Patterns of Physical Growth
The most rapid period of growth occurs immediately after birth, and then the growth rate slows to a modest, steady process during childhood. This is followed by an adolescent growth spurt and then by deceleration until growth finally stops. There is little difference in the relative growth rates of boys and girls during childhood. However, during childhood boys are slightly taller and heavier than girls of the same age. This difference is a relatively minor one and of no real practical significance for sport performance.
When girls experience the rapid growth spurt that occurs be¬tween the ages of ten and a half and thirteen, they become taller than boys. During pubescence, tall girls will be taller than tall boys, and all girls will be taller than the shortest 3 to 5 percent of boys. This is a temporary situation that changes when boys begin to experience their adolescent growth spurt in height some two years after girls have experienced their peak velocity in gaining height.
Prior to adolescence, sex differences in body composition are minor. However, boys do have slightly more bone and muscle tissue and less fat than girls. Following the period of maximum gain in height that occurs in early adolescence (about age twelve to thirteen for girls and fourteen to fifteen for boys), there is a period of maximum gain in weight. In girls this is due primarily to a large increase in body fat, with a relatively small increase in muscle tissue. In boys the rapid gain in body weight that follows a rapid gain in height is due to a decrease in body fatness and a striking in¬crease in muscle mass. Consequently, post-adolescent girls have only about two-thirds as much muscle as males, and young adult females have almost twice the amount of body fat.
Because boys on the average begin their rapid gain in height at the age of twelve and a half, they have about two more years of preadolescent growth than girls have. During this two-year period, they continue to grow, and at age fourteen or fifteen they are about Four inches taller than girls were when they began their rapid growth. In the immediate preadolescent period, boys' legs grow much faster than their trunks. Thus, the longer period of preadolescent growth for males is ¬responsible for the fact that legs of adult males are longer than those of females.
The age at which the adolescent growth spurt begins varies widely from one child to another. The variation is so great in a sample of normal males, for example, that some boys may have their most rapid growth as early as their twelfth birthday, whereas others will not have this growth experience until they are nearly sixteen. These slower-maturing boys will not have their muscle growth and rapid gain in body weight until more than fourteen months later, at seventeen or eighteen years of age. A very normal but slowly maturing young male will have completed high school, before he is physically suited to compete in many sports requiring large size, strength, and a mature skeleton.
The differences in age at which adolescent growth and physical development occur are most evident during junior high school or middle school, or at twelve to fifteen years of age. Normal boys can vary as much as fifteen inches in height, ninety pounds in weight, and five years in maturation status, that is, biologic age. (Biologic age is commonly determined by an X-ray examination of skeletal maturation.) Most youth sport programs match competitors on the basis of calendar age. Therefore, large numbers of boys who do not experience their growth and maturation close to the average risk some very significant problems. This is true for both the slow later maturer and the advanced early maturer.