Visual-vestibular influences on locomotor adjustments for stepping over an obstacle

Combined visual and vestibular influences on locomotor control, particularly in changing environments, are little understood. We studied such influences on body orientation and foot trajectory control during level walking and obstacle avoidance. Six young adults walked on the level and over an obstacle while vision was present or occluded as well as while vestibular information was intact or perturbed using galvanic vestibular stimulation (GVS). The occlusion of vision caused a slowing of gait during obstacle avoidance as well as increased clearance of the leading limb over the obstruction. GVS caused lateral deviations in head and trunk roll angles as well as in foot and trunk displacements, but these lateral deviations were the same during both level walking and obstacle avoidance. In addition, GVS had no effect at all on sagittal plane factors such as speed, foot proximity to the obstacle and vertical clearance over the obstacle. Overall, there is a complex visual control of bilateral obstacle avoidance, but the lack of differences in GVS effects between level and obstructed walking shows that vestibular information is not upregulated for obstacle avoidance. In addition, the robust indifference of anterior foot placement and body displacement to significant lateral deviations from GVS suggests an orthogonally based sensori-locomotor control.     

Evidence for interactive locomotor and oculomotor deficits in cerebellar patients during visually guided stepping

Eight patients suffering from primary cerebellar degenerative diseases undertook a walkway task, demanding precise foot placement at each step, and a visual fixation task, requiring only eye movements. Step cycle and horizontal eye movements were recorded throughout the tasks and compared to those of healthy adults (including age- and sex-matched controls). Cerebellar patients displayed both locomotor and oculomotor deficits. Increases in duration of the stance, swing and double support phases of the step cycle were all shown to contribute to ataxic gait. Dysmetric saccades to fixate the footfall targets were seen more frequently in patients than in controls. These hypometric saccades were followed by one or more corrective saccades (patients: >45% accompanied by one or more corrective saccades; controls: <10% accompanied by a single corrective saccade). Similarities between the oculomotor deficits displayed by patients during the visual fixation task and when walking indicate that the latter are not merely a consequence of ataxic gait. The existence of several links between these locomotor and oculomotor deficits provides evidence for considerable interaction between the two control systems in the production of patterned eye and stepping movements. These results also suggest that the cerebellum plays an active role in the co-ordination of visually guided eye and limb movements during visually guided stepping.     

Lesions of area 5 of the posterior parietal cortex in the cat produce errors in the accuracy of paw placement during visually guided locomotion

We developed a novel locomotor task in which cats step over obstacles that move at a different speed from that of the treadmill on which the cat is walking: we refer to this as a visual dissociation locomotion task. Slowing the speed of the obstacle with respect to that of the treadmill sometimes led to a major change in strategy so that cats made two steps with the hindlimbs before stepping over the obstacle (double step strategy) instead of the single step (standard strategy) observed when the obstacle was at the same speed as the treadmill. In addition, in the step preceding the step over the obstacle, the paws were placed significantly closer to the obstacle in the visual dissociation task than when the treadmill and the obstacle were at the same speed. After unilateral lesion of area 5 of the posterior parietal cortex (PPC), the cats frequently hit the obstacle as they stepped over it, especially in the visual dissociation task. This locomotor deficit was linked to significant differences in the location in which the forelimbs were placed in the step preceding that over the obstacle compared with the prelesion control. Cats also frequently hit the obstacle with their hindlimbs even when the forelimbs negotiated the obstacle successfully; this suggests an important role for the posterior parietal cortex in the coordination of the forelimbs and hindlimbs. Together, these results suggest an important contribution of the PPC to the planning of visually guided gait modifications.     

What guides the selection of alternate foot placement during locomotion in humans

Our goal was to understand the bases for selection of alternate foot placement during locomotion when the normal landing area is undesirable. In this study, a light spot of different shapes and sizes simulated an undesirable landing area. Participants were required to avoid stepping on this spot under different time constraints. Alternate chosen foot placements were categorised into one of eight choices. Results showed that selection of alternate foot placement is systematic. There is a single dominant choice for each combination of light spot and normal landing spot. The dominant choice minimises the displacement of the foot from its normal landing spot (less than half a foot length). If several response choices satisfy this criterion, three selection strategies are used to guide foot placement: placing the foot in the plane of progression, choosing to take a longer step over a shorter step and selecting a medial rather than lateral foot placement. All these alternate foot-placement choices require minimal changes to the ongoing locomotor muscle activity, pose minimal threat to dynamic stability, allow for quick initiation of change in ongoing movement and ensure that the locomotor task runs without interruption. Thus, alternate foot-placement choices are dependent not only on visual input about the location, size and shape of the undesirable surface, but also on the relationship between the characteristics of the undesirable surface and the normal landing area. Received: 27 October 1998 / Accepted: 3 May 1999     

The effects of distant and on-line visual information on the control of approach phase and step over an obstacle during locomotion

One of the goals of this study was to examine the nature and role of distant visual information sampled during locomotion in the feedforward control of leading and trailing limb while an individual is required to step over an obstacle in the travel path. In addition we were interested in whether or not on-line visual information available while the limb (lead or trail) is stepping over the obstacle influences limb trajectory control and whether the information provided during lead limb cross would be used to calibrate movement of the trail limb. Towards this end, we manipulated availability of vision following an initial dynamic sampling period during the approach phase in proximity to the obstacle and during the lead and trail limb stepping over the obstacle. Ten participants completed 40 trials of obstacle crossing in 8 testing conditions. Initial dynamic visual sampling was sufficient to ensure successful task performance in the absence of vision in the approach phase and during both lead and trail limb stepping over the obstacle. Despite successful task performance, foot placement of the lead and trail limb before obstacle crossing and limb elevation over the obstacle were increased after withdrawal of vision in the approach area. Furthermore, the correlation between toe clearance and foot placement was diminished. While both limbs require feedforward visual information to control the step over the obstacle, only lead limb elevation was influenced by availability of on-line visual information during obstacle crossing. Results were in agreement with the notion of primacy of information inherent in the optic array over those from static samples of the environment in guiding locomotion. It is suggested that the expected proprioceptive feedback information associated with the limb posture before the obstacle, reconstructed using visual memory from dynamic sampling of the environment, mismatched with those from the actual limb position. Accordingly, participants adopted a different strategy that enabled them to clear the obstacle with a higher safety margin.Financial assistance was provided by a grant from the Office of Naval Research, USA, NSERC/Canada, and CAPES/Brazil. We would like to thank Milad G. Ishac, Mike Greig, Zinat Shafaei-Shirazi, and Candida T. Goncalves for their assistance     

Stepping over an obstacle on a compliant travel surface reveals adaptive and maladaptive changes in locomotion patterns

Adaptive human locomotion is dependent on safe clearance of obstacles encountered in the path of locomotion. When the terrain is uneven or compliant, stability along with safe obstacle clearance are competing demands presented to the central nervous system (CNS). To examine how the CNS deals with the two competing demands, six participants walked under four conditions: normal ground walking, normal ground walking with an obstacle in the travel path, compliant surface walking, and compliant surface walking with an obstacle in the travel path. Full body kinematics were measured and swing limb kinetics were derived from these measurements. Results showed that on a compliant surface, the CNS was able to decrease foot placement variability at foot contact when approaching an obstacle, similar to the normal ground terrain. Limb trajectory over the obstacle showed that toe elevation was maintained while clearance over the obstacle was lower in the compliant surface condition due to depression of the surface during push off. This illustrates that the CNS controls toe elevation, not toe clearance when stepping over an obstacle. Work done in the knee during elevation and hip during lowering was similar in the compliant and ground conditions even though a lower clearance over the obstacle was achieved in the complaint condition. This shows the inability of the CNS to account for compression of the surface prior to obstacle clearance and provides further evidence the CNS controls toe elevation, not clearance when stepping over an obstacle.An erratum to this article can be found at     

Determinants guiding alternate foot placement selection and the behavioral responses are similar when avoiding a real or a virtual obstacle

In this study we validate the use of a virtual planar obstacle paradigm to study the avoidance of a real obstacle, such as a hole, during locomotion. Also we further validate the economy determinant implicated with the minimization of foot displacement from its normal landing position during alternate foot placement. Participants were asked to perform two blocks of trials: real (a real hole was embedded in the pathway and participants were requested to avoid stepping into it) and virtual (a virtual planar obstacle was displayed on the screen of a liquid crystal display monitor). Trunk and feet kinematics were monitored, as well as electromyography (EMG) activity of 14 muscles of both lower limbs and trunk. The results of this study showed that the dominant choice for each obstacle investigated was not different between the real and virtual conditions. In addition, dynamic stability, economy and forward progression determinants guiding alternate foot placement were similarly satisfied. Thus the use of virtual planar obstacle in adaptive locomotion study is appropriate. EMG data were used to compute an index relating the changes in muscle activity relative to the normal walking profile. This EMG index was significantly and positively correlated with the amount of foot displacement for the adaptive step. The fact that the dominant choice resulted in minimum foot displacement from its normal landing spot combined with minimal changes in muscle activity validates conclusively the economy determinant.     

Strategies and determinants for selection of alternate foot placement during human locomotion: influence of spatial and temporal constraints

During locomotion in a cluttered terrain, certain terrain surfaces such as an icy one are not appropriate for foot placement; an alternate choice is required. In a previous study we showed that the selection of foot placement is not random but systematic; the dominant choices made are not uniquely defined by the available or predicted sensory inputs. We argued that selection is guided by specific rules and involves minimal displacement of the foot from its normal landing spot. The experimental protocol involved implicit spatial constraint by requiring individuals to step on the force plate that could trigger a lighted area to be avoided, thereby requiring individuals to respond within one step-cycle. Alternate foot placement was visually identified, but not measured. The purpose of this study was to directly measure foot placement, validate and/or refine the rules used to guide selection, and identify whether the alternate foot placement choices are influenced by spatial and temporal constraints on response selection. The area to be avoided was visible from the start and therefore individuals could plan and implement appropriate avoidance strategies without any temporal constraint. Spatial constraint introduced in this experiment included requirement both to step on a specific location and to avoid stepping on a specific location on the next step. The results provide support for the rules previously identified in guiding foot placement to an alternate location. Minimal displacement of the foot from its normal landing spot was validated as an important factor for selecting alternate foot placement. When several choices satisfied this factor, additional factors guide alternate foot placement. Modifications in the plane of progression are preferred while stepping wide is avoided. When no temporal constraints are imposed on the response selection, enhancing forward progression of the body becomes the dominant determinant followed by stability and lastly by energy costs associated with the modifications. A decision algorithm for selecting foot placement is proposed based on these findings. It is clear that while visual input plays a critical role in guiding foot placement, it is not entirely based on reactive control. This has implications for implementing visually guided adaptive locomotion in legged robots.     

Asymmetrical after-effects of prism adaptation during goal oriented locomotion

In healthy subjects, sensorimotor after-effects of prism adaptation are known to be symmetric (they appear after using leftward and rightward optical deviations), whereas cognitive after-effects are asymmetric (they appear after using a leftward optical deviation) and rightward oriented. Sensorimotor and cognitive after-effects have been classically studied using different specific tasks. The purpose of the current study was to investigate whether both after-effects may be involved in a same visuo-spatial task. Therefore we compared the amplitude of after-effects following adaptation to a rightward or leftward optical deviation. After-effects were assessed by manual pointing or goal oriented locomotor task. The main result showed a greater amplitude for rightward locomotor after-effects (after adaptation to a leftward deviation) than for leftward locomotor after-effects (after adaptation to a rightward deviation). This means that cognitive after-effects may add to sensorimotor after-effects following adaptation to a leftward optical deviation. This asymmetry challenges the classical distinction between sensorimotor and cognitive after-effects of prism adaptation. Implications for the functional mechanisms and the neuroanatomical substrate of prism adaptation are discussed.     

Gait modification during approach phase when stepping over an obstacle in rats

Stepping over obstacles to avoid tripping is an essential component in safe and smooth locomotion. Obstacle avoidance during locomotion is completed in several steps during the approach phase toward the obstacle and stepping over the obstacle. The purpose of this study was to investigate gait modification during the approach phase when stepping over obstacles of different heights in rats. In all four limbs, the toe height when the toe was just above the obstacle increased depending on the obstacle height, leaving a safe margin. However, the horizontal distance between toe and obstacle just prior to stepping over was not influenced by obstacle height. In the fore- and hindlimbs that served as trailing limbs, it was found that the stride length and its related swing phase duration in the final step were significantly shorter than those in both the penultimate step and overground locomotion. These results suggest that adjustment of trailing limb in the final step during the approach phase is important in preparation for the stepping movement over an obstacle.     

Visual guidance of the human foot during a step

When the intended foot placement changes during a step, either due to an obstacle appearing in our path or the sudden shift of a target, visual input can rapidly alter foot trajectory. However, previous studies suggest that when intended foot placement does not change, the path of the foot is fixed after it leaves the floor and vision has no further influence. Here we ask whether visual feedback can be used to improve the accuracy of foot placement during a normal, unperturbed step. To investigate this we measured foot trajectory when subjects made accurate steps, at fast and slow speeds, to stationary floor-mounted targets. Vision was randomly occluded in 50% of trials at the point of foot-off. This caused an increase in foot placement error, reflecting lower accuracy and higher variability. This effect was greatest for slow steps. Trajectory heading analysis revealed that visually guided corrections occurred as the foot neared the target (on average 64 mm away). They occurred closer to the target for the faster movements thus allowing less time and space to execute corrections. However, allowing for a fixed reaction time of 120 ms, movement errors were detected when the foot was approximately halfway to the target. These results suggest that visual information can be used to adjust foot trajectory during the swing phase of a step when stepping onto a stationary target, even for fast movements. Such fine control would be advantageous when environmental constraints place limitations on foot placement, for example when hiking over rough terrain.     

Cerebellar damage produces context-dependent deficits in control of leg dynamics during obstacle avoidance

It has been suggested that the cerebellum is an important contributor to CNS prediction and control of intersegmental dynamics during voluntary multijoint reaching movements. Leg movements subserve different behavioral goals, e.g., locomotion versus voluntary stepping, which may or may not be under similar dynamic control. The objective was to determine whether cerebellar leg hypermetria (excessive foot elevation) during obstacle avoidance in locomotion and voluntary stepping could be attributed to a particular deficit in appropriately controlling intersegmental dynamics. We compared the performance of eight individuals with cerebellar damage to eight healthy controls as they walked or voluntarily stepped in place over a small obstacle. Joint kinematics and dynamics were calculated during swing phase for both movement contexts. The kinematic analysis showed that hypermetria occurred during both walking and stepping and was associated with excessive knee flexion. When present, the amplitude of hypermetria was greater during stepping compared to walking. During stepping, subjects with cerebellar damage produced excessive knee flexor muscle torques and consequently overcompensated for interaction and gravitational torques normally used to decelerate the limb. During walking, the torque pattern was very similar to that of control subjects walking over a taller obstacle, and therefore might be a voluntary compensatory strategy to avoid tripping. Our results show that the extent of kinematic and dynamic abnormalities associated with cerebellar leg hypermetria is context-specific, with more fundamental abnormalities of leg dynamics being apparent during stepping as opposed to walking.     

Infants adapt their stepping to repeated trip-inducing stimuli

This study examined whether human infants under the age of 12 mo learn to modify their stepping pattern after repeated trip-inducing stimuli. Thirty three infants aged from 5 to 11 mo were studied. The infants were held over a moving treadmill belt to induce stepping. Occasionally, a mechanical tap was applied to the dorsum of the left foot during the early swing phase to elicit a high step. In some trials, the stimulus was applied for only one step. In other trials, the foot was stimulated for a few consecutive steps. We determined whether the infants continued to show high stepping immediately after the removal of the stimuli. The results showed that after the foot was touched for two or more consecutive steps, some infants continued to demonstrate high stepping for a few steps after the removal of the stimuli (i.e., aftereffect). Such adaptation was achieved by an increase in hip and knee flexor muscle torque, which led to greater hip and knee flexion during the early swing phase. Aftereffects were more commonly seen in older infants (9 mo or older). The results indicated that before the onset of independent walking, the locomotor circuitry in human infants is capable of adaptive locomotor plasticity. The increased incidence of aftereffect in older infants also suggests that the ability to adapt to repeated trip-inducing stimuli may be related to other factors such as experience in stepping and maturation of the nervous system.     

Characteristics of voluntary visual sampling of the environment for safe locomotion over different terrains

The characteristics of visual sampling required for successful locomotion over various terrains is the focus of this work. In the first experiment we directly address the role of continuous visual monitoring of the environment in guiding locomotion by allowing the subjects to choose when and where to take a visual sample of the terrain and examine the effects of different terrains on characteristics of visual sampling. Young subjects walked over travel paths of varying difficulties while wearing opaque liquid crystal eyeglasses and pressed a hand-held switch to make the glasses transparent when they needed to sample the environment. Travel time and visual sampling characteristics were recorded. Results show that intermittent sampling (less than 50%) of the environment is adequate for safe locomotion, even over a novel travel path. The frequency, duration and timing of visual samples are dependent on terrain characteristics. Visual sampling of the environment is unaffected by preview restriction of the travel path and is increased when specific foot placement is required and there is a potential hazard in the travel path. In the second experiment we dissociated steering control and obstacle avoidance from specific foot placement and examined visual sampling demands prior to the initiation of the swing phase and during the swing phase. The results show that steering control and obstacle avoidance do influence the visual sampling time, which is scaled to the magnitude of change. Vision was used in a feedforward control mode to plan for and initiate appropriate changes in the swing limb trajectory: its use during the swing phase to provide on-line control was minimal.     

Environmental constraints on foot trajectory reveal the capacity for modulation of anticipatory postural adjustments during rapid triggered stepping reactions

This study used environmental restrictions on foot movement to challenge the capacity of the central nervous system (CNS) to counter the lateral instability that arises after foot-lift during rapid triggered stepping reactions evoked by unpredictable postural perturbation. The objective was to determine the extent to which lateral stability could be regulated via modulation of the mediolateral (m-l) anticipatory postural adjustment (APA) that precedes foot-lift. A high frontal obstacle was used to double the required swing duration, and thereby increase the potential for the center of mass (COM) to fall laterally toward the unsupported side, during forward-step reactions. The capacity to use lateral step placement to recover lateral stability was restricted by means of lateral barriers. Six healthy young adults were tested. In obstacle-only trials, the APA was insufficient to prevent increased lateral COM motion during the prolonged swing phase; hence, lateral step placement was necessitated. However, when lateral stepping was obstructed, the CNS was able to upregulate the APA amplitude so as to prevent this increase in lateral COM motion. The swing foot was placed medially, with no detriment to clearing the frontal obstacle or recovering equilibrium. There was no change in step timing or anteroposterior (a-p) COM motion. While previous studies have suggested that the a-p COM progression may determine the extent to which the m-l APA is expressed or truncated during triggered stepping reactions evoked by unpredictable perturbation, the present findings demonstrate that prior knowledge of environmental demands can lead to predictive efforts to modulate the APA during such reactions. An apparent preference to underscale anticipatory efforts when lateral step placement is permitted suggests that the CNS may be acting to avoid some potential risk or cost associated with the execution of a large APA.     

Anticipatory locomotor adjustments for accommodating versus avoiding level changes in humans

 The control of locomotion has been studied from various perspectives related to the tasks of pattern generation, equilibrium control or adaptation to the environment. The last of these locomotor components has received comparably less attention, specifically pertaining to anticipatory adjustments. Continuing the work which has been conducted on both humans and cats, the present paper explores the nature of the differences in anticipatory locomotor adjustments for obstacle avoidance versus the accommodation to level changes. Six subjects walked in six different environments including no obstructions, a simple obstacle, two different level changes (a platform and stairs), and a combination of an obstacle with each respective level change. Full dynamic analyses allowed comparison of muscle torques as well as muscle power generated and absorbed at the lower limb joints across conditions. It was found that the previously shown robust lower limb reorganization characterized by a knee flexor generation strategy was upheld in all conditions when the obstacle was present. Pure level changes involved an augmentation of the ongoing hip strategy inherent in normal level walking. In the compound environment of obstructed level changes, subjects chose to combine an augmentation of hip flexor power with a reorganization to active knee flexion. The results are discussed from the point of view of general principles of mechanical coordination and the exploitation of intersegmental dynamics for foot transport. Received: 31 May 1996 / Accepted: 1 November 1996     

Validating determinants for an alternate foot placement selection algorithm during human locomotion in cluttered terrain

The goal of this study was to validate dynamic stability and forward progression determinants for the alternate foot placement selection algorithm. Participants were asked to walk on level ground and avoid stepping, when present, on a virtual white planar obstacle. They had a one-step duration to select an alternate foot placement, with the task performed under two conditions: free (participants chose the alternate foot placement that was appropriate) and forced (a green arrow projected over the white planar obstacle cued the alternate foot placement). To validate the dynamic stability determinant, the distance between the extrapolated center of mass (COM) position, which incorporates the dynamics of the body, and the limits of the base of support was calculated in both anteroposterior (AP) and mediolateral (ML) directions in the double support phase. To address the second determinant, COM deviation from straight ahead was measured between adaptive and subsequent steps. The results of this study showed that long and lateral choices were dominant in the free condition, and these adjustments did not compromise stability in both adaptive and subsequent steps compared with the short and medial adjustments, which were infrequent and adversely affected stability. Therefore stability is critical when selecting an alternate foot placement in a cluttered terrain. In addition, changes in the plane of progression resulted in small deviations of COM from the endpoint goal. Forward progression of COM was maintained even for foot placement changes in the frontal plane, validating this determinant as part of the selection algorithm.     

Segmental control for adaptive locomotor adjustments during obstacle clearance in healthy young adults

Anticipatory locomotor adjustments (ALAs) are used during locomotion to perform tasks, such as obstacle clearance, although not much is known as to how these ALAs are implemented by the central nervous system (CNS). The current study applied the planar law of intersegmental coordination to both leading and trailing limbs in a paradigm in which obstacle height and depth were manipulated to propose how ALAs are controlled. Ten healthy young adults stepped over nine obstacle conditions. Full-body 3D kinematic data were collected and elevation angles of the foot, shank, and thigh in the sagittal plane were calculated. For each limb within each trial, a principal component analysis was applied to limb segment trajectories. As well, a Fourier harmonic series was used to represent segment elevation angle trajectories, and phase differences between adjacent segments were determined. Planarity was consistently high in both limbs for all obstacle conditions, although significant differences between obstacle heights were observed. Increases in covariance loop width and rotation of the covariance plane accompanied changes in planarity. As observed in previous studies, fundamental harmonic phase differences between adjacent segments were highly correlated to plane characteristics and these phase differences changed systematically with increases in obstacle height. From the results, it is proposed that if a given environment requires a change in locomotion, the CNS adjusts a basic locomotor pattern if needed through the manipulation of the phase differences in the fundamental harmonics of the elevation angles between adjacent segments and elevation angle amplitude (with a constraint being intersegmenal elevation angle planarity).     

Asymmetric generalization between the arm and leg following prism-induced visuomotor adaptation

We have previously shown an asymmetric generalization following a prism-induced visuomotor adaptation. Subjects who adapt to laterally deviating prism lenses during walking show a broad generalization to an arm pointing task, while subjects who adapt to prisms during arm pointing do not show generalization to walking. It is not known whether this broad generalization persists with other movements outside of walking or what specific features of the walking task, e.g. lower extremity involvement, allow it to be so broadly generalizable. In the current study, we tested healthy adult subjects performing one of three forms of prism adaptation and subsequently measured generalization. In Experiment 1 we tested whether a seated arm pointing prism adaptation would generalize to the leg. In Experiment 2 we tested whether a seated leg pointing prism adaptation would generalize to the arm. In Experiment 3 we tested whether standing influenced the extent of generalization from leg to arm. Results were surprising. We found a clear and consistent generalization from arm to leg, but much less so from leg to arm during either the seated or the standing task. These findings indicate that prism adaptations during arm movements are not limb-specific, as has been previously suggested. Further, the lack of generalization from leg to arm suggests that neither the adaptation of leg movements specifically, nor standing posture, nor the bilateral component of walking could be the salient feature allowing for its broad generalization across body parts.     

Age-related kinematic changes in late visual-cueing during obstacle circumvention

On a daily basis, we are challenged by common environmental obstacles (e.g. street posts) that require simple and often rapid modifications to our gait patterns to avoid collisions. Poor vision appears to be responsible for important reductions in postural stability during gait; and therefore, individuals with impaired vision, such as the elderly, may be at a greater risk of falling, especially under conditions where stepping avoidance strategies may be constrained by the environment. The purpose of the current study was to examine the body segment and eye-gaze reorientation strategy, role of base of support, as well as visual areas of interest attended to by healthy young (YA) and older adults (OA) when only given limited time, one stride, to prepare for an obstacle circumvention task. Six YA and six OA were asked to perform ten walking trials which required them to circumvent an obstacle in their travel path. Participants used one of two avoidance strategies, either lead leg crossing-over trail leg (narrow base of support) or lead leg stepping-out (wide base of support). Results indicate that base of support constraints did not affect segment reorientation sequence in either age group. The general segment reorientation sequence in YA was initiated by trunk yaw and head yaw, followed by gaze and finally, by M-L foot deviation. No trunk roll deviations were observed. In OA, the general segment reorientation sequence was the following: trunk yaw and trunk roll, gaze and finally, M-L foot deviation. No head yaw deviations were observed. Our findings suggest that YA utilized a foot placement strategy to perform the transient change in travel direction while OA relied on a hip strategy. In addition, YA spent more time gazing straight ahead at the obstacle and the wall, while OA spent more time looking at the ground. This strategy indicates that OA use a more cautious strategy to safely avoid the obstacle. Findings from the present work contribute further knowledge regarding locomotor adjustments during a common, and complex, everyday task in young and older adults.