These skills emerge directly from reflexes and proceed in a cephalocaudal (head-down) and proximodistal (center-out) direction. Infants first control their heads, lifting them up to look around. Then they control their upper bodies, their arms, and finally their legs and feet.
Sitting develops gradually; it is a matter of developing the muscles to steady the top half of the body. By 3 months, most babies can sit propped up in someone’s lap. By 6 months, they can usually sit unsupported. Crawling is another example of the head-down and center-out direction of skill mastery. When placed on their stomachs, many newborns reflexively try to lift their heads and move their arms as if they were swimming. As they gain muscle strength, infants wiggle, attempting to move forward by pushing their arms, shoulders, and upper bodies against whatever surface they are lying on.
Usually by 5 months, infants add their legs to this effort, inching forward (or backward) on their bellies. Exactly when this occurs depends partly on how much “tummy time” the infant has had, which is affected by culture (Zachry & Kitzmann, 2011).
Between 8 and 10 months after birth, most infants lift their midsections and crawl (or creep, as the British call it) on “all fours”, coordinating the movements of their hands and knees. Crawling depends on experience as well as maturation. Some normal babies never do it, especially if the floor is cold, hot, or rough, or if they have always lain on their backs (Pin et al., 2007) It is not true that babies must crawl to develop normally.
All babies find some way to move before they can walk (inching, bear-walking, scooting, creeping, or crawling), but many resist being placed on their stomachs (Adolph & Berger, 2005) Overweight babies master gross motor skills later than thinner ones: Practice and balance is harder when the body is (Slining et al., 2010). As soon as they are able, babies walk, falling frequently but getting up undaunted and trying again, because walking is much quicker than crawling, and it has another advantage--- free hands (Adolph et al., 2012).
The dynamic system underlying every motor skill has three interacting elements. We illustrate those three here with walking.
- Muscle strength. Newborns with skinny legs and 3-month-olds buoyed by water make stepping movements, but 6-month-olds on dry land do not; their legs are too chubby for their underdeveloped muscles. As they gain strength they stand and then walk.
- Brain maturation. The first leg movements---kicking (alternating legs at birth and then both legs together or one leg repeatedly at about 3 months) ---occur without much thought. As the brain matures, deliberate leg action becomes possible.
- Practice. Unbalanced, wide-legged, short strides become a steady, smooth gait.
The last item, practice, is powerfully affected by caregiving before the first independent step. Some adults spend hours helping infants walk (holding their hands or the back of their shirts) or providing walkers (dangerous if not supervised).
Once toddlers are able to walk by themselves, they practice obsessively, barefoot or not, at home or in stores, on sidewalks or streets, on lawns or in mud. They fall often, but that does not stop them---“they average between 500 and 1,500 walking steps per hour so that by the end of each day, they have taken 9,000 walking steps and traveled the length of 29 football fields” (Adolph et al., 2003, p. 494).