gait enhancing mobile shoe improves upon the previous version's drawbacks. This version of the GEMS has motion that is continuous, smooth, and regulated with on-board electronics. A vital component of this new design is the Archimedean spiral wheel shape that redirects the wearer's downward force into a horizontal backward motion. The design is passive and does not utilize any motors. Its motion is regulated only by a small magnetic particle brake. Further experimentation is needed to evaluate the long-term after-effects.

Biomechanical strategies to accommodate expected slips in different directions during walking

The aim of the study was to verify whether heel kinematics, ground reaction forces and electromyography (EMG) during walking are affected when anticipating slips in anterior-posterior (AP) and medial-lateral directions (ML). Eight healthy men walked through a 7-m walkway, stepping on a robotic force platform. Initially, baseline (BASE) gait mechanics were assessed with the platform at rest. Subsequently, two sets of randomized perturbations (10-cm translations with at different platform movement velocities) in the AP and ML direction were applied. Perturbations were interspersed with unperturbed walking (i.e., catch-trials C-AP and C-ML). Heel accelerations, ground reaction forces and activities from the perturbed leg and trunk muscles were analyzed. EMG was analysed in four epochs: PRE (-100 ms to heel strike [HS]), EARLY (HS to 150 ms after HS), MID (150-300 ms after HS) and LATE (300 ms to toe-off). Comparisons were made between BASE, C-AP and C-ML. The first peak of the vertical force component (Fz) was decreased for C-AP and C-ML (p<0.05) but no changes were found for braking and propulsion impulses. EMG showed effects of expected slips on tibialis anterior, gastrocnemius lateralis, soleus and peroneus longus, especially for EARLY and MID epochs, with direction-specific increases in activity. In conclusion, expected slips in different directions determine only marginal changes in terms of kinetics and heel kinematics, but selective activation after HS indicates that direction-dependent strategies are adopted when anticipating perturbations.

Developing a Gait Enhancing Mobile Shoe to Alter Over-Ground Walking Coordination

This paper presents a Gait Enhancing Mobile Shoe (GEMS) that mimics the desirable kinematics of a split-belt treadmill except that it does so over ground. Split-belt treadmills, with two separate treads running at different speeds, have been found useful in the rehabilitation of persons with asymmetric walking patterns. Although in preliminary testing, beneficial after-effects have been recorded, various drawbacks include the stationary nature of the split-belt treadmill and the inability to keep a person on the split-belt treadmill for an extended period of time. For this reason, the after-effects for long-term gait training are still unknown. The mobile ability of the GEMS outlined in this paper enables it to be worn in different environments such as in one's own house and also enables it to be worn for a longer period of time since the GEMS is completely passive. Healthy subject testing has demonstrated that wearing this shoe for twenty minutes can alter the wearer's gait and will generate after-effects in a similar manner as a split-belt treadmill does.

Enhanced locomotor adaptation aftereffect in the "broken escalator" phenomenon using anodal tDCS

The everyday experience of stepping onto a stationary escalator causes a stumble, despite our full awareness that the escalator is broken. In the laboratory, this "broken escalator" phenomenon is reproduced when subjects step onto an obviously stationary platform (AFTER trials) that was previously experienced as moving (MOVING trials) and attests to a process of motor adaptation. Given the critical role of M1 in upper limb motor adaptation and the potential for transcranial direct current stimulation (tDCS) to increase cortical excitability, we hypothesized that anodal tDCS over leg M1 and premotor cortices would increase the size and duration of the locomotor aftereffect. Thirty healthy volunteers received either sham or real tDCS (anodal bihemispheric tDCS; 2 mA for 15 min at rest) to induce excitatory effects over the primary motor and premotor cortex before walking onto the moving platform. The real tDCS group, compared with sham, displayed larger trunk sway and increased gait velocity in the first AFTER trial and a persistence of the trunk sway aftereffect into the second AFTER trial. We also used transcranial magnetic stimulation to probe changes in cortical leg excitability using different electrode montages and eyeblink conditioning, before and after tDCS, as well as simulating the current flow of tDCS on the human brain using a computational model of these different tDCS montages. Our data show that anodal tDCS induces excitability changes in lower limb motor cortex with resultant enhancement of locomotor adaptation aftereffects. These findings might encourage the use of tDCS over leg motor and premotor regions to improve locomotor control in patients with neurological gait disorders.

Design and Pilot Study of a Gait Enhancing Mobile Shoe

Hemiparesis is a frequent and disabling consequence of stroke and can lead to asymmetric and inefficient walking patterns. Training on a split-belt treadmill, which has two separate treads driving each leg at a different speed, can correct such asymmetries post-stroke. However, the effects of split-belt treadmill training only partially transfer to everyday walking over ground and extended training sessions are required to achieve long-lasting effects. Our aim is to develop an alternative device, the Gait Enhancing Mobile Shoe (GEMS), that mimics the actions of the split-belt treadmill, but can be used during overground walking and in one's own home, thus enabling long-term training. The GEMS does not require any external power and is completely passive; all necessary forces are redirected from the natural forces present during walking. Three healthy subjects walked on the shoes for twenty minutes during which one GEMS generated a backward motion and the other GEMS generated a forward motion. Our preliminary experiments suggest that wearing the GEMS did cause subjects to modify coordination between the legs and these changes persisted when subjects returned to normal over-ground walking. The largest effects were observed in measures of temporal coordination (e.g., duration of double-support). These results suggest that the GEMS is capable of altering overground walking coordination in healthy controls and could potentially be used to correct gait asymmetries post-stroke.

Context-specific saccadic adaptation in monkeys

When environmental or sensory conditions change suddenly, the brain must be capable of learning different behavioral modes to produce accurate movements under multiple circumstances. A form of this dual-state adaptation known as "context-specific adaptation" has been widely investigated using the saccade gain adaptation paradigm in humans. In this study, we asked whether or not context-specific adaptation of saccade gain exists in monkeys and if so to explore its properties. Here, vertical eye position was used as a context cue for adaptation of horizontal saccade gain. We asked for a gain-increase in one context and gain-decrease in another context, and then determined if a change in the context would invoke switching between the adapted states. After training, our monkeys developed context-specific adaptation: in most cases gain-decrease adaptation could be induced, but there was little or no gain-increase adaptation. This context-specific adaptation developed gradually and switching of gains was evident on the first saccades with each change in context. Along with these results, the retention of an adaptation aftereffect overnight indicates that contextual-specific adaptation in monkeys is not a strategy, but involves a true adaptive process of reorganization in the brain. We suggest that context-specific adaptation in monkeys could be an important tool to provide insights into the mechanisms of saccade adaptation that occurs during the more natural circumstances of daily life.

What does autonomic arousal tell us about locomotor learning?

Walking onto a stationary sled previously experienced as moving induces locomotor aftereffects (LAE, or "broken escalator phenomenon"). This particular form of aftereffect can develop after a single adaptation trial and occurs despite subjects being fully aware that the sled will not move. Here, we investigate whether such strong LAE expression may relate to arousal or fear related to instability during the gait adaptation process. Forty healthy subjects were allocated to three sled velocity groups; SLOW (0.6 m/s), MEDIUM (1.3 m/s), or FAST (2.0 m/s). Subjects walked onto the stationary sled for five trials (BEFORE), then onto the moving sled for 15 trials (adaptation or MOVING trials) and, finally, again onto the stationary sled for five trials (AFTER). Explicit warning regarding sled status was given. Trunk position, foot-sled contact timing, autonomic markers (electrodermal activity [EDA], ECG, respiratory movements) in addition to self-reported task-related confidence and state/trait anxiety were recorded. Trunk sway, EDA, and R-R interval shortening were greatest during the first MOVING trial (MOVING_1), progressively attenuating during subsequent MOVING trials. A LAE, recorded as increased gait velocity and trunk sway during AFTER_1, occurred in both MEDIUM and FAST sled velocity groups. The amplitude of forward trunk sway in AFTER_1 (an indicator of aftereffect magnitude) was related to EDA during the final adaptation trial (MOVING_15). AFTER_1 gait velocity (also an indicator of aftereffect magnitude) was related to MOVING_1 trunk sway. Hence, gait velocity and trunk sway components of the LAE are differentially related to kinematic and autonomic parameters during the early and late adaptation phase. The finding that EDA is a predictor of LAE expression indicates that autonomic arousal or fear-based mechanisms can promote locomotor learning. This could in turn explain some unusual characteristics of this LAE, namely its resistance to explicit knowledge and its generation with just a single adaptation trial.

The trampoline aftereffect: the motor and sensory modulations associated with jumping on an elastic surface

After repeated jumps over an elastic surface (e.g. a trampoline), subjects usually report a strange sensation when they jump again overground (e.g. they feel unable to jump because their body feels heavy). However, the motor and sensory effects of exposure to an elastic surface are unknown. In the present study, we examined the motor and perceptual effects of repeated jumps over two different surfaces (stiff and elastic), measuring how this affected maximal countermovement vertical jump (CMJ). Fourteen subjects participated in two counterbalanced sessions, 1 week apart. Each experimental session consisted of a series of maximal CMJs over a force plate before and after 1 min of light jumping on an elastic or stiff surface. We measured actual motor performance (height jump and leg stiffness during CMJ) and how that related to perceptual experience (jump height estimation and subjective sensation). After repeated jumps on an elastic surface, the first CMJ showed a significant increase in leg stiffness (P < or = 0.01), decrease in jump height (P < or = 0.01) increase in perceptual misestimation (P < or = 0.05) and abnormal subjective sensation (P < or = 0.001). These changes were not observed after repeated jumps on a rigid surface. In a complementary experiment, continuous surface transitions show that the effects persist across cycles, and the effects over the leg stiffness and subjective experience are minimized (P < or = 0.05). We propose that these aftereffects could be the consequence of an erroneous internal model resulting from the high vertical forces produced by the elastic surface.

Thinking about walking: effects of conscious correction versus distraction on locomotor adaptation

Control of the human walking pattern normally requires little thought, with conscious control used only in the face of a challenging environment or a perturbation. We have previously shown that people can adapt spatial and temporal aspects of walking to a sustained perturbation generated by a split-belt treadmill. Here we tested whether conscious correction of walking, versus distraction from it, modifies adaptation. Conscious correction of stepping may expedite the adaptive process and help to form a new walking pattern. However, because walking is normally an automatic process, it is possible that conscious effort could interfere with adaptation, whereas distraction might improve it by removing competing voluntary control. Three groups of subjects were studied: a control group was given no specific instructions, a conscious correction group was instructed how to step and given intermittent visual feedback of stepping during adaptation, and a distraction group performed a dual-task during adaptation. After adaptation, retention of aftereffects was assessed in all groups during normal treadmill walking without conscious effort, feedback, or distraction. We found that conscious correction speeds adaptation, whereas distraction slows it. Subjects trained with distraction retained aftereffects longest, suggesting that the training used during adaptation predicts the time course of deadaptation. An unexpected finding was that these manipulations affected the adaptation rate of spatial but not temporal elements of walking. Thus conscious processes can preferentially access the spatial walking pattern. It may be that spatial and temporal controls of locomotion are accessible through distinct neural circuits, with the former being most sensitive to conscious effort or distraction.

Locomotor adaptation and aftereffects in patients with reduced somatosensory input due to peripheral neuropathy

We studied 12 peripheral neuropathy patients (PNP) and 13 age-matched controls with the "broken escalator" paradigm to see how somatosensory loss affects gait adaptation and the release and recovery ("braking") of the forward trunk overshoot observed during this locomotor aftereffect. Trunk displacement, foot contact signals, and leg electromyograms (EMGs) were recorded while subjects walked onto a stationary sled (BEFORE trials), onto the moving sled (MOVING or adaptation trials), and again onto the stationary sled (AFTER trials). PNP were unsteady during the MOVING trials, but this progressively improved, indicating some adaptation. During the after trials, 77% of control subjects displayed a trunk overshoot aftereffect but over half of the PNP (58%) did not. The PNP without a trunk aftereffect adapted to the MOVING trials by increasing distance traveled; subsequently this was expressed as increased distance traveled during the aftereffect rather than as a trunk overshoot. This clear separation in consequent aftereffects was not seen in the normal controls suggesting that, as a result of somatosensory loss, some PNP use distinctive strategies to negotiate the moving sled, in turn resulting in a distinct aftereffects. In addition, PNP displayed earlier than normal anticipatory leg EMG activity during the first after trial. Although proprioceptive inputs are not critical for the emergence or termination of the aftereffect, somatosensory loss induces profound changes in motor adaptation and anticipation. Our study has found individual differences in adaptive motor performance, indicative that PNP adopt different feed-forward gait compensatory strategies in response to peripheral sensory loss.

What the "broken escalator" phenomenon teaches us about balance

Gait adaptation is crucial for coping with varying terrain and biological needs. It is also important that any acquired adaptation is expressed only in the appropriate context. Here we review a recent series of experiments that demonstrate inappropriate expression of gait adaptation. We show that a brief period of walking onto a platform previously experienced as moving results in a large forward sway aftereffect, despite full awareness of the changing context. The adaptation mechanisms involved in this paradigm are extremely fast, just 1-2 discrete exposures to the moving platform result in the motor aftereffect. This aftereffect occurs even if subjects deliberately attempt to suppress it. However, it disappears when the location or method of gait is altered, indicating that aftereffect expression is context dependent. Conversely, making gait self-initiated increases sway during the aftereffect. This aftereffect demonstrates a profound dissociation between knowledge and action. The absence of generalization suggests a relatively simple form of motor learning, albeit involving high-level processing by cortical and cerebellar structures.

Odd sensation induced by moving-phantom which triggers subconscious motor program

Our motor actions are sometimes not properly performed despite our having complete understanding of the environmental situation with a suitable action intention. In most cases, insufficient skill for motor control can explain the improper performance. A notable exception is the action of stepping onto a stopped escalator, which causes clumsy movements accompanied by an odd sensation. Previous studies have examined short-term sensorimotor adaptations to treadmills and moving sleds, but the relationship between the odd sensation and behavioral properties in a real stopped-escalator situation has never been examined. Understanding this unique action-perception linkage would help us to assess the brain function connecting automatic motor controls and the conscious awareness of action.

Here we directly pose a question: Does the odd sensation emerge because of the unfamiliar motor behavior itself toward the irregular step-height of a stopped escalator or as a consequence of an automatic habitual motor program cued by the escalator itself. We compared the properties of motor behavior toward a stopped escalator (SE) with those toward moving escalator and toward a wooden stairs (WS) that mimicked the stopped escalator, and analyzed the subjective feeling of the odd sensation in the SE and WS conditions. The results show that moving escalator-specific motor actions emerged after participants had stepped onto the stopped escalator despite their full awareness that it was stopped, as if the motor behavior was guided by a “phantom” of a moving escalator. Additionally, statistical analysis reveals that postural forward sway that occurred after the stepping action is directly linked with the odd sensation.

The results suggest a dissociation between conscious awareness and subconscious motor control: the former makes us perfectly aware of the current environmental situation, but the latter automatically emerges as a result of highly habituated visual input no matter how unsuitable the motor control is. This dissociation appears to yield an attribution conflict, resulting in the odd sensation.

Generalization of gait adaptation for fall prevention: from moveable platform to slippery floor

A person's ability to transfer the acquired improvements in the control of center of mass (COM) state stability to slips induced in everyday conditions can have profound theoretical and practical implications for fall prevention. This study investigated the extent to which such generalization could take place. A training group (n=8) initially experienced 24 right-side slips in blocked-and-random order (from the 1st unannounced, novel slip, S-1 to the last, S-24) resulting from release of a low-friction moveable platform in walking. They then experienced a single unannounced slip while walking on an oil-lubricated vinyl floor surface (V-T). A control group (n=8) received only one unannounced slip on the same slippery floor (V-C). Results demonstrated that the incidence of balance loss and fall on V-T was comparable to that on S-24. In both trials, fall and balance-loss incidence was significantly reduced in comparison with that on S-1 or on V-C, resulting from significant improvements in the COM state stability. The observed generalization indicates that the control of COM stability can be optimally acquired to accommodate alterations in environmental constraints, and it may be broadly coded and easily modifiable within the CNS. Because of such mechanisms, it is possible that the locomotor-balance skills acquired with the aid of low-friction moveable platforms can translate into resisting falls encountered in daily living.

Visuo-vestibular Influences on the Moving Platform Locomotor Aftereffect

After walking onto a moving platform subjects experience a locomotor aftereffect (LAE), including a self-generated stumble, when walking again onto a stationary platform. Thus this LAE affords examination of the role of vestibular input during an internally generated postural challenge. The experiments involved walking onto the stationary sled (BEFORE trials), walking onto the moving sled (MOVING), and a second set of stationary trials (AFTER). We investigated 9 bilateral labyrinthine defective subjects (LDS) and 13 age-matched normal controls (NC) with eyes open. We repeated the experiment in 5 NC and 5 LDS but this time the AFTER trials were performed twice, first eyes closed and then on eye reopening. During MOVING trials, LDS were considerably unstable, thus confirming the established role of the vestibular system during externally imposed postural perturbations. During AFTER trials, both groups experienced an aftereffect with eyes open and closed, shown as higher approach gait velocity, a forward trunk overshoot, and increased leg EMG. However, there were no significant group differences due to the fact that stopping the forward trunk overshoot was accomplished by anticipatory EMG bursts. On eye reopening the aftereffect reemerged, significantly larger in LDS than that in NC. The lack of group differences in AFTER trials suggests that when facing internally generated postural perturbations, as in this adaptation process, the CNS relies less on vestibular feedback and more on anticipatory mechanisms. Reemergence of the aftereffect on eye reopening indicates the existence of a feedforward visuo-contextual mechanism for locomotor learning, which is adaptively enhanced in the absence of vestibular function.

Repeated-slip training: an emerging paradigm for prevention of slip-related falls among older adults

Falls frequently cause injury-related hospitalization or death among older adults. This article reviews a new conceptual framework on dynamic stability and weight support in reducing the risk for falls resulting from a forward slip, based on the principles of motor control and learning, in the context of adaptation and longer-term retention induced by repeated-slip training. Although an unexpected slip is severely destabilizing, a recovery step often is adequate for regaining stability, regardless of age. Consequently, poor weight support (quantified by reduction in hip height), rather than instability, is the major determinant of slip-related fall risk. Promisingly, a single session of repeated-slip training can enhance neuromechanical control of dynamic stability and weight support to prevent falls, which can be retained for several months or longer. These principles provide the theoretical basis for establishing task-specific adaptive training that facilitates the development of protective strategies to reduce falls among older adults.

The moving platform after-effect reveals dissociation between what we know and how we walk

Gait adaptation is crucial for coping with varying terrain and biological needs. It is also important that any acquired adaptation is expressed only in the appropriate context. Here we review a recent series of experiments which demonstrate inappropriate expression of gait adaptation. We showed that a brief period of walking onto a platform previously experienced as moving results in a large forward sway despite full awareness of the changing context. The adaptation mechanisms involved in this paradigm are extremely fast, just 1-2 discrete exposures to the moving platform results in a motor after-effect. This after-effect still occurs even if subjects deliberately attempt to suppress it. However it disappears when the location or method of gait is altered, indicating that after-effect expression is context dependent. Conversely, making gait self-initiated increased sway during the after-effect. This after-effect demonstrates a profound dissociation between knowledge and action. The absence of generalisation suggests a simple form of motor learning. However, persistent expression of gait after-effects may be dependent on an intact cerebral cortex. The fact that the after-effect is greater during self-initiated gait, and is context dependent, would be consistent with the involvement of supraspinal areas.

Self-initiated gait increases susceptibility to the moving platform after-effect

Walking onto a stationary platform previously experienced as moving results in a large forward sway, despite awareness of the changing context. This after-effect demonstrates aberrant motor prediction. Here, we compared after-effect magnitude when gait initiation was either externally triggered or self-initiated. Both participant groups adapted their walking speed similarly when walking onto the moving platform. When it was kept stationary however, after-effect magnitude was more than doubled in the self-initiated group. We attribute this to a stronger association made between the action (gait initiation) and its consequence (platform movement) when the action is initiated by oneself. This increased sense of agency reduces the ability to switch off the acquired motor response when it is no longer appropriate.