Human locomotion through a multiple obstacle environment: strategy changes as a result of visual field limitation

Gender Differences in Joint Biomechanics During Obstacle Crossing with Different Heights

Identifying gender-related gait changes offers valuable insights into the role of gender in motor control. It is anticipated that more difficult gait tasks (obstacle crossing) may reveal gender-specific effects on gait parameters. The present study aimed to explore the gait adaptations of male and female participants when stepping over obstacles of 0 cm, 13 cm, 19 cm, and 26 cm in height. A total of 12 male and 12 female participants were recruited. The Vicon motion capture system and AMTI force plates were utilized to obtain the gait parameters. Moreover, spatiotemporal parameters were investigated. Two-way repeated ANOVA (gender × obstacle height) and three-way repeated ANOVA (gender × obstacle height × leg) were performed to compare gait parameters, respectively. Correlations between maximum joint angle and obstacle height were also evaluated. Significant interactions were observed for leading leg swing time, maximum hip extension angle, maximum knee flexion angle, and maximum ankle plantarflexion angle (gender × obstacle height). There were some differences in gait parameters between males and females in the unobstructed gait, and these changes became more evident as obstacle height increased. This study also identified significant differences in gait parameters between leading and trailing legs when stepping over the obstacle.

The allocentric nature of ground-surface representation: A study of depth and location perception

When observers perceive 3D relations, they represent depth and spatial locations with the ground as a reference. This frame of reference could be egocentric, that is, moving with the observer, or allocentric, that is, remaining stationary and independent of the moving observer. We tested whether the representation of relative depth and of spatial location took an egocentric or allocentric frame of reference in three experiments, using a blind walking task. In Experiments 1 and 2, participants either observed a target in depth, and then straightaway blind walked for the previously seen distance between the target and the self; or walked to the side or along an oblique path for 3 m and then started blind walking for the previously seen distance. The difference between the conditions was whether blind walking started from the observation point. Results showed that blind walking distance varied with the starting locations. Thus, the represented distance did not seem to go through spatial updating with the moving observer and the frame of reference was likely allocentric. In Experiment 3, participants observed a target in space, then immediately blind walked to the target, or blind walked to another starting point and then blind walked to the target. Results showed that the end location of blind walking was different for different starting points, which suggested the representation of spatial location is likely to take an allocentric frame of reference. Taken together, these experiments convergingly suggested that observers used an allocentric frame of reference to construct their mental space representation.

Generalizing stepping concepts to non-straight walking

People rarely walk in straight lines. Instead, we make frequent turns or other maneuvers. Spatiotemporal parameters fundamentally characterize gait. For straight walking, these parameters are well-defined for the task of walking on a straight path. Generalizing these concepts to non-straight walking, however, is not straightforward. People follow non-straight paths imposed by their environment (sidewalk, windy hiking trail, etc.) or choose readily-predictable, stereotypical paths of their own. People actively maintain lateral position to stay on their path and readily adapt their stepping when their path changes. We therefore propose a conceptually coherent convention that defines step lengths and widths relative to predefined walking paths. Our convention simply re-aligns lab-based coordinates to be tangent to a walker's path at the mid-point between the two footsteps that define each step. We hypothesized this would yield results both more correct and more consistent with notions from straight walking. We defined several common non-straight walking tasks: single turns, lateral lane changes, walking on circular paths, and walking on arbitrary curvilinear paths. For each, we simulated idealized step sequences denoting "perfect" performance with known constant step lengths and widths. We compared results to path-independent alternatives. For each, we directly quantified accuracy relative to known true values. Results strongly confirmed our hypothesis. Our convention returned vastly smaller errors and introduced no artificial stepping asymmetries across all tasks. All results for our convention rationally generalized concepts from straight walking. Taking walking paths explicitly into account as important task goals themselves thus resolves conceptual ambiguities of prior approaches.

Generalizing Stepping Concepts To Non-Straight Walking

People rarely walk in straight lines. Instead, we make frequent turns or other maneuvers. Spatiotemporal parameters fundamentally characterize gait. For straight walking, these parameters are well-defined for that task of walking on a straight path. Generalizing these concepts to non-straight walking, however, is not straightforward. People also follow non-straight paths imposed by their environment (store aisle, sidewalk, etc.) or choose readily-predictable, stereotypical paths of their own. People actively maintain lateral position to stay on their path and readily adapt their stepping when their path changes. We therefore propose a conceptually coherent convention that defines step lengths and widths relative to known walking paths. Our convention simply re-aligns lab-based coordinates to be tangent to a walker's path at the mid-point between the two footsteps that define each step. We hypothesized this would yield results both more correct and more consistent with notions from straight walking. We defined several common non-straight walking tasks: single turns, lateral lane changes, walking on circular paths, and walking on arbitrary curvilinear paths. For each, we simulated idealized step sequences denoting "perfect" performance with known constant step lengths and widths. We compared results to path- independent alternatives. For each, we directly quantified accuracy relative to known true values. Results strongly confirmed our hypothesis. Our convention returned vastly smaller errors and introduced no artificial stepping asymmetries across all tasks. All results for our convention rationally generalized concepts from straight walking. Taking walking paths explicitly into account as important task goals themselves thus resolves conceptual ambiguities of prior approaches.

A Fully-Immersive Virtual Reality Setup to Study Gait Modulation

Objective: Gait adaptation to environmental challenges is fundamental for independent and safe community ambulation. The possibility of precisely studying gait modulation using standardized protocols of gait analysis closely resembling everyday life scenarios is still an unmet need.

Methods: We have developed a fully-immersive virtual reality (VR) environment where subjects have to adjust their walking pattern to avoid collision with a virtual agent (VA) crossing their gait trajectory. We collected kinematic data of 12 healthy young subjects walking in real world (RW) and in the VR environment, both with (VR/A+) and without (VR/A-) the VA perturbation. The VR environment closely resembled the RW scenario of the gait laboratory. To ensure standardization of the obstacle presentation the starting time speed and trajectory of the VA were defined using the kinematics of the participant as detected online during each walking trial.

Results: We did not observe kinematic differences between walking in RW and VR/A-, suggesting that our VR environment per se might not induce significant changes in the locomotor pattern. When facing the VA all subjects consistently reduced stride length and velocity while increasing stride duration. Trunk inclination and mediolateral trajectory deviation also facilitated avoidance of the obstacle.

Conclusions: This proof-of-concept study shows that our VR/A+ paradigm effectively induced a timely gait modulation in a standardized immersive and realistic scenario. This protocol could be a powerful research tool to study gait modulation and its derangements in relation to aging and clinical conditions.

Continuous Classification of Locomotion in Response to Task Complexity and Anticipatory State

Objective

Intent recognition in lower-extremity assistive devices (e.g., prostheses and exoskeletons) is typically limited to either recognition of steady-state locomotion or changes of terrain (e.g., level ground to stair) occurring in a straight-line path and under anticipated condition. Stability is highly affected during non-steady changes of direction such as cuts especially when they are unanticipated, posing high risk of fall-related injuries. Here, we studied the influence of changes of direction and user anticipation on task recognition, and accordingly introduced classification schemes accommodating such effects.

Methods

A linear discriminant analysis (LDA) classifier continuously classified straight-line walking, sidestep/crossover cuts (single transitions), and cuts-to-stair locomotion (mixed transitions) performed under varied task anticipatory conditions. Training paradigms with varying levels of anticipated/unanticipated exposures and analysis windows of size 100–600 ms were examined.

Results

More accurate classification of anticipated relative to unanticipated tasks was observed. Including bouts of target task in the training data was necessary to improve generalization to unanticipated locomotion. Only up to two bouts of target task were sufficient to reduce errors to <20% in unanticipated mixed transitions, whereas, in single transitions and straight walking, substantial unanticipated information (i.e., five bouts) was necessary to achieve similar outcomes. Window size modifications did not have a significant influence on classification performance.

Conclusion

Adjusting the training paradigm helps to achieve classification schemes capable of adapting to changes of direction and task anticipatory state.

Significance

The findings could provide insight into developing classification schemes that can adapt to changes of direction and user anticipation. They could inform intent recognition strategies for controlling lower-limb assistive to robustly handle “unknown” circumstances, and thus deliver increased level of reliability and safety.

How can basic research on spatial cognition enhance the visual accessibility of architecture for people with low vision?

People with visual impairment often rely on their residual vision when interacting with their spatial environments. The goal of visual accessibility is to design spaces that allow for safe travel for the large and growing population of people who have uncorrectable vision loss, enabling full participation in modern society. This paper defines the functional challenges in perception and spatial cognition with restricted visual information and reviews a body of empirical work on low vision perception of spaces on both local and global navigational scales. We evaluate how the results of this work can provide insights into the complex problem that architects face in the design of visually accessible spaces.

Does active learning benefit spatial memory during navigation with restricted peripheral field?

Spatial learning of real-world environments is impaired with severely restricted peripheral field of view (FOV). In prior research, the effects of restricted FOV on spatial learning have been studied using passive learning paradigms - learners walk along pre-defined paths and are told the location of targets to be remembered. Our research has shown that mobility demands and environmental complexity may contribute to impaired spatial learning with restricted FOV through attentional mechanisms. Here, we examine the role of active navigation, both in locomotion and in target search. First, we compared effects of active versus passive locomotion (walking with a physical guide versus being pushed in a wheelchair) on a task of pointing to remembered targets in participants with simulated 10° FOV. We found similar performance between active and passive locomotion conditions in both simpler (Experiment 1) and more complex (Experiment 2) spatial learning tasks. Experiment 3 required active search for named targets to remember while navigating, using both a mild and a severe FOV restriction. We observed no difference in pointing accuracy between the two FOV restrictions but an increase in attentional demands with severely restricted FOV. Experiment 4 compared active and passive search with severe FOV restriction, within subjects. We found no difference in pointing accuracy, but observed an increase in cognitive load in active versus passive search. Taken together, in the context of navigating with restricted FOV, neither locomotion method nor level of active search affected spatial learning. However, the greater cognitive demands could have counteracted the potential advantage of the active learning conditions.

Interdependence of balance mechanisms during bipedal locomotion

Our main interest is to identify how humans maintain upright while walking. Balance during standing and walking is different, primarily due to a gait cycle which the nervous system must contend with a variety of body configurations and frequent perturbations (i.e., heel-strike). We have identified three mechanisms that healthy young adults use to respond to a visually perceived fall to the side. The lateral ankle mechanism and the foot placement mechanism are used to shift the center of pressure in the direction of the perceived fall, and the center of mass away from the perceived fall. The push-off mechanism, a systematic change in ankle plantarflexion angle in the trailing leg, results in fine adjustments to medial-lateral balance near the end of double stance. The focus here is to understand how the three basic balance mechanisms are coordinated to produce an overall balance response. The results indicate that lateral ankle and foot placement mechanisms are inversely related. Larger lateral ankle responses lead to smaller foot placement changes. Correlations involving the push-off mechanism, while significant, were weak. However, the consistency of the correlations across stimulus conditions suggest the push-off mechanism has the role of small adjustments to medial-lateral movement near the end of the balance response. This verifies that a fundamental feature of human bipedal gait is a highly flexible balance system that recruits and coordinates multiple mechanisms to maintain upright balance during walking to accommodate extreme changes in body configuration and frequent perturbations.

A Hierarchical View of Gecko Locomotion: Photic Environment, Physiological Optics, and Locomotor Performance

Terrestrial animals move in complex habitats that vary over space and time. The characteristics of these habitats are not only defined by the physical environment, but also by the photic environment, even though the latter has largely been overlooked. For example, numerous studies of have examined the role of habitat structure, such as incline, perch diameter, and compliance, on running performance. However, running performance likely depends heavily on light level. Geckos are an exceptional group for analyzing the role of the photic environment on locomotion as they exhibit several independent shifts to diurnality from a nocturnal ancestor, they are visually-guided predators, and they are extremely diverse. Our initial goal is to discuss the range of photic environments that can be encountered in terrestrial habitats, such as day versus night, canopy cover in a forest, fog, and clouds. We then review the physiological optics of gecko vision with some new information about retina structures, the role of vision in motor-driven behaviors, and what is known about gecko locomotion under different light conditions, before demonstrating the effect of light levels on gecko locomotor performance. Overall, we highlight the importance of integrating sensory and motor information and establish a conceptual framework as guide for future research. Several future directions, such as understanding the role of pupil dynamics, are dependent on an integrative framework. This general framework can be extended to any motor system that relies on sensory information, and can be used to explore the impact of performance features on diversification and evolution.

Circumvention of Pedestrians While Walking in Virtual and Physical Environments

Virtual environments (VEs) are increasingly used in the context of scientific inquiries and rehabilitation for tasks that are otherwise difficult to control or perform safely in physical environments (PEs), such as avoiding other pedestrians during locomotion. The usefulness of VEs, however, remains constrained by the extent to which they can elicit natural responses. The objectives of the study were to examine circumvention strategies in response to pedestrians approaching from different directions in the VE versus PE and to determine the effects of repeated practice on the circumvention strategies. Twelve participants were assessed over five blocks of eight trials that consisted of walking toward a target while circumventing pedestrians approaching from different directions (0°, ± 30° right or left or none) in the VE and the PE. Similar onset distances of circumvention strategy and preferred side of circumvention were observed between the two environments. Participants, however, maintained enlarged minimum distances from the interferer (13%) and walked slower (11.5%) in the VE. Repeated practice resulted in walking speed increments of 7.4% over the entire session that were similar in the VE versus PE. While the changes observed in VE may reflect the use of more cautious circumvention strategies, the similarities in strategies between the two environments and the advantages of VEs (e.g., controlled exposure, reproduction of ecologically valid conditions, and safety) suggest that virtual reality is a valuable tool to study visually guided locomotor tasks, such as pedestrian circumvention, and shows great potential for assessment and intervention in physical rehabilitation.

Dynamic feet distance: A new functional assessment during treadmill locomotion in normal and thoracic spinal cord injured rats

Of all the detrimental effects of spinal cord injury (SCI), one of the most devastating is the disruption of the ability to perform functional movement. Very little is known on the recovery of hindlimb joint kinematics after clinically-relevant contusive thoracic lesion in experimental animal models. A new functional assessment instrument, the dynamic feet distance (DFD) was used to describe the distance between the two feet throughout the gait cycle in normal and affected rodents. The purpose of this investigation was the evaluation and characterization of the DFD during treadmill locomotion in normal and T9 contusion injured rats, using three-dimensional (3D) instrumented gait analysis. Despite that normal and injured rats showed a similar pattern in the fifth metatarsal head joints distance excursion, we found a significantly wider distance between the feet during the entire gait cycle following spinal injury. This is the first study to quantify the distance between the two feet, throughout the gait cycle, and the biomechanical adjustments made between limbs in laboratory rodents after nervous system injury.

Strategies for obstacle avoidance during walking in the cat

Avoiding obstacles is essential for successful navigation through complex environments. This study aimed to clarify what strategies are used by a typical quadruped, the cat, to avoid obstacles during walking. Four cats walked along a corridor 2.5 m long and 25 or 15 cm wide. Obstacles, small round objects 2.5 cm in diameter and 1 cm in height, were placed on the floor in various locations. Movements of the paw were recorded with a motion capture and analysis system (Visualeyez, PTI). During walking in the wide corridor, cats' preferred strategy for avoiding a single obstacle was circumvention, during which the stride direction changed while stride duration and swing-to-stride duration ratio were preserved. Another strategy, stepping over the obstacle, was used during walking in the narrow corridor, when lateral deviations of walking trajectory were restricted. Stepping over the obstacle involved changes in two consecutive strides. The stride preceding the obstacle was shortened, and swing-to-stride ratio was reduced. The obstacle was negotiated in the next stride of increased height and normal duration and swing-to-stride ratio. During walking on a surface with multiple obstacles, both strategies were used. To avoid contact with the obstacle, cats placed the paw away from the object at a distance roughly equal to the diameter of the paw. During obstacle avoidance cats prefer to alter muscle activities without altering the locomotor rhythm. We hypothesize that a choice of the strategy for obstacle avoidance is determined by minimizing the complexity of neuro-motor processes required to achieve the behavioral goal.NEW & NOTEWORTHY In a study of feline locomotor behavior we found that the preferred strategy to avoid a small obstacle is circumvention. During circumvention, stride direction changes but length and temporal structure are preserved. Another strategy, stepping over the obstacle, is used in narrow walkways. During overstepping, two strides adjust. A stride preceding the obstacle decreases in length and duration. The following stride negotiating the obstacle increases in height while retaining normal temporal structure and nearly normal length.

Adaptive Gaze Strategies for Locomotion with Constricted Visual Field

In retinitis pigmentosa (RP), loss of peripheral visual field accounts for most difficulties encountered in visuo-motor coordination during locomotion. The purpose of this study was to accurately assess the impact of peripheral visual field loss on gaze strategies during locomotion, and identify compensatory mechanisms. Nine RP subjects presenting a central visual field limited to 10-25° in diameter, and nine healthy subjects were asked to walk in one of three directions-straight ahead to a visual target, leftward and rightward through a door frame, with or without obstacle on the way. Whole body kinematics were recorded by motion capture, and gaze direction in space was reconstructed using an eye-tracker. Changes in gaze strategies were identified in RP subjects, including extensive exploration prior to walking, frequent fixations of the ground (even knowing no obstacle was present), of door edges, essentially of the proximal one, of obstacle edge/corner, and alternating door edges fixations when approaching the door. This was associated with more frequent, sometimes larger rapid-eye-movements, larger movements, and forward tilting of the head. Despite the visual handicap, the trajectory geometry was identical between groups, with a small decrease in walking speed in RPs. These findings identify the adaptive changes in sensory-motor coordination, in order to ensure visual awareness of the surrounding, detect changes in spatial configuration, collect information for self-motion, update the postural reference frame, and update egocentric distances to environmental objects. They are of crucial importance for the design of optimized rehabilitation procedures.

How do walkers avoid a mobile robot crossing their way?

Robots and Humans have to share the same environment more and more often. In the aim of steering robots in a safe and convenient manner among humans it is required to understand how humans interact with them. This work focuses on collision avoidance between a human and a robot during locomotion. Having in mind previous results on human obstacle avoidance, as well as the description of the main principles which guide collision avoidance strategies, we observe how humans adapt a goal-directed locomotion task when they have to interfere with a mobile robot. Our results show differences in the strategy set by humans to avoid a robot in comparison with avoiding another human. Humans prefer to give the way to the robot even when they are likely to pass first at the beginning of the interaction.

Light level impacts locomotor biomechanics in a secondarily diurnal gecko, Rhoptropus afer

Locomotion through complex habitats relies on the continuous feedback from a number of sensory systems, including vision. Animals face a visual trade-off between acuity and light sensitivity that depends on light levels, which will dramatically impact the ability to process information and move quickly through a habitat, making ambient illumination an incredibly important ecological factor. Despite this, there is a paucity of data examining ambient light in the context of locomotor dynamics. There have been several independent transitions from the nocturnal ancestor to a diurnal activity pattern among geckos. We examined how ambient light level impacted the locomotor performance and high-speed three-dimensional kinematics of a secondarily diurnal, and cursorial, gecko (Rhoptropus afer) from Namibia. This species is active under foggy and sunny conditions, indicating that a range of ambient light conditions is experienced naturally. Locomotor speed was lowest in the 'no-light' condition compared with all other light intensities, occurring via a combination of shorter stride length and lower stride frequency. Additionally, the centre of mass was significantly lower, and the geckos were more sprawled, in the no-light condition relative to all of the higher light intensities. Locomotor behaviour is clearly sub-optimal under lower light conditions, suggesting that ecological conditions, such as very dense fog, might preclude the ability to run quickly during predator-prey interactions. The impact of ambient light on fitness should be explored further, especially in those groups that exhibit multiple transitions between diel activity patterns.

The Effects of Restricted Peripheral Field-of-View on Spatial Learning while Navigating

Recent work with simulated reductions in visual acuity and contrast sensitivity has found decrements in survey spatial learning as well as increased attentional demands when navigating, compared to performance with normal vision. Given these findings, and previous work showing that peripheral field loss has been associated with impaired mobility and spatial memory for room-sized spaces, we investigated the role of peripheral vision during navigation using a large-scale spatial learning paradigm. First, we aimed to establish the magnitude of spatial memory errors at different levels of field restriction. Second, we tested the hypothesis that navigation under these different levels of restriction would use additional attentional resources. Normally sighted participants walked on novel real-world paths wearing goggles that restricted the field-of-view (FOV) to severe (15°, 10°, 4°, or 0°) or mild angles (60°) and then pointed to remembered target locations using a verbal reporting measure. They completed a concurrent auditory reaction time task throughout each path to measure cognitive load. Only the most severe restrictions (4° and blindfolded) showed impairment in pointing error compared to the mild restriction (within-subjects). The 10° and 4° conditions also showed an increase in reaction time on the secondary attention task, suggesting that navigating with these extreme peripheral field restrictions demands the use of limited cognitive resources. This comparison of different levels of field restriction suggests that although peripheral field loss requires the actor to use more attentional resources while navigating starting at a less extreme level (10°), spatial memory is not negatively affected until the restriction is very severe (4°). These results have implications for understanding of the mechanisms underlying spatial learning during navigation and the approaches that may be taken to develop assistance for navigation with visual impairment.

Echoic Sensory Substitution Information in a Single Obstacle Circumvention Task

Accurate motor control is required when walking around obstacles in order to avoid collisions. When vision is unavailable, sensory substitution can be used to improve locomotion through the environment. Tactile sensory substitution devices (SSDs) are electronic travel aids, some of which indicate the distance of an obstacle using the rate of vibration of a transducer on the skin. We investigated how accurately such an SSD guided navigation in an obstacle circumvention task. Using an SSD, 12 blindfolded participants navigated around a single flat 0.6 x 2 m obstacle. A 3-dimensional Vicon motion capture system was used to quantify various kinematic indices of human movement. Navigation performance under full vision was used as a baseline for comparison. The obstacle position was varied from trial to trial relative to the participant, being placed at two distances 25 cm to the left, right or directly ahead. Under SSD guidance, participants navigated without collision in 93% of trials. No collisions occurred under visual guidance. Buffer space (clearance between the obstacle and shoulder) was larger by a factor of 2.1 with SSD guidance than with visual guidance, movement times were longer by a factor of 9.4, and numbers of velocity corrections were larger by a factor of 5 (all p<0.05). Participants passed the obstacle on the side affording the most space in the majority of trials for both SSD and visual guidance conditions. The results are consistent with the idea that SSD information can be used to generate a protective envelope during locomotion in order to avoid collisions when navigating around obstacles, and to pass on the side of the obstacle affording the most space in the majority of trials.

Vertical adaptation of the center of mass in human running on uneven ground

In running we are frequently confronted with different kinds of disturbances. Some require quick reactions and adaptations while others, like moderate changes in ground level, can be compensated passively. Monitoring the kinematics of the runner's center of mass (CoM) in such situations can reveal what global locomotion control strategies humans use and can help to distinguish between active and passive compensation methods. In this study single and permanent upward steps of 10 cm as well as drops of the same height were used as mechanical disturbances and the adaptations in the vertical oscillation of the runners CoM were analyzed. We found that runners visually perceiving uneven ground ahead substantially adapted their CoM in preparation by lifting it about 50% of step height or lowering it by about 40% of drop height, respectively. After contact on the changed ground level different adaptations depending on the situation occur. For persisting changes the adaptation to the elevated ground is completed after the first step on the new level. For single steps part of the adaptation takes place while returning to the ground. The consistent adaptations for the different situations support the idea that controlling the CoM by adapting leg parameters is a general control principle in running.

Interactions of age and leg muscle fatigue on unobstructed walking and obstacle crossing

Older adults commonly report muscle fatigue, which may be associated with reduced walking ability. Elderly may have insufficient awareness of the balance threat caused by muscle fatigue. The aim of this study was to analyze the interaction effects of aging and leg muscle fatigue on gait parameters in walking and obstacle crossing. One hundred and twenty men, who were divided in six groups according to their age (20-29 years, 30-39 years, 40-49 years, 50-59 years, 60-69 years, above 70 years), participated in this study. Participants performed three trials of unobstructed level ground walking and obstacle crossing during walking before and after quadriceps muscle fatigue. To induce fatigue, participants performed a repeated sit-to-stand task from a chair with arms across the chest to a pre-determined cadence (30 cycles/min) using a metronome. Spatial-temporal gait parameters (stride length, duration, and speed, step width, and trailing and leading heel-clearance) were analyzed, and compared by two-way ANOVA (group and fatigue). The results confirmed our hypothesis, showing age-related effects of leg muscles fatigue in both gait conditions. From 40 years old, participants modulated spatial-temporal and vertical impulses in both tasks more in response to fatigue than younger participants, apparently to improve balance and safety. Leg muscle fatigue caused age-dependent changes in both unobstructed level ground walking and obstacle crossing during walking, which appeared to reflect an attempt to maintain balance and safety, probably to counteract adverse fatigue effects.

Adjustments of speed and path when avoiding collisions with another pedestrian

When walking in open space, collision avoidance with other pedestrians is a process that successfully takes place many times. To pass another pedestrian (an interferer) walking direction, walking speed or both can be adjusted. Currently, the literature is not yet conclusive of how humans adjust these two parameters in the presence of an interferer. This impedes the development of models predicting general obstacle avoidance strategies in humans’ walking behavior. The aim of this study was to investigate the adjustments of path and speed when a pedestrian is crossing a non-reactive human interferer at different angles and speeds, and to compare the results to general model predictions. To do so, we designed an experiment where a pedestrian walked a 12 m distance to reach a goal position. The task was designed in such a way that collision with an interferer would always occur if the pedestrian would not apply a correction of movement path or speed. Results revealed a strong dependence of path and speed adjustments on crossing angle and walking speed, suggesting local planning of the collision avoidance strategy. Crossing at acute angles (i.e. 45° and 90°) seems to require more complex collision avoidance strategies involving both path and speed adjustments than crossing at obtuse angles, where only path adjustments were observed. Overall, the results were incompatible with predictions from existing models of locomotor collision avoidance. The observed initiations of both adjustments suggest a collision avoidance strategy that is temporally controlled. The present study provides a comprehensive picture of human collision avoidance strategies in walking, which can be used to evaluate and adjust existing pedestrian dynamics models, or serve as an empirical basis to develop new models.

Collision avoidance between two walkers: role-dependent strategies

This paper studies strategies for collision avoidance between two persons walking along crossing trajectories. It has been previously demonstrated that walkers are able to anticipate the risk of future collision and to react accordingly. The avoidance task has been described as a mutual control of the future distance of closest approach, MPD (i.e., Mininum Predicted Distance). In this paper, we studied the role of each walker in the task of controlling MPD. A specific question was: does the walker giving way (2nd at the crossing) and the one passing first set similar and coordinated strategies? To answer this question, we inspected the effect of motion adaptations on the future distance of closest approach. This analysis is relevant in the case of collision avoidance because subtle anticipatory behaviors or large last moment adaptations can finally yield the same result upon the final crossing distance. Results showed that collision avoidance is performed collaboratively and the crossing order impacts both the contribution and the strategies used: the participant giving way contributes more than the one passing first to avoid the collision. Both walkers reorient their path but the participant giving way also adapts his speed. Future work is planned to investigate the influence of crossing angle and TTC on adaptations as well as new types of interactions, such as intercepting or meeting tasks.

Birds achieve high robustness in uneven terrain through active control of landing conditions

We understand little about how animals adjust locomotor behaviour to negotiate uneven terrain. The mechanical demands and constraints of such behaviours likely differ from uniform terrain locomotion. Here we investigated how common pheasants negotiate visible obstacles with heights from 10 to 50% of leg length. Our goal was to determine the neuro-mechanical strategies used to achieve robust stability, and address whether strategies vary with obstacle height. We found that control of landing conditions was crucial for minimising fluctuations in stance leg loading and work in uneven terrain. Variation in touchdown leg angle (θTD) was correlated with the orientation of ground force during stance, and the angle between the leg and body velocity vector at touchdown (βTD) was correlated with net limb work. Pheasants actively targeted obstacles to control body velocity and leg posture at touchdown to achieve nearly steady dynamics on the obstacle step. In the approach step to an obstacle, the birds produced net positive limb work to launch themselves upward. On the obstacle, body dynamics were similar to uniform terrain. Pheasants also increased swing leg retraction velocity during obstacle negotiation, which we suggest is an active strategy to minimise fluctuations in peak force and leg posture in uneven terrain. Thus, pheasants appear to achieve robustly stable locomotion through a combination of path planning using visual feedback and active adjustment of leg swing dynamics to control landing conditions. We suggest that strategies for robust stability are context specific, depending on the quality of sensory feedback available, especially visual input.