Slow maturation of planning in obstacle avoidance in humans

Dynamic stability during level walking and obstacle crossing in children aged 2–5 years estimated by marker-less motion capture

In the present study, dynamic stability during level walking and obstacle crossing in typically developing children aged 2–5 years (n = 13) and healthy young adults (n = 19) was investigated. The participants were asked to walk along unobstructed and obstructed walkways. The height of the obstacle was set at 10% of the leg length. Gait motion was captured by three RGB cameras. 2D body landmarks were estimated using OpenPose, a marker-less motion capture algorithm, and converted to 3D using direct linear transformation (DLT). Dynamic stability was evaluated using the margin of stability (MoS) in the forward and lateral directions. All the participants successfully crossed the obstacles. Younger children crossed the obstacle more carefully to avoid falls, as evidenced by obviously decreased gait speed just before the obstacle in 2-year-olds and the increased in maximum toe height with younger age. There was no significant difference in the MoS at the instant of heel contact between children and adults during level walking and obstacle crossing in the forward direction, although children increased the step length of the lead leg to a greater extent than the adults to ensure base of support (BoS)-center of mass (CoM) distance. In the lateral direction, children exhibited a greater MoS than adults during level walking [children: 9.5%, adults: 6.5%, median, W = 39.000, p < .001, rank-biserial correlation = −0.684]; however, some children exhibited a smaller MoS during obstacle crossing [lead leg: −5.9% to 3.6% (min–max) for 4 children, 4.7%–6.4% [95% confidence interval (CI)] for adults, p < 0.05; trail leg: 0.1%–4.4% (min–max) for 4 children, 4.7%–6.4% (95% CI) for adults, p < 0.05]]. These results indicate that in early childhood, locomotor adjustment needed to avoid contact with obstacles can be observed, whereas lateral dynamic stability is frangible.

Perceptual distortion in virtual reality and its impact on dynamic postural control

BACKGROUND

Voluntary movement such as lifting a foot in preparation to stepping acts as a self-initiated perturbation that disturbs postural equilibrium. To maintain and restore equilibrium, humans utilize early, anticipatory, and compensatory postural adjustments. Despite technological progress in accessible virtual reality (VR) devices, little is known on the usage of VR in control and maintenance of balance while standing.

RESEARCH QUESTION

How does VR modulate early, anticipatory, and compensatory postural adjustments during a dynamic task of leg lifting while avoiding an obstacle?

METHODS

First, the postural adjustments in a single-leg obstacle avoidance were compared between real and VR settings, where a statistical reanalysis was performed with data subsets that minimize the difference of foot elevation speed. Second, the effect of simple foot elevation was examined to identify the fundamental nature of leg lifting motion as a self-initiated perturbation. Lastly, perceptual distortion in VR was assessed by evaluating how the spatial scale of the virtual scene used in the single-leg obstacle avoidance experiment was recognized by participants.

RESULTS

The VR setting reduced the activities of lower leg muscles on the supporting side not only in the compensatory phase but also in the preparatory early and anticipatory phases. On the other hand, simple foot elevation resulted in a significant increase of muscle activities with lifting height only found in the compensatory phase. Furthermore, it is suggested that the VR induced perceptual distortion in estimating the sizes of the virtual objects.

SIGNIFICANCE

The findings provide more definitive evidence that VR presentation modulates the components of postural adjustments for maintaining upright stance while being perturbed. One of the potential psychophysical factors is perceptual distortion in VR, and this provides critical information for further development of VR based training system.

Locomotor patterns during obstacle avoidance in children with cerebral palsy

Previous studies mainly evaluated the neuromuscular pattern generation in cerebral palsy (CP) during unobstructed gait. Here we characterized impairments in the obstacle task performance associated with a limited adaptation of the task-relevant muscle module timed to the foot lift during obstacle crossing. Impaired task performance in children with CP may reflect basic developmental deficits in the adaptable control of gait when the locomotor task is superimposed with the voluntary movement.

Mind your step: learning to walk in complex environments

In everyday contexts, children must respond to both self-related constraints (their own skills and abilities) and environmental constraints (external obstacles and goals). How do young children simultaneously accommodate these to support skilled and flexible behaviour? We used walking in a complex environment as a testbed for two hypotheses. Hypothesis 1: children will accommodate the self-related constraint of high foot placement variability via dynamic scaling. Hypothesis 2: children will plan ahead, even in complex environments. In our task, 3- to 5-year-olds and adults walked over obstacle sequences of varying complexity. We measured foot placement around the first obstacle in the sequence. Hypothesis 1 was partially supported. In simple, single obstacle environments, children engaged in dynamic scaling like adults. Those with more variable foot placement left greater margins of error between the feet and the obstacle. However, in complex, multiple obstacle settings, children employed large, un-tailored margins of error. This parallels other multisensory tasks in which children do not rely on the relative variability of sensory inputs. Hypothesis 2 was supported. Like adults, children planned ahead for environmental constraints. Children adjusted foot placement around the first obstacle depending on the upcoming obstacle sequence. In doing so, they demonstrate surprisingly sophisticated planning. We, therefore, show that in the motor domain, even very young children simultaneously control both self-related and environmental constraints. This allows flexible, safe and efficient behaviour.

Infants' Age and Walking Experience Shapes Perception-Action Coupling When Crossing Obstacles

This study examined the effects of age and walking experience on infants' ability to step over an obstacle. We videotaped 30 infants with one (mean [ M] age = 12.6 months), three ( M age = 14.7 months), and six months ( M age = 17.7 months) of walking experience walking on a pathway with and without an obstacle. We found a shorter stride and slower velocity for infants with one month of walking experience and for the walking condition with an obstacle than for other experience groups or for walking without an obstacle. Across all groups, the horizontal distance between an infant's foot and the obstacle was larger for the trailing leg than for the leading leg. The vertical distance for both legs was similar among 1-month walkers, increased for 3-month walkers, and was similar for the trailing leg of the 6-month walker group. The percentage of the interlimb coordination relative phase for the leading limb was smaller for 3- and 6-month walker groups. In conclusion, age and walking experience contribute to improving coupling between sensory information and motor action and to organization for stepping over an obstacle in infants.

Transitioning from the level surface to stairs in children with and without Down syndrome: Motor strategy and anticipatory locomotor adjustments

BACKGROUND

Children with Down syndrome (DS) show underdeveloped motor strategy and anticipatory locomotor adjustments (ALA) before crossing an obstacle. Stairs presents another important setting to study environment navigation and motor adaptation. Inclusion of external ankle load is often used to perturb the stability of a system and observe the emergence of new patterns.

RESEARCH QUESTION

How do stair height and external ankle load affect motor strategy and ALA in 5-to-11-year-old children with typical development (TD) and with DS when approaching the stairs?

METHODS

Fourteen children with DS and 14 age- and sex-matched children with TD participated in the study. They walked along a 5-meter walkway and ascended 3-step staircases. There were three staircases (low, moderate, and high heights) and 2 loading conditions (no load and ankle load). A 3D motion capture system was used to collect data. Motor strategy was coded for each trial. Step length, width, time, and velocity, minimum toe clearance, and horizontal toe velocity were calculated for the last four steps before stair ascent. Mixed ANOVAs with repeated measures were conducted for statistical analysis.

RESULTS

The TD group walked up all the stairs, while the DS group displayed a strategy shift from walking to crawling when the stairs became higher. While the TD group maintained the values of most spatiotemporal variables, the DS group continuously decreased step length and velocity but not step width over the last four approaching steps. Ankle load decreased step length, step velocity, minimum toe clearance, and horizontal toe velocity in the DS group, to a greater extent, than in the TD group.

SIGNIFICANCE

Children with DS show underdeveloped motor strategy and ALA when approaching the stairs, and external ankle load further disrupts these patterns. Stair negotiation appears to be an effective assessment tool for evaluating motor adaptation in children with DS.

Body representation does not lag behind in updating for the pubertal growth spurt

Both making perceptual judgments about your own body and successfully moving your body through the world depend on a mental representation of the body. However, there are indications that moving might be challenging when your body is changing. For instance, the pubertal growth spurt has been reported to be negatively correlated to motor competence. A possible explanation for this clumsiness would be that when the body is growing fast, updating the body representation may lag behind, resulting in a mismatch between internal body representation and actual body size. The current study investigated this hypothesis by testing participants ranging from aged 6 to 50 years on both a tactile body image task and a motor body schema task. Separate groups of participants, including those in the age range when pubertal growth spurt occurs, were asked to estimate the distance between two simultaneously applied tactile stimuli on the arm and to move their hand through apertures of different widths. Tactile distance estimations were equal between participants before, during, and after the age range where the pubertal growth spurt is expected. Similarly, Bayesian evaluation of informative hypotheses showed that participants in the age range of the growth spurt did not move through the apertures as if their representation of the hand was smaller than its physical size. These results suggest that body representations do not lag behind in updating for the pubertal growth spurt.

Different neural substrates for precision stepping and fast online step adjustments in youth

Humans can navigate through challenging environments (e.g., cluttered or uneven terrains) by modifying their preferred gait pattern (e.g., step length, step width, or speed). Growing behavioral and neuroimaging evidence suggests that the ability to modify preferred step patterns requires the recruitment of cognitive resources. In children, it is argued that prolonged development of complex gait is related to the ongoing development of involved brain regions, but this has not been directly investigated yet. Here, we aimed to elucidate the relationship between structural brain properties and complex gait in youth aged 9–18 years. We used volumetric analyses of cortical grey matter (GM) and whole-brain voxelwise statistical analyses of white matter (WM), and utilized a treadmill-based precision stepping task to investigate complex gait. Moreover, precision stepping was performed on step targets which were either unperturbed or perturbed (i.e., unexpectedly shifting to a new location). Our main findings revealed that larger unperturbed precision step error was associated with decreased WM microstructural organization of tracts that are particularly associated with attentional and visual processing functions. These results strengthen the hypothesis that precision stepping on unperturbed step targets is driven by cortical processes. In contrast, no significant correlations were found between perturbed precision stepping and cortical structures, indicating that other (neural) mechanisms may be more important for this type of stepping.