Adolph, K. E., Berger, S. E., & Leo, A. J. (2011). Developmental continuity? Crawling, cruising, and walking. Developmental science, 14(2), 306–318. https://doi.org/10.1111/j.1467-7687.2010.00981.x

This research examined developmental continuity between "cruising" (moving sideways holding onto furniture for support) and walking. Because cruising and walking involve locomotion in an upright posture, researchers have assumed that cruising is functionally related to walking. Study 1 showed that most infants crawl and cruise concurrently prior to walking, amassing several weeks of experience with both skills. Study 2 showed that cruising infants perceive affordances for locomotion over an adjustable gap in a handrail used for manual support, but despite weeks of cruising experience, cruisers are largely oblivious to the dangers of gaps in the floor beneath their feet. Study 3 replicated the floor-gap findings for infants taking their first independent walking steps, and showed that new walkers also misperceive affordances for locomoting between gaps in a handrail. The findings suggest that weeks of cruising do not teach infants a basic fact about walking: the necessity of a floor to support their body. Moreover, this research demonstrated that developmental milestones that are temporally contiguous and structurally similar might have important functional discontinuities.

Adolph, K. E., Hoch, J. E., & Cole, W. G. (2018). Development (of Walking): 15 Suggestions. Trends in cognitive sciences, 22(8), 699–711. https://doi.org/10.1016/j.tics.2018.05.010

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.

Ossmy, O., & Adolph, K. E. (2020). Real-Time Assembly of Coordination Patterns in Human Infants. Current biology : CB, 30(23), 4553–4562.e4. https://doi.org/10.1016/j.cub.2020.08.073

Flexibility and generativity are fundamental aspects of functional behavior that begin in infancy and improve with experience. How do infants learn to tailor their real-time solutions to variations in local conditions? On a nativist view, the developmental process begins with innate prescribed solutions, and experience elaborates on those solutions to suit variations in the body and the environment. On an emergentist view, infants begin by generating a variety of strategies indiscriminately, and experience teaches them to select solutions tailored to the current relations between their body and the environment. To disentangle these accounts, we observed coordination patterns in 11-month-old pre-walking infants with a range of cruising (moving sideways in an upright posture while holding onto a support) and crawling experience as they cruised over variable distances between two handrails they held for support. We identified infants' coordination patterns using a novel combination of computer-vision, machine-learning, and time-series analyses. As predicted by the emergentist view, the least experienced infants generated multiple coordination patterns inconsistently regardless of body size and handrail distance, whereas the most experienced infants tailored their coordination patterns to body-environment relations and switched solutions only when necessary. Moreover, the beneficial effects of experience were specific to cruising and not crawling, although both skills involve anti-phase coordination among the four limbs. Thus, findings support an emergentist view and suggest that everyday experience with the target skill may promote "learning to learn," where infants learn to assemble the appropriate solution for new problems on the fly.

Berger, S. E., & Adolph, K. E. (2007). Learning and development in infant locomotion. Progress in brain research, 164, 237–255. https://doi.org/10.1016/S0079-6123(07)64013-8

The traditional study of infant locomotion focuses on what movements look like at various points in development, and how infants acquire sufficient strength and balance to move. We describe a new view of locomotor development that focuses on infants' ability to adapt their locomotor decisions to variations in the environment and changes in their bodily propensities. In the first section of the chapter, we argue that perception of affordances lies at the heart of adaptive locomotion. Perceiving affordances for balance and locomotion allows infants to select and modify their ongoing movements appropriately. In the second section, we describe alternative solutions that infants devise for coping with challenging locomotor situations, and various ways that new strategies enter their repertoire of behaviors. In the third section, we document the reciprocal developmental relationship between adaptive locomotion and cognition. Limits and advances in means-ends problem solving and cognitive capacity affect infants' ability to navigate a cluttered environment, while locomotor development offers infants new opportunities for learning.

Atun-Einy O, Berger SE, Scher A. Pulling to stand: common trajectories and individual differences in development. Dev Psychobiol. 2012 Mar;54(2):187-98.

This longitudinal study of 27 infants examined the development of pulling-to-stand (PTS). In general, infants began PTS using a two-leg strategy and transitioned to a half-kneel strategy. As a group, infants showed no preference for either strategy at the onset of PTS, switching between strategies until half-kneeling became the dominant pattern about 1 month after the onset of PTS. Examination of individual developmental trajectories revealed variability in age at PTS onset, time between PTS onset and half-kneel strategy onset, duration of the two-leg strategy as the dominant pattern, time until the half-kneel strategy became the dominant pattern, shape of the transition between strategies (gradual vs. abrupt), and timing of PTS relative to onset of other motor milestones. We discuss variation in developmental trajectory in terms of adaptive behavior during the acquisition of new skills and as a process shaped by infants' unique experiences prior to and during the acquisition period.

Dewolf, A. H., Sylos Labini, F., Ivanenko, Y., & Lacquaniti, F. (2021). Development of Locomotor-Related Movements in Early Infancy. Frontiers in cellular neuroscience, 14, 623759. https://doi.org/10.3389/fncel.2020.623759

This mini-review focuses on the emergence of locomotor-related movements in early infancy. In particular, we consider multiples precursor behaviors of locomotion as a manifestation of the development of the neuronal networks and their link in the establishment of precocious locomotor skills. Despite the large variability of motor behavior observed in human babies, as in animals, afferent information is already processed to shape the behavior to specific situations and environments. Specifically, we argue that the closed-loop interaction between the neural output and the physical dynamics of the mechanical system should be considered to explore the complexity and flexibility of pattern generation in human and animal neonates.

Cappellini, G., Sylos-Labini, F., Dewolf, A. H., Solopova, I. A., Morelli, D., Lacquaniti, F., & Ivanenko, Y. (2020). Maturation of the Locomotor Circuitry in Children With Cerebral Palsy. Frontiers in bioengineering and biotechnology, 8, 998. https://doi.org/10.3389/fbioe.2020.00998

The first years of life represent an important phase of maturation of the central nervous system, processing of sensory information, posture control and acquisition of the locomotor function. Cerebral palsy (CP) is the most common group of motor disorders in childhood attributed to disturbances in the fetal or infant brain, frequently resulting in impaired gait. Here we will consider various findings about functional maturation of the locomotor output in early infancy, and how much the dysfunction of gait in children with CP can be related to spinal neuronal networks vs. supraspinal dysfunction. A better knowledge about pattern generation circuitries in infancy may improve our understanding of developmental motor disorders, highlighting the necessity for regulating the functional properties of abnormally developed neuronal locomotor networks as a target for early sensorimotor rehabilitation. Various clinical approaches and advances in biotechnology are also considered that might promote acquisition of the locomotor function in infants at risk for locomotor delays.

Dewolf, A. H., Sylos-Labini, F., Cappellini, G., Lacquaniti, F., & Ivanenko, Y. (2020). Emergence of Different Gaits in Infancy: Relationship Between Developing Neural Circuitries and Changing Biomechanics. Frontiers in bioengineering and biotechnology, 8, 473. https://doi.org/10.3389/fbioe.2020.00473

How does gait-specific pattern generation evolve in early infancy? The idea that neural and biomechanical mechanisms underlying mature walking and running differ to some extent and involve distinct spinal and supraspinal neural circuits is supported by various studies. Here we consider the issue of human gaits from the developmental point of view, from neonate stepping to adult mature gaits. While differentiating features of the walk and run are clearly distinct in adults, the gradual and progressive developmental bifurcation between the different gaits suggests considerable sharing of circuitry. Gaits development and their biomechanical determinants also depend on maturation of the musculoskeletal system. This review outlines the possible overlap in the neural and biomechanical control of walking and running in infancy, supporting the idea that gaits may be built starting from common, likely phylogenetically conserved elements. Bridging connections between movement mechanics and neural control of locomotion could have profound clinical implications for technological solutions to understand better locomotor development and to diagnose early motor deficits. We also consider the neuromuscular maturation time frame of gaits resulting from active practice of locomotion, underlying plasticity of development.

Brandone, A. C., Stout, W., & Moty, K. (2020). Intentional action processing across the transition to crawling: Does the experience of self-locomotion impact infants' understanding of intentional actions?. Infant behavior & development, 60, 101470. https://doi.org/10.1016/j.infbeh.2020.101470

Brandone, A. C., Stout, W., & Moty, K. (2020). Intentional acMotor developmental milestones in infancy, such as the transition to self-locomotion, have cascading implications for infants' social and cognitive development. The current studies aimed to add to this literature by exploring whether and how crawling experience impacts a key social-cognitive milestone achieved in infancy: the development of intentional action understanding. Study 1 used a cross-sectional, age-held-constant design to examine whether locomotor (n = 36) and prelocomotor (n = 36) infants differ in their ability to process a failed intentional reaching action. Study 2 (n = 124) further probed this question by assessing how variability in locomotor infants' experience maps onto variability in their failed intentional action understanding. Both studies also assessed infants' tendency to engage in triadic interactions to shed light on whether self-locomotion impacts intentional action understanding directly or indirectly via changes in infants' interactions with social partners. Altogether, results showed no evidence for the role of self-locomotion in the development of intentional action understanding. Locomotor and prelocomotor infants did not differ in their faileClearfield M. W. (2011). Learning to walk changes infants' social interactions. Infant behavior & development, 34(1), 15–25. https://doi.org/10.1016/j.infbeh.2010.04.008d action understanding or levels of triadic engagement (Study 1) and individual differences in days of crawling experience, propensity to crawl during play, and maximum crawling speed failed to predict infants' intentional action understanding or triadic engagement (Study 2). Explanations for these null findings and alternative influences on the development of intentional action understanding are considered.processing across the transition to crawling: Does the experience of self-locomotion impact infants' understanding of intentional actions?. Infant behavior & development, 60, 101470. https://doi.org/10.1016/j.infbeh.2020.101470

Kornafel, T., Paremski, A. C., & Prosser, L. A. (2022). Unweighting infants reveals hidden motor skills. Developmental science, e13279. Advance online publication. https://doi.org/10.1111/desc.13279

We investigated the role of rate limiting factors in development using walking as a model system. The achievement of bipedal posture and locomotion are among the most significant achievements in an infant's first year, with poor balance and weak muscles long proposed as the rate limiting factors. Compensating for either may reveal upright motor skill that has not yet emerged in the infant's natural repertoire. To probe this question, we unweighted prewalking infants and measured their performance in various standing and walking behaviors while unweighted compared to baseline. Our secondary objective was to determine if the influence of unweighting was related to infants' locomotor experience. Infants stood unsupported for longer durations with 20% or 40% unweighting. Infants took more independent steps and more steps with one hand held with 40% unweighting. No differences in transition to/from standing were observed. Locomotor experience was related to the influence of unweighting during cruising and walking with a push toy. This is the first report of more advanced motor skills-longer periods of unsupported standing and the emergence of independent walking-revealed by unweighting infants. We interpret our observations to suggest that the refinement of motor control needed to support bipedal posture and locomotion precedes the functional emergence of these skills in infants. In other words, the musculoskeletal components required for walking are slower to develop than the neurological factors - and consequently may be the rate limiters. We further suggest that training regimens including unweighting should be explored in infants with motor delays.

Chen, L. C., Metcalfe, J. S., Chang, T. Y., Jeka, J. J., & Clark, J. E. (2008). The development of infant upright posture: sway less or sway differently?. Experimental brain research, 186(2), 293–303. https://doi.org/10.1007/s00221-007-1236-1

Postural control is an important factor for early motor development; however, compared with adults, little is known about how infants control their unperturbed upright posture. This lack of knowledge, particularly with respect to spatial and temporal characteristics of infants' unperturbed independent standing, represents a significant gap in the understanding of human postural control and its development. Therefore, our first analysis offers a thorough longitudinal characterization of infants' quiet stance through the 9 months following the onset of independent walking. Second, we examined the influence of sensory-mechanical context, light touch contact, on infants' postural control. Nine typically developing infants were tested monthly as they stood on a small pedestal either independently or with the right hand lightly touching a stationary contact surface. In addition to the longitudinal study design, an age-constant sample was analyzed to verify the influence of walking experience in infant postural development without the confounding effect of chronological age. Center of pressure excursions were recorded and characterized by distance-related, velocity, and frequency domain measures. The results indicated that, with increasing experience in the upright, as indexed by walk age, infants' postural sway exhibited shifts in rate-related characteristics toward lower frequency and slower, less variable velocity oscillations without changing the spatial characteristics of sway. Additional touch contact stabilized infants' postural sway as revealed by decrease in sway position variance, amplitude, and area as well as lower frequency and velocity. These results were confirmed by the age-constant analysis. Taken together, our findings suggest that instead of progressively reducing the sway magnitude, infants sway differently with increasing upright experience or with additional somatosensory information. These differences suggest that early development of upright stance, particularly as it relates to increasing postural and locomotor experience, involves a refinement of sensorimotor dynamics that enhances estimation of self-motion for controlling upright stance.

Metcalfe, J. S., Chen, L. C., Chang, T. Y., McDowell, K., Jeka, J. J., & Clark, J. E. (2005). The temporal organization of posture changes during the first year of independent walking. Experimental brain research, 161(4), 405–416. https://doi.org/10.1007/s00221-004-2082-z

Although the development of upright posture has received considerable attention, the quiet stance of infants in their first months of learning this fundamental behavior has not been well studied. The purpose of the present study was to characterize the time evolutionary properties, or temporal organization, of these infants' unperturbed upright stance as well as to elucidate how somatosensory information influences that organization. Six healthy, full-term infants were tested monthly from walk onset until 9 months of independent walking experience while standing either independently or touching a static surface. The structure of sway was assessed through stabilogram-diffusion analysis using an exponential Ornstein-Uhlenbeck characterization. The results of this analysis revealed two new insights into postural development. First, the developmental changes in quiet stance involved a decreased rate at which sway decays to maximal variance, rather than an attenuation of the magnitude of that variance. Specifically, measures indexing amount of sway variance were significantly reduced when touching a static surface as compared with an independent stance condition, but revealed no change with increased walking experience. Further, a reduction in the average rate constant of decay indicated an increased influence of long time-scale sway corrections on the overall sway trajectory. Second, it was shown that, at early walk ages, the use of touch both reduced the amount of variance and shifted the rate constant of sway towards longer time-scale displacements. Taken in the context of previous research, these results support our conclusion that early postural development embodies the dual tasks of calibrating sensorimotor relations for estimation of self-motion as well as identification and tuning of control system properties.

Blanchet, M., Prince, F., & Messier, J. (2019). Development of postural stability limits: Anteroposterior and mediolateral postural adjustment mechanisms do not follow the same maturation process. Human movement science, 63, 164–171. https://doi.org/10.1016/j.humov.2018.11.016

There is increasing evidence that indicates a critical transition period for the maturation of postural control from the ages of 6-7 years. Some studies suggest that this transitional period may be explained by a change from a ballistic toward a sensory strategy, but the cause remains unknown. The purpose of this study was to investigate the influence of the transition period on dynamic postural control in a natural self-initiated leaning task under different sensory conditions. We evaluated the center of pressure (COP) displacement during maximum leaning in four directions (forward, backward, rightward, leftward) under three sensory conditions (eyes open, eyes closed and eyes closed standing on a foam). Three groups were tested: young children (4 years old), older children (8-10 years old) and adults (21-42 years old). The maximum COP excursion along the anteroposterior and mediolateral axes and the COP amplitude were analyzed. Young children showed smaller maximum anteroposterior and mediolateral COP excursion than other groups. Older children also exhibited a significantly smaller maximum excursion along the mediolateral direction but performed similar to adults along the anteroposterior direction. In a similar manner, the analysis of the COP amplitude did not indicate any differences between the groups along the anteroposterior axis. In contrast, along the mediolateral axis, the results showed developmental differences. Furthermore, the effect of sensory conditions was similar across the children's groups. Our results suggest an important plasticity period for the maturation of postural control mechanisms. Notably, our findings support the idea that the postural mechanisms controlling the anteroposterior axis reach maturity before the mechanisms involved in controlling the mediolateral axis.

Sway

Verbecque, E., Vereeck, L., & Hallemans, A. (2016). Postural sway in children: A literature review. Gait & posture, 49, 402–410. https://doi.org/10.1016/j.gaitpost.2016.08.003

Background and aim: Static posturography can serve as an easy and fast way to determine whether a child possesses sufficient balance control in different sensory conditions. Therefore, the aim of this review is to map age-related differences in postural sway during childhood in typically developing children, using static posturography and to provide an overview of the available (age-specific) reference values in scientific literature.

Methods: The search strategy was performed in five databases (Pubmed, Web of Science, ScienceDirect, Cochrane Clinical Trials, Medline (Ovid)) November 2nd 2014 and updated twice: March 16th 2015 and July 20th 2015. The following keywords were used: (children OR child) AND ("postural control" OR "postural stability" OR equilibrium OR posture OR "postural balance"[mesh]) AND ("quiet stance" OR standing OR stance OR "quiet stance" OR static) AND ("postural sway" OR posturography OR "body sway" OR stabilography OR "trunk sway" OR "medio-lateral sway" OR "antero-posterior sway"). Relevant studies were identified using predefined selection criteria, applied on title and abstract (phase 1) and on full text (phase 2), supplemented with reference screening after the second phase.

Results: A total of 14 studies met the criteria. This review showed three main findings: 1) during natural bipedal stance with the eyes open, all studies reported a decrease in postural sway with increasing age, with conflicting results on the (non-)linearity of its development, 2) with eyes closed, all children show more sway than with eyes open and 3) only four studies reported numeric sway values that could serve as reference values, mainly focusing on children aged five and older.

Conclusion: Considerable disagreement exists on the (non-)linearity of the development of postural sway in children. By choosing arbitrary age categories, it remains unclear between which age groups differences are situated. Future research is necessary to determine for which age groups age-specific reference values are relevant.

Aim: To study changes in muscular postural strategies and general motor behaviour during the transition to independent walking. Postural control was assessed at its two functional levels: (1) direction specificity, in which dorsal muscles are primarily activated when reaching forward; and (2) fine-tuning of direction specificity.

Method: In an explorative longitudinal study, surface electromyograms of the arm, trunk, and neck muscles of 28 typically developing infants were recorded during reaching while sitting. Each infant was assessed in three developmental phases: during pull-to-stand (T0), first independent steps (T1), and 1 month after T1 (T2). Motor behaviour was assessed using the Infant Motor Profile (IMP). The effect on developmental outcome measures (postural parameters and IMP) of the developmental phases (T0, T1, T2) was estimated using linear mixed-effects models.

Results: None of the postural parameters changed significantly over time. However, individual developmental trajectories showed infant-specific postural reorganizational changes. Total IMP score decreased between T0 and T1 (mean IMP score 95% and 91% respectively; p<0.001); between T1 and T2 IMP scores did not change (91% and 93%; p=0.073).

Interpretation: Typically developing infants do not show consistent patterns of postural reorganization but show individual muscular strategies during the transition to independent walking. However, signs of reorganization of general motor behaviour are present.

What this paper adds: Infants show signs of reorganization of motor behaviour when learning to walk. Infants show individual strategies of postural reorganization when learning to walk.

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