The Importance of Motor Skills for Development

Motor skills are important because they constitute infants’ behaviors. People typically think of motor development as the items on a standard milestone chart: the parade of basic postural and locomotor skills from rolling and sitting to crawling, standing, and walking (Fig. 1). But milestone charts do not represent the scope of motor development. Infants’ motor skills include every part of the body, from the eyes and other parts of the face down to the toes [for reviews, see 1–3]. Looking, smiling, suckling, eating, and using the mouth to “blow raspberries” and speak are all motor skills. Ditto for manual actions such as reaching, grasping, and exploring objects with hands and fingers, and likewise for actions with artifacts such as scribbling with a crayon, using a spoon, wielding a hammer, and pressing the buttons on a toy. Social and communicative actions (e.g., hugging a caregiver, petting a dog, pointing and nodding, or pretending to drink from a cup) also involve motor skills. Infant locomotion includes a mind-boggling
array of behaviors – pivoting in circles, belly crawling, inchworming, bum shuffling, knee walking, log rolling, pulling to stand, cruising, sliding, backing, and climbing. Similarly, infants’ postures (e.g., side-lying, mermaid posture, Wsit, ring-sit, long-sit, and short-sit positions) are so diverse that many postures commonly displayed by infants do not even have names. In short, all behavior is motor behavior. Thus, to the extent that behavior is important, motor skills are important. Moreover, behavior develops. So, motor development is really behavioral development. From day to day, new skills enter
and exit infants’ repertoires, and frequently used skills continually improve [for reviews, see 1–3]. As illustrated in Figure 1, newborns are stuck with their head in the mattress; 18 months later, toddlers are racing across the living room floor. Newborns track a proffered block with their eyes, 4-month-old infants swat at it, 8-month-old infants grasp and explore it, and 18-month-old toddlers build block towers [4]. Banging the high chair tray transforms into hammering a peg [5]. Coos and babbles become fully articulated speech. The coordination between breathing and sucking to nurse is replaced with new patterns of coordination to chew and swallow solid foods [6]. Overall, motor skill acquisition reflects dramatic improvements in coordination, strength, balance control, and – given the diversity of infants’ solutions – a great deal of ingenuity.
 

Motor skills are important because they provide a window into development. Generally, motor development is age related, as represented by the progression of skills in Figure 1. Age norms for skill onset provide a useful diagnostic tool because motor behavior is directly observable, skills are salient to caregivers and clinicians, and anomalies are correlated with many types of developmental disabilities. Indeed, most pediatricians’ offices and parenting books contain a milestone chart or table of age norms, and when infants’ onset ages fall beyond the normative range, clinicians and caregivers have cause for concern. The World Health Organization (WHO) even published “standards” (prescriptive age bands rather than descriptive norms) for infants’ postural and locomotor milestones [7].

However, age norms must be interpreted with caution. Age and motor experience
are highly correlated (older infants are also more experienced). When age and experience are unconfounded, experience trumps age as the key predictor 


of skill emergence and improvement [for reviews, see 1– 3] . The implications are
twofold. First, motor skill acquisition is not a direct readout of neuromuscular maturation. Atypical development (earlier or later skill onset ages) reflects a history of motor experiences interacting with physiology. Second, because cultural differences in child-rearing practices affect infants’ motor experiences, age norms should reflect worldwide diversity in child-rearing practices. But they do not. In Western and industrialized cultures, for example, caregivers handle newborns gently and support their head and trunk against gravity; caregivers allow infants freedom of movement through much of the 24-h day; and they expect infants’ motor skills to develop naturally without special training. But such child-rearing practices are not universal. In cultures that consider motor development as the result of exercise, caregivers deliberately train skills such as sitting and walking ( Fig. 2 a). Accordingly, infants achieve those milestones at earlier ages than would be expected based on western norms and the WHO standards [for reviews, see 1, 2, 8, 9] . Similarly, infants acquire postural and locomotor skills at earlier ages in cultures where caregivers use rough handling (lift babies by an arm or suspend infants by their ankles, for example), require infants to withstand gravity (e.g., hold infants without supporting their head), and expose infants to high-amplitude vestibular stimulation (e.g., carry them in slings while engaging in vigorous activities). Moreover, in experiments with random assignment to experimental and control groups, a few minutes of daily practice with upright stepping results in earlier onset of independent walking [10], and a few minutes of daily postural training
leads to accelerated postural, manual, and locomotor skills [11].

Whereas most western caregivers assume that freedom to move is important
for motor development, caregivers in some cultures do not [for reviews, see 1, 2,
8]. In rural China, caregivers bury supine infants up to their chests in sandbags [12], and in central Asia, caregivers bind infants neck to toe in a gahvora cradle for large parts of the 24-h day (Fig. 2b) [13]. Such constraint – even without social deprivation – delays postural and locomotor skills relative to western norms and the WHO standards. Even within the same culture, historical changes in child-rearing affect motor development. In western cultures, for example, the “Back to Sleep” campaign to put infants to sleep on their backs instead of their stomachs caused widespread delays in the emergence of rolling and crawling [14]. Merely wearing a diaper impedes walking compared with going naked, and old-fashioned cloth diapers present greater impediments than modern disposables [15]. Thus, attributions of “acceleration” or “delay” make sense only if the normative age bands include infants who experienced the same child-rearing practices. The first age norms were constructed from homogeneous groups of US infants. Although more racially and ethnically diverse, subsequent age norms and screening tests perpetuate the bias toward western, educated, middle-income populations [4]. The WHO standards [7] are based on data from 5 geographically, racially, and economically diverse countries (USA, Norway, India, Ghana, and Oman), but none have cultures that exercise or restrict infants’ movements.
 

As Piaget [16], Gibson [17], and others pointed out, motor skills are important
because they lay the foundation for psychological development. The developmental story does not end with the ability to sit, walk, reach, and so on. New motor skills are only the beginning. Each motor achievement unlocks new parts of the environment for exploration and alters infants’ interactions with objects, people, and places. New opportunities for learning, in turn, cascade into developments far afield from motor behavior [for reviews, see 1, 2, 8, 17, 18]. Advances in postural control literally broaden infants’ world view. Before infants can lift their heads, their visual field is limited to what is already in sight [19]. Head control allows infants to decide for themselves what merits
visual exploration. Sitting postural control expands infants’ view to include the whole room and the people and objects in it [20]. The ability to crawl allows infants to see what is around the corner and in the next room [17]. The transition from crawling to walking provides even greater visual access to the environment (Fig. 2c). While crawling, infants mostly see the ground in front of their hands; while walking, they can spot far-off objects and places to explore [20]. The benefits of upright posture and walking do not end with improved visual access [for reviews, see 1, 2, 8]. Compared with crawling, walking infants spend more time in motion, travel farther distances, visit more places, access more distant objects, carry objects to new locations, and venture farther away from caregivers. Walkers are also more likely to carry objects to share with their caregivers, to initiate more joint engagement with caregivers, and to pay more attention to caregivers’ gaze and points. Caregivers, in turn, are more likely to respond with language about what infants can do with the objects. Likely due to changes in language input, walking experience predicts infants’ receptive and productive language, controlling for infants’ age [21]. In addition, the onset of walking instigates surges in infant autonomy, self-efficacy, and control. Such links between locomotion and psychological development prompted a “mobility revolution” aimed at providing “ride-on cars” to children with disabilities to enable independent mobility [22]. Postural control also jump-starts a cascade of developments for manual actions [for reviews, see 1, 2]. Sitting postural control facilitates reaching, grasping, and visual-manual-oral object exploration. Object exploration, in turn, leads to new object knowledge. For example, infants with more sitting experience spontaneously produce more exploratory behaviors that reveal the threedimensional form of objects. These exploration skills predict longer looking at displays that reveal unexpectedly hollow three-dimensional shapes [23]. Object exploration skills also facilitate cross-modal perception of object properties [24], discrimination of object boundaries [25], and mental rotation of objects [26]. Moreover, the ability to grasp objects (developed over time or experimentally boosted) helps infants understand the intent of others’ manual actions [27]. Motor skill acquisition may also affect infants’ body growth and health. Babies who move more have less belly fat [28]. But movement may also come at a cost. Crawling and walking momentarily churn up dust and debris, and some particles are detrimental if inhaled. Because crawlers’ heads are so close to the  ground [20], they are more likely to inhale particulates than walking infants or adults [29]. Thus, particulates related to respiratory infections and asthma can land deep in the respiratory system [29].

The Development of Motor Skills Makes Behavior Functional and Flexible

Perhaps most important, the development of motor skills makes behavior more functional and flexible [for reviews, see 1—3, 8]. Function is critical for the activities of daily living, and behavioral flexibility is imperative because bodies and environments are continually in flux. Infants can grow up to 2 cm on a single day [30]. Infants' clothing, footwear, and the objects they carry can change from hour to hour and moment to moment. Ground surfaces can be slippery, sloping, or cluttered. The layout includes doorways, elevations, and barriers. Objects vary in size, shape, and texture. Changes in either body or environment alter the biomechanics of movement, and thus alter the possibilities for action. Social partners introduce further variability into the mix.

In sum, for behavior to be functional and flexible, infants must tailor action to changes in local conditions [for reviews, see 1-3, 8]. They must select, modify, and, in many instances, create appropriate actions on the fly. Perception and cognition are required to guide actions adaptively. Infants must perceive what is out there and decide what to do about it. Behavioral flexibility is an essential part of motor skill acquisition and requires immense amounts of practice and learning.

For example, when infants first begin crawling and walking, they do not perceive possibilities for locomotion. In laboratory experiments, they plunge headlong over the brink of impossibly steep slopes and high drop-offs. Over weeks of crawling and walking, infants' motor decisions become increasingly accurate (Fig. 2d). After 20 weeks or so of everyday crawling or walking experience, infants perceive possibilities for locomotion with impressive precision. They distinguish safe from risky slopes based on 2° differences in slant, and safe from risky drop-offs based on 1-cm differences in height. Infants can even update their assessments to take experimentally induced changes in their bodies into account (e.g., lead-weighted shoulder packs and Teflon-soled shoes that decrease their ability to walk down slopes). Moreover, infants can create new means to cope with novel tasks - learning in the moment to slide down steep slopes in a sitting or backing position, and so on.

How do they do it? Through massive amounts of everyday, time-distributed, variable practice and by generating the requisite perceptual information through exploratory actions [for reviews, see 1-3, 8]: each hour, the average toddler accumulates 2,400 steps and visits most of the surfaces and places in the accessible environment. In real time, infants direct their gaze at the obstacle, modify their gait as they approach the precipice, obtain tactile and proprioceptive information from touching the obstacle, and they test various means before deciding whether and how to go.

Conclusions

Motor skills are part and parcel of development. They both constitute behavior and instigate the changes that comprise behavioral development. Although motor skills are a window into typical development, skills are malleable and shaped by experience. Motor skills also create new experiences that facilitate developments in psychological domains seemingly remote from motor behavior. Finally, motor development ensures that behavior can be sufficiently functional and flexible to cope with a variable body in a variable world.

Conflict of Interest Statement

The authors report no conflicts of interest. Research support to K.E.A. from 2017-2020 includes: Templeton Foundation, DARPA HR001119S0005-MCS-FP-035, NICHD F32 NRSA, NICHD R01-HD-094830, NICHD R01-HD-033486, NIDCD R01-DC016557, NICHD R01-HD086034, James S. McDonnell Foundation JSMF-220020558, KEEN Foundation, LEGO Foundation, Sloan Foundation, NSF/SBE-BSF-1627993, NSF BCS- 1528831, NICHD U01-HD076595, NSF BCS-1238599. 

 
References
1    Adolph KE, Robinson SR: Motor development; in Liben L, Muller U (eds): Handbook of Child Psy-chology and Developmental Science, ed 7. Hobo-ken, Wiley, 2015, pp 113-157.
2    Adolph KE, Hoch JE: Motor development: em-bodied, embedded, enculturated, and enabling. Annu Rev Psychol 2019;70:141-164.
3    Adolph KE, Berger SE: Physical and motor devel-opment; in Bornstein MH, Lamb ME (eds): De-velopment Science: An Advanced Textbook, ed 7. Milton Park, Psychology Press/Taylor & Francis, 2015, pp 261-333.
4    Bayley N: Bayley Scales of Infant and Toddler Development, ed 3. San Antonio, The Psychologi-cal Corporation, 2006.
5    Lockman JJ, Kahrs BA: New insights into the de-velopment of human tool use. Curr Dir Psychol Sci 2017; 26: 330-334.
6    Wilson EM, Green JR, Yunosova Y, et al: Task specificity in early oral motor development. Semin Speech Lang 2008;29:257-266.
7    Martorell R, de Onis M, Martines J, et al: WHO motor development study: windows of achievement for six gross motor development milestones. Acta Paediatr 2006; 95: 86-95.
8    Adolph KE, Hoch JE, Cole WG: Development (of walking): 15 suggestions. Trends Cogn Sci 2018; 22: 699-711.
9    Super CM: Environmental effects on motor de-velopment: the case of “African infant precocity”. Dev Med Child Neurol 1976; 18: 561-567. 
10   Zelazo PR, Zelazo NA, Kolb S: “Walking” in the newborn. Science 1972;176: 314-315.
11   Lobo MA, Galloway JC: Enhanced handling and positioning in early infancy advances development throughout the first year. Child Dev 2012; 83: 1290-1302.
12   Mei J: The northern Chinese custom of rearing babies in sandbags: implications for motor and intellectual development; in van Rossum JHA, Laszlo JI (eds): Motor Development: Aspects of Normal and Delayed Development. Amsterdam, VU Uitgeverij, 1994, pp 41-48.
13   Karasik LB, Tamis-LeMonda CS, Ossmy O, et al: The ties that bind: cradling in Tajikistan. PLoS One 2018; 13:e0204428.
14   Davis BE, Moon RY, Sachs HC, et al: Effects of sleep position on infant motor development. Pediatrics 1998;102: 1135-1140.
15   Cole WG, Lingeman JM, Adolph KE: Go naked: diapers affect infant walking. Dev Sci 2012; 15: 783-790.
16   Piaget J: The Origins of Intelligence in Children. New York, International Universities Press, 1952.
17  Gibson EJ: Exploratory behavior in the development of perceiving, acting, and the acquiring of knowledge. Annu Rev Psychol 1988; 39: 1-41.
18  Campos JJ, Anderson DI, Barbu-Roth MA, et al: Travel broadens the mind. Infancy 2000; 1: 149219.
19 Jayaraman S, Fusey CM, Smith LB: Why are faces denser in the visual experiences of younger than older infants? Dev Psychol 2017; 53:38-49.
20  Kretch KS, Franchak JM, Adolph KE: Crawling and walking infants see the world differently. Child Dev 2014; 85:1503-1518.
21    Walle EA, Campos JJ: Infant language development is related to the acquisition of walking. Dev Psychol 2014; 50: 336-348.
22    Logan SW, Hospodar CM, Feldner HA, et al: Modifed ride-on car use by young children with disabilities. Pediatr Phys Ther 2018; 30: 50-56.
23    Soska KC, Adolph KE, Johnson SP: Systems in development: motor skill acquisition facilitates three-dimensional object completion. Dev Psychol 2010; 46: 29-138.
24    Eppler MA: Development of manipulatory skills and the deployment of attention. Infant Behav Dev 1995;18: 391-405.
25    Needham AW: Improvements in object exploration skills may facilitate the development of object segregation in early infancy. J Cogn Dev 2000; 1: 131-156.
26    Mohring W, Frick A: Touching up mental rotation: effects of manual experience on 6-month- old infants' mental object rotation. Child Dev 2013; 84: 1554-1565.
27    Sommerville JA, Woodward AL, Needham A: Action experience alters 3-month-old infants' perception of others' actions. Cognition 2005; 96:B1-B11.
28    Benjamin-Neelon SE, Bai J, Ostbye T, et al: Physi-cal activity and adiposity in a racially diverse co-hort of US infants. Obesity (Silver Spring) 2020; 28: 631-637.
29    Wu T, Taubel M, Holopainen R, et al: Infant and adult inhalation exposure to resuspeded biological particulate matter. Environ Sci Technol 2018; 52:237-247.
30    Lampl M: Evidence of saltatory growth in infancy. Am J Hum Biol 1993;5: 641-652.