Learning to tune the antero-posterior propulsive forces during walking: a necessary skill for mastering upright locomotion in toddlers

An update of the development of motor behavior

This primer describes research on the development of motor behavior. We focus on infancy when basic action systems are acquired-posture, locomotion, manual actions, and facial actions-and we adopt a developmental systems perspective to understand the causes and consequences of developmental change. Experience facilitates improvements in motor behavior and infants accumulate immense amounts of varied everyday experience with all the basic action systems. At every point in development, perception guides behavior by providing feedback about the results of just prior movements and information about what to do next. Across development, new motor behaviors provide new inputs for perception. Thus, motor development opens up new opportunities for acquiring knowledge and acting on the world, instigating cascades of developmental changes in perceptual, cognitive, and social domains. This article is categorized under: Cognitive Biology > Cognitive Development Psychology > Motor Skill and Performance Neuroscience > Development.

Biomechanical Characteristics of the Typically Developing Toddler Gait: A Narrative Review

Independent ambulation is one of the most important motor skills in typically developing toddlers. Gait analysis is a key evaluation method in basic and clinical research. A narrative review on the literature of toddler gait development was conducted following inclusion criteria, explicitly including the factors of English article, age range, no external intervention during the experimental process of studies involved, the non-symptomatic toddler, and no pathological gait. Studies about toddlers' morphological, physiological, and biomechanical aspects at this developmental stage were identified. Remarkable gait characteristics and specific development rules of toddlers at different ages were reported. Changes in gait biomechanics are age and walking experience-dependent. Gait patterns are related to the maturation of the neuro and musculoskeletal systems. This review thus provides critical and theoretical information and the nature of toddler walking development for clinicians and other scientific researchers. Future studies may systematically recruit subjects with more explicit criteria with larger samples for longitudinal studies. A particular design could be conducted to analyze empirically before practical application. Additionally, the influence of external interventions on the development of toddler gait may need consideration for gait development in the toddler cohort.

The Impact of Different Periods of Walking Experience on Kinematic Gait Parameters in Toddlers

This study aimed to analyse the kinematic differences in gait between three groups of toddlers who differed in their weeks of independent walking (IW) experience, but not in anthropometrical characteristics, to determine the relationship between walking experience without the side effect of morphological differences on gait parameters. Twenty-six toddlers participated in this study. Depending on the week of their IW, toddlers were divided into three groups: Group 1 (1-5 weeks of IW), Group 2 (6-10 weeks of IW), and Group 3 (11-15 weeks of IW). Each toddler walked barefooted over a 2-m long pathway, and 3D kinematic data were obtained. A decrease in the upper limb position, hip flexion, and step width, i.e., changes towards the adult gait pattern, were observed in Group 3. Less experienced walkers exhibited a wider step width despite no statistically significant difference in body mass and height between groups. Results of this study show no statistically significant difference in step length between groups, suggesting that step length is more related to height than to the walking experience. The increased step length in more experienced walkers reported in previous studies may therefore be a result of different heights and not walking experience.

Simple models highlight differences in the walking biomechanics of young children and adults

Adults conserve metabolic energy during walking by minimizing the step-to-step transition work performed by the legs during double support and by utilizing spring-like mechanisms in their legs, but little is known as to whether children utilize these same mechanisms. To gain a better understanding, we studied how children (5-6 years) and adults modulate the mechanical and metabolic demands of walking at their preferred speed, across slow (75%), preferred (100%) and fast (125%) step frequencies. We quantified (1) the positive mass-specific work done by the trailing leg during step-to-step transitions and (2) the leg's spring-like behavior during single support. On average, children walked with a 36% greater net cost of transport (COT; J kg-1 m-1) than adults (P=0.03), yet both groups increased their net COT at varying step frequencies. After scaling for speed, children generated ∼2-fold less trailing limb positive scaled mechanical work during the step-to-step transition (P=0.02). Unlike adults, children did not modulate their trailing limb positive work to meet the demands of walking at 75% and 125% of their preferred step frequency. In single support, young children operated their stance limb with much greater compliance than adults ( versus 11.35; P=0.023). Our observations suggest that the mechanics of walking in children aged 5-6 years are fundamentally distinct from the mechanics of walking in adults and may help to explain a child's higher net COT. These insights have implications for the design of assistive devices for children and suggest that children cannot be simply treated as scaled-down versions of adults.

Toe walking in children with cerebral palsy: a possible functional role for the plantar flexors

Equinus and toe walking are common locomotor disorders in children with cerebral palsy (CP) walking barefoot or with normal shoes. We hypothesized that, regardless of the type of footwear, the plantar flexors do not cause early equinus upon initial foot contact but decelerate ankle dorsiflexion during weight acceptance (WA). This latter action promoted by early flat-foot contact is hypothesized to be functional. Hence, we performed an instrumented gait analysis of 12 children with CP (Gross Motor Function Classification System class: I or II; mean age: 7.2 yr) and 11 age-matched typically developing children. The participants walked either barefoot, with unmodified footwear (4° positive-heel shoes), or with 10° negative-heel shoes (NHSs). In both groups, wearing NHSs was associated with greater ankle dorsiflexion upon initial foot contact, and greater tibialis anterior activity (but no difference in soleus activity) during the swing phase. However, the footwear condition did not influence the direction and amplitude of the first ankle movement during WA and the associated peak negative ankle power. Regardless of the footwear condition, the CP group displayed 1) early flattening of the foot and ample dorsiflexion (decelerated by the plantar flexors) during WA and 2) low tibialis anterior and soleus activities during the second half of the swing phase (contributing to passive equinus upon foot strike). In children with CP, the early action of plantar flexors (which typically decelerate the forward progression of the center of mass) may be a compensatory mechanism that contributes to the WA's role in controlling balance during gait.NEW & NOTEWORTHY Adaptation to walking in negative-heel shoes was similar in typically developing children and children with cerebral palsy: it featured ankle dorsiflexion upon initial contact, even though (in the latter group) the soleus was always spastic in a clinical examination. Hence, in children with cerebral palsy, the early deceleration of ankle dorsiflexion by the plantar flexors (promoted by early flattening of the foot, and regardless of the type of footwear) may have a functional role.

Automated Measures of Force and Motion Can Improve Our Understanding of Infants’ Motor Persistence

Every day, young learners are confronted with challenges. The degree to which they persist in overcoming those challenges, and the different ways they persist, provides critical insights into the various cognitive, motoric, and affective processes that drive behavior. Here, we present a systematic overview of the methodologies that have been traditionally used to study persistence, and offer suggestions for new approaches to the study of persistence that will make strides in moving the field forward. We argue that automated measures of force and motion, which have long been used in the study of infants’ motoric behavior, can provide a means to unravel the psychological processes that guide infants’ trying behavior. To illustrate this, we present a case study that highlights the novel lessons to be learned by the use of automated measures of force and motion regarding infants’ persistence, along with an analysis of the benefits and drawbacks of this approach, as well as detailed instructions for application. In sum, we conclude that these measures, when used in conjunction with more traditional approaches, will provide creative new insights into the nature and development of early persistence.

Motor Development: Embodied, Embedded, Enculturated, and Enabling

Motor development and psychological development are fundamentally related, but researchers typically consider them separately. In this review, we present four key features of infant motor development and show that motor skill acquisition both requires and reflects basic psychological functions. ( a) Motor development is embodied: Opportunities for action depend on the current status of the body. ( b) Motor development is embedded: Variations in the environment create and constrain possibilities for action. ( c) Motor development is enculturated: Social and cultural influences shape motor behaviors. ( d) Motor development is enabling: New motor skills create new opportunities for exploration and learning that instigate cascades of development across diverse psychological domains. For each of these key features, we show that changes in infants' bodies, environments, and experiences entail behavioral flexibility and are thus essential to psychology. Moreover, we suggest that motor development is an ideal model system for the study of psychological development.

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.

Development (of Walking): 15 Suggestions

Although a fundamental goal of developmental science is to identify general processes of change, developmental scientists rarely generalize beyond their specific content domains. As a first step toward a more unified approach to development, we offer 15 suggestions gleaned from a century of research on infant walking. These suggestions collectively address the multi-leveled nature of change processes, cascades of real-time and developmental events, the diversity of developmental trajectories, inter- and intraindividual variability, starting and ending points of development, the natural input for learning, and the roles of body, environment, and sociocultural context. We argue that these 15 suggestions are not limited to motor development, and we encourage researchers to consider them within their own areas of research.

Variety Wins: Soccer-Playing Robots and Infant Walking

Although both infancy and artificial intelligence (AI) researchers are interested in developing systems that produce adaptive, functional behavior, the two disciplines rarely capitalize on their complementary expertise. Here, we used soccer-playing robots to test a central question about the development of infant walking. During natural activity, infants' locomotor paths are immensely varied. They walk along curved, multi-directional paths with frequent starts and stops. Is the variability observed in spontaneous infant walking a "feature" or a "bug?" In other words, is variability beneficial for functional walking performance? To address this question, we trained soccer-playing robots on walking paths generated by infants during free play and tested them in simulated games of "RoboCup." In Tournament 1, we compared the functional performance of a simulated robot soccer team trained on infants' natural paths with teams trained on less varied, geometric paths-straight lines, circles, and squares. Across 1,000 head-to-head simulated soccer matches, the infant-trained team consistently beat all teams trained with less varied walking paths. In Tournament 2, we compared teams trained on different clusters of infant walking paths. The team trained with the most varied combination of path shape, step direction, number of steps, and number of starts and stops outperformed teams trained with less varied paths. This evidence indicates that variety is a crucial feature supporting functional walking performance. More generally, we propose that robotics provides a fruitful avenue for testing hypotheses about infant development; reciprocally, observations of infant behavior may inform research on artificial intelligence.

The cost of simplifying complex developmental phenomena: a new perspective on learning to walk

Researchers can study complex developmental phenomena with all the inherent noise and complexity or simplify behaviors to hone in on the essential aspects of a phenomenon. We used the development of walking as a model system to compare the costs and benefits of simplifying a complex, noisy behavior. Traditionally, researchers simplify infant walking by recording gait measures as infants take continuous, forward steps along straight paths. Here, we compared the traditional straight-path task with spontaneous walking during 20 minutes of free play in 97 infants (10.75-19.99 months of age). We recorded infants' footfalls on an instrumented floor to calculate gait measures in the straight-path and free-play tasks. In addition, we scored videos for other critical aspects of spontaneous walking-steps per bout, shape of walking path, and step direction. Studying infant walking during free play incurred no cost compared with the straight-path task, but considerable benefits. Straight-path gait was highly correlated with spontaneous gait and both sets of measures improved with walking age, validating use of the straight-path task as an index of development. However, a large proportion of free-play bouts were too short to permit standard gait measures, and most bouts were curved with omnidirectional steps. The high prevalence of these "non-canonical" bouts was constant over development. We propose that a focus on spontaneous walking, the phenomenon we ostensibly wish to explain, yields important insights into the problems infants solve while learning to walk. Other areas of developmental research may also benefit from retaining the complexity of complex phenomena.

Mobility as the Purpose of Postural Control

Counteracting the destabilizing force of gravity is usually considered to be the main purpose of postural control. However, from the consideration of the mechanical requirements for movement, we argue that posture is adjusted in view of providing impetus for movement. Thus, we show that the posture that is usually adopted in quiet standing in fact allows torque for potential movement. Moreover, when performing a movement—either voluntarily or in response to an external perturbation—we show that the postural adjustments are organized both spatially and temporally so as to provide the required torque for the movement. Thus, when movement is performed skillfully, the force of gravity is not counteracted but actually used to provide impetus to movement. This ability to move one's weight so as to exploit the torque of gravity seems to be dependent on development and skill learning, and is impaired in aging.

The development of motor behavior

This article reviews research on the development of motor behavior from a developmental systems perspective. We focus on infancy when basic action systems are acquired. Posture provides a stable base for locomotion, manual actions, and facial actions. Experience facilitates improvements in motor behavior and infants accumulate immense amounts of experience with all of their basic action systems. At every point in development, perception guides motor behavior by providing feedback about the results of just prior movements and information about what to do next. Reciprocally, the development of motor behavior provides fodder for perception. More generally, motor development brings about new opportunities for acquiring knowledge about the world, and burgeoning motor skills can instigate cascades of developmental changes in perceptual, cognitive, and social domains. WIREs Cogn Sci 2017, 8:e1430. doi: 10.1002/wcs.1430 For further resources related to this article, please visit the WIREs website.