Modeling dynamic resource utilization in populations with unique constraints: preadolescents with and without Down syndrome

In this study we used a damped inverted pendulum and spring with an escapement function model to compare the global levels of stiffness and forcing used by 12 preadolescents with Down syndrome (DS) and 12 with typical development (TD). Participants walked overground at their self-selected speed and on a treadmill at speeds slower and faster than overground. Children with DS, who are characterized as hypotonic with reduced capacity for producing muscle force, exhibited significantly higher levels of stiffness and forcing (angular impulse) when walking on the treadmill and higher forcing but not stiffness overground, than children with TD. Both groups adapted to imposed speed increases similarly by increasing their global stiffness and angular impulse. We propose children with DS increased stiffness in order to overcome their hypotonia and joint laxity, thus, optimizing on stability, rather than metabolic efficiency. Higher angular impulse values for children with DS may reflect higher energy cost associated with increasing stiffness and their inherent biomechanical and physiological characteristics that reduce efficiency. We conclude that the inverted pendulum and spring with escapement function model is a useful tool for uncovering solutions to movement problems-solutions that reflect the dynamic resources of the individual and ones that are discovered, rather than prescribed.     

Changes in lower limb co-contraction and stiffness by toddlers with Down syndrome and toddlers with typical development during the acquisition of independent gait

During gait acquisition, children learn to use their changing resources to meet the requirements of the task. Compared to typically developing toddlers (TD), toddlers with Down syndrome (DS) have functionally different musculoskeletal characteristics, such as hypotonia, and joint and ligament laxity, that could produce a reduced passive stiffness. The interplay between this inherently lower passive stiffness and the demands of walking may result in different strategies during gait acquisition. This study compared normalized global stiffness and lower limb's co-contraction indices (CCI) used by toddlers with TD (n=12) and with DS (n=12), during the early stages of gait acquisition. Stiffness and CCI were normalized by gravitational torque (mLg) in both phases of gait (stance, swing). Five longitudinal evaluations were conducted from the onset of locomotion until three months post-acquisition. All children were video taped and had electromyographic (EMG) recordings from muscle pairs of one leg, which were used to calculate CCI of hip, knee, ankle, and total leg CCI. Body and lower limb stiffness were calculated according to a hybrid pendulum resonance equation. Results from ANOVAs revealed no group differences on stiffness or on CCI's during stance but children with DS showed greater CCI during swing. Despite the structural musculoskeletal differences between toddlers with TD and with DS, the similarities observed in their processes of gait development suggest functional equivalences.     

Early changes in muscle activation patterns of toddlers during walking

Early locomotor behavior has been the focus of considerable attention by developmentalists over several decades. Few studies have addressed explicitly patterns of muscle activity that underlie this coordination pattern. Our purposes were to illustrate a method to determine objectively the onset and offset of muscle firings during early walking and to investigate the emergence of patterns of activation of the core locomotor muscles. We tested eight toddlers as they walked overground at walking onset (max. of 3-6 independent steps) and after three months of walking experience. Surface electrodes monitored activity of the gastrocnemius, tibialis anterior, quadriceps, and hamstrings. We reduced EMG signals to a frame-by-frame designation of "on-off," followed by muscle state and co-contraction analyses, and probability distributions for each muscle's activity across multiple cycles. Our results clearly show that at walking onset muscle activity was highly variable with few, if any, muscles showing recurring patterns of behavior, within or among toddlers. Variability and co-activation decreased with walking experience but remained inconsistent, in contrast to the significant increase in stability shown for joint coordination and endpoint (foot placement) parameters. We propose this trend emerges because of the high number of options (muscle combinations) available. Toddlers learn first to marshal sufficient force to balance and make forward progress but slowly discover how to optimize these resources.     

Level walking in adults with and without Developmental Coordination Disorder: An analysis of movement variability

Several studies have shown that Developmental Coordination Disorder (DCD) is a condition that continues beyond childhood. Although adults with DCD report difficulties with dynamic balance, as well as frequent tripping and bumping into objects, there have been no specific studies on walking in this population. Some previous work has focused on walking in children with DCD but variation in the tasks and measures used has led to inconsistent findings. The aim of the current study therefore was to examine the characteristics of level walking in adults with and without DCD. Fifteen adults with DCD and 15 typically developing (TD) controls walked barefoot at a natural pace up and down an 11 m walkway for one minute. Foot placement measures and velocity and acceleration of the body were recorded, as well as measures of movement variability. The adults with DCD showed similar gait patterns to the TD group in terms of step length, step width, double support time and stride time. The DCD group also showed similar velocity and acceleration to the TD group in the medio-lateral, anterior–posterior and vertical direction. However, the DCD group exhibited greater variability in all foot placement and some body movement measures. The finding that adults with DCD have a reduced ability to produce consistent movement patterns is discussed in relation to postural control limitations and compared to variability of walking measures found in elderly populations.     

Cerebral specialization and verbal-motor integration in adults with and without Down syndrome

Persons with Down syndrome (DS) tend to exhibit an atypical left ear-right hemisphere advantage (LEA) for the perception of speech sounds. In the present study, a recent adaptation of the dichotic listening procedure was employed to examine interhemispheric integration during the performance of a lateralized verbal-motor task. Although adults with DS (n = 13) demonstrated a right ear-left hemisphere advantage in the dichotic-motor task similar to their peers with (n = 14) and without undifferentiated developmental disabilities (n = 14), they showed an LEA in a free recall dichotic listening task. Based on a comparison of the laterality indices obtained from both dichotic listening procedures, it appears that the manifestation of lateral ear advantages in persons DS may dependent on the response requirements of the task.     

Adaptations to walking on an uneven terrain for individuals with and without Developmental Coordination Disorder

Given the importance of walking in everyday life, understanding why this is challenging for some populations is particularly important. Studies focusing on gait patterns of individuals with Developmental Coordination Disorder (DCD) have shown that whilst increased variability is characteristic of walking patterns for this group, differences in spatio-temporal gait variables seem only to arise when task demands increase. However, these differences occur under rather artificial conditions, for example using a treadmill. The aim of this study, therefore was to examine the step characteristics of individuals with and without DCD whilst walking along an irregular terrain. Thirty-five individuals with DCD aged 8–32 years and 35 age and gender-matched controls participated in this study. Participants were divided into 3 age groups; 8–12 years (n = 12), 13–17 years (n = 12) and 18–32 years (n = 11). Participants walked up and down a 6 m walkway for two minutes on two terrains: level and irregular. VICON 3D motion analysis was used to extract measures of foot placement, velocity and angle of the head and trunk. Results showed that both groups adapted their gait to negotiate the irregular terrain, but the DCD group was more affected than their TD peers; walking significantly slower with shorter, wider steps and inclining their head more towards the ground. This suggests an adaptive approach used by individuals with DCD to preserve stability and increase visual sampling whilst negotiating an irregular terrain.     

A biomechanical analysis of the 'high guard' position of arms during walking in toddlers

In this study, we investigated biomechanical contributions of the high guard position of the arms observed only in a subgroup of toddlers at very early stages of gait development. Six healthy toddlers who showed this peculiar arm posture were involved in this study. They participated in two data collection sessions (1 month apart). We used three-dimensional analysis of arm posture during gait to estimate the changes in forces and torques generated by the arms and acting on the upper trunk segment. Across visits, toddlers’ increase in walking speeds coincided with lowering arm postures. Despite the apparent trend of changes in arm posture in this group of toddlers, the interaction between arm posture and upper trunk position created a variety of changes in forces and torques among individuals. Findings of this study exhibited an example of the exploration of dynamics by toddlers in the early stage of gait development.     

Measurement of range of motion in individuals with mental retardation and with or without Down syndrome

The purpose of this study was to assess differences in variability of three joints' range of motion in the lower extremity among individuals with Down syndrome, mentally retarded individuals without Down syndrome, and sedentary subjects without mental retardation (ns = 13, 25, and 30, respectively). Range of motion for hip and knee flexion was obtained using a Myrin goniometer. For hip abduction the range of motion was obtained using a double protractor goniometer (Brodin type). Three test repetitions were carried out, and the greatest value was recorded. As no significant differences were found between left and right sides for each motion, the average was used to represent the range of motion. The Down syndrome group had significantly higher mean range of motion in hip flexion than the mentally retarded group. No significant differences in mean range of motion were found between Down syndrome and sedentary groups, but a significant difference was observed between the control and mentally retarded groups. In hip abduction, the Down syndrome group showed significantly higher mean range of motion than the control and mentally retarded groups. The control group had significantly a higher mean range of motion than the mentally retarded group. No significant differences were found in knee flexion between the two mentally disabled groups, but significant differences in mean range of motion were found between each of the two groups of mentally retarded individuals and the control group. Because differences exist in mean range of motion between the two mentally disabled groups, individualized and differentiated training programs to improve flexibility must be designed based on the type of handicap.     

Visual search in typically developing toddlers and toddlers with Fragile X or Williams syndrome

Visual selective attention is the ability to attend to relevant visual information and ignore irrelevant stimuli. Little is known about its typical and atypical development in early childhood Experiment 1 investigates typically developing toddlers' visual search for multiple targets on a touch-screen. Time to hit a target, distance between successively touched items, accuracy and error types revealed changes in 2- and 3-year-olds' vulnerability to manipulations of the search display. Experiment 2 examined search performance by toddlers with Fragile X syndrome (FXS) or Williams syndrome (WS). Both of these groups produced equivalent mean time and distance per touch as typically developing toddlers matched by chronological or mental age; but both produced a larger number of errors. Toddlers with WS confused distractors with targets more than the other groups; while toddlers with FXS perseverated on previously found targets. These findings provide information on how visual search typically develops in toddlers, and reveal distinct search deficits for atypically developing toddlers.     

Longitudinal assessment of leg motor activity and sleep patterns in infants with and without Down syndrome

Whether infants with Down syndrome (DS) perform leg movements with the same frequency and quality as their typical development (TD) counterparts is equivocal. Furthermore, the relationship between these early leg movements and later onset of locomotor milestones has only been partially explored. The aims of this study were two-fold: (1) to describe the longitudinal leg activity in infants with and without DS (3-6 months), and (2) to examine sleeping patterns and leg activity during the night. In addition, the relationships between leg activities and sleep patterns with locomotor development were explored. An activity monitor was placed monthly on the infant's ankle for 48 h. Data were analyzed to separate day-night, high-low activity, and sleep fragmentation. The results indicate that infants with DS produced more low intensity activity and more fragmented sleep. These findings are discussed in relation to the influence of early motor activity on achievement of functional motor behavior.     

Changes in dynamic balance control over time in children with and without Developmental Coordination Disorder

The aim of this study was to examine differences in underlying adaptations of dynamic balance in children with and without Developmental Coordination Disorder (DCD) during a Wii Fit game and to measure changes over time and after intervention.Twenty-eight children with DCD and 21 typically developing (TD) children participated in the study. Analyses of force plate variables showed that the TD group initially used a longer path length for the ski slope descent and tended toward more variation in Center of Pressure (CoP) displacement in lateral direction than the children with DCD. In contrast, the TD group showed a trend of fewer reversals per cm in both AP and lateral direction. After the nonintervention period, the TD group improved performance by decreasing the path length, while the DCD group improved by increasing the path length and by decreasing the number of reversals. After intervention, no changes were found in sway characteristics. Individual analyses within the DCD group showed that the path length per run fell more often within the 95% confidence Interval of the faultless runs. In conclusion both TD and DCD children modify the underlying kinetics of dynamic balance control, but in different ways and both lead to better performance.     

Sensory information affords exploration of posture in newly walking infants and toddlers

The development of posture and locomotion provides a valuable window for understanding the ontogeny of perception-action relations. In this study, 13 infants were examined cross-sectionally while standing quietly either hands-free or while lightly touching a contact surface. Mean sway amplitude results indicate that infants use light touch for sway attenuation (≈28–40%) as has been seen previously with adults (Jeka & Lackner, 1994). Additionally, while using the contact surface, movement patterns of the head and trunk show reduced temporal coordination (≈25–40%), as well as increased temporal variability, as compared to no touch conditions. These findings are discussed with regard to the ontogeny of perception-action relations, with the overall conclusion that infants use somatosensory information in an exploratory manner to aid in the development of an accurate internal model of upright postural control.     

Variability in parenting behaviours during play and during mealtimes with toddlers

Few empirical studies have explored variability in parenting behaviours observed in a mealtime setting as well as in a play setting with young children. We analysed data from 282 parent–toddler dyads who participated in the Play & Grow study in the United States in 2017–2019. Parent–child interactions were video recorded during play in the laboratory when the child was 18 months old and then during mealtime in the home 6 months later. Standardised coding procedures were used to rate parenting behaviours (sensitivity, cognitive stimulation, positive regard, detachment, intrusive control and negative regard). We describe correlations and differences and variability in parenting behaviour between the two assessments and relative to child sex, gestational age, parent education level, household composition and household income. Detachment was lower on average during play at 18 months compared to during mealtime at 24 months (Cohen's d = −0.40), and the other five parenting behaviours were greater on average during play than during mealtime (d = 0.18–0.43). Sociodemographic characteristics were unrelated to the magnitude of individual differences across contexts in any of the parenting behaviours. Implications for research on associations between parent–child interactions and risk for childhood obesity are discussed.     

At home with Down syndrome and gender

I argue that there is an important analogy between sex selection and selective abortion of fetuses diagnosed with Down syndrome. There are surprising parallels between the social construction of Down syndrome as a disability and the deeply entrenched institutionalization of sexual difference in many societies. Prevailing concepts of gender and mental retardation exert a powerful influence in constructing the sexual identities and life plans of people with Down syndrome, and also affect their families' lives.     

Prospective dynamic balance control during the swing phase of walking: Stability boundaries and time-to-contact analysis

This study examined the prospective control of the swing phase in young healthy adults while walking at preferred speed over unobstructed ground and during obstacle clearance. Three aspects of swing were examined: (1) the relation of the body Center of Mass (CoM) to the stability boundaries at the base of support; (2) a dynamic time-to-contact analysis of the CoM and swing foot to these boundaries; and (3) the role of head movements in the prospective control of gait and field of view assessment. The time-to-contact analysis of CoM and swing foot showed less stable swing dynamics in the trail foot compared to the lead foot in the approach to the unstable equilibrium, with the CoM leading the swing foot and crossing the anterior stability boundary before the swing foot. Compensations in temporal coupling occurred in the trail limb during the late swing phase. Time-to-contact analysis of head movement showed stronger prospective control of the lead foot, while fixation of the field of view occurred earlier in swing and was closer to the body in the obstacle condition compared to unobstructed walking. The dynamic time-to-contact analysis offers a new approach to assessing the unstable swing phase of walking in different populations.     

Behavioral phenotype of individuals with Down syndrome

Evidence is reviewed for a developmentally-emerging behavioral phenotype in individuals with Down syndrome that includes significant delay in nonverbal cognitive development accompanied by additional, specific deficits in speech, language production, and auditory short-term memory in infancy and childhood, but fewer adaptive behavior problems than individuals with other cognitive disabilities. Evidence of dementia emerges for up to half the individuals studied after age 50. Research issues affecting control group selection in establishing phenotypic characteristics are discussed, as well as the possible genetic mechanisms underlying variation in general cognitive delay, specific language impairment, and adult dementia. MRDD Research Reviews 2000;6:84-95. Wiley-Liss, Inc.     

Preschoolers with Down syndrome do not yet show the learning and memory impairments seen in adults with Down syndrome

Individuals with Down syndrome (DS) exhibit a behavioral phenotype of specific strengths and weaknesses, in addition to a generalized cognitive delay. In particular, adults with DS exhibit specific deficits in learning and memory processes that depend on the hippocampus, and there is some suggestion of impairments on executive function tasks that depend on the prefrontal cortex. While these functions have been investigated in adults with DS, it is largely unclear how these processes develop in young children with DS. Here we tested preschoolers with DS and typically developing children, age‐matched on either receptive language or non‐verbal scores as a proxy for mental age (MA), on a battery of eye‐tracking and behavioral measures that have been shown to depend on the hippocampus or the prefrontal cortex. Preschoolers with DS performed equivalently to MA‐matched controls, suggesting that the disability‐specific memory deficits documented in adults with DS, in addition to a cognitive delay, are not yet evident in preschoolers with DS, and likely emerge progressively with age. Our results reinforce the idea that early childhood may be a critical time frame for targeted early intervention. A video abstract of this article can be viewed at https://www.youtube.com/watch?v=r6GUA6my22Q&list=UU3FIcom6UpITHZOIEa8Onnw     

Examining the trajectory of parent emotion talk in mothers of toddlers: A predominantly Latine sample

Supportive parent emotion socialization has been associated with greater child emotion understanding and expression and lower levels of externalizing behavior problems, with limited understanding on parent emotion socialization in toddlerhood. The current study examined the developmental trajectory of emotion socialization via emotion talk in mothers of toddlers from a predominantly Latine sample. Participants were 101 mother-toddler dyads assessed over three time points from ages 12–25 months. Overall, maternal emotion talk remained relatively stable over time, although there was a significant decrease between the first and second assessments before returning to initial rates at the third assessment. Maternal emotion talk did not predict child externalizing behavior over time. Interestingly, however, greater toddler externalizing behavior problems was associated with an increase in maternal emotion talk over time. These findings suggest maternal emotion talk is relatively stable for parents of children who are low on externalizing behaviors and may fluctuate (i.e., slowly increase) for mothers of children who are high in externalizing behaviors. Understanding these mechanisms further could help inform how we implement and personalize parenting interventions.     

Small and large number processing in infants and toddlers with Williams syndrome

Previous studies have suggested that typically developing 6‐month‐old infants are able to discriminate between small and large numerosities. However, discrimination between small numerosities in young infants is only possible when variables continuous with number (e.g. area or circumference) are confounded. In contrast, large number discrimination is successful even when variables continuous with number are systematically controlled for. These findings suggest the existence of different systems underlying small and large number processing in infancy. How do these develop in atypical syndromes? Williams syndrome (WS) is a rare neurocognitive developmental disorder in which numerical cognition has been found to be impaired in older children and adults. Do impairments of number processing have their origins in infancy? Here this question is investigated by testing the small and large number discrimination abilities of infants and toddlers with WS. While infants with WS were able to discriminate between 2 and 3 elements when total area was confounded with numerosity, the same infants did not discriminate between 8 and 16 elements, when number was not confounded with continuous variables. These findings suggest that a system for tracking the features of small numbers of object (object‐file representation) may be functional in WS, while large number discrimination is impaired from an early age onwards. Finally, we argue that individual differences in large number processing in infancy are more likely than small number processing to be predictive of later development of numerical cognition.