How people grow in size and in shape
The images here help to understand the dynamic of growth and development after birth. Figure ‘A’ is the average distance curves of growth in height for healthy girls (dashed lines) and boys (solid lines). Distance is the amount of height achieved at a given age. In part ‘A’, the cartoon shows a child’s height being measured. The average length at birth of girls and boys is between 49-51 cm and their growth in length after birth is quite similar until puberty, which is the onset of sexual maturation. Girls puberty takes place, on average, two years earlier than puberty for boys. Until the boys achieve puberty they are shorter in height, on average, than girls. Part ‘B’ shows the velocity curves of growth in height for girls and boys. Velocity is the rate of growth at a given time, in this case shown as centimeters per year.
In figure ‘B’ the running figure represents velocity. The velocity curves show the five primary stages of the postnatal pattern of human growth. The postnatal stages are: ‘I’, Infancy; ‘C’, Childhood; ‘J’, Juvenile; ‘A’, Adolescence; ‘M’, Mature adult. Note the very rapid velocity of growth during Infancy and the steep deceleration in velocity with time. Another rapid increase in velocity takes place during adolescence, followed by another steep deceleration. There is also a small growth spurt for at the end of the Childhood stage. The small Childhood height spurt and the large Adolescence height spurt are unique features of human growth.
Figure ‘C’ is an illustration of body proportion changes from birth (‘0 ann.’), to age 2 years (‘2 ann.’), and 6 years (‘6 ann.’) elaborated by Carl Heinrich Stratz (1858-1924). The images are drawn to the same length/height so that the relative sizes of body segments are readily apparent. At birth, the infant’s head is about 25% of total body length. At birth, the relative length of the arms is greater than that of the legs. Relative length of the head and arms reduce progressively as the legs grow longer. These changes in relative lengths relate to behavior, as the infant cannot walk on two legs. By two years of age most infants are ‘toddlers’ and by age 6 they are almost as proficient in walking as are adults. [Barry Bogin]