During the adolescent spurt in height, for a year or more, the velocity of growth approximately doubles; a boy is likely to be growing again at the rate he last experienced about age two. The peak velocity of height (P.H.V., a point much used in growth studies) averages about 10.5 centimetres per year in boys and 9.0 centimetres in girls (about 4 and 3.4 inches, respectively), but this is the “instantaneous” peak given by a smooth curve drawn through the observations. The velocity over the whole year encompassing the six months before and after the peak is naturally somewhat less. During this year a boy usually grows between 7 and 12 centimetres (2.75 and 4.75 inches) and a girl between 6 and 11 centimetres (2.35 and 4.35 inches). Children who have their peak early reach a somewhat higher peak than those who have it late.

The average age at which the peak is reached depends on the nature and circumstances of the group studied more, probably, than does the height of the peak. In moderately well-off British or North American children at present the peak occurs on average at about 14.0 years in boys and 12.0 years in girls. Though the absolute average ages differ from population to population, the two-year sex difference always persists.

Practically all skeletal and muscular dimensions take part in the spurt, though not to an equal degree. Most of the spurt in height is due to acceleration of trunk length rather than of length of legs. There is a fairly regular order in which the dimensions accelerate; leg length as a rule reaches its peak first, followed by the body breadths, with shoulder width last. The earliest structures to reach their adult status are the head, hands, and feet.

The spurt in muscle, of both limbs and heart, coincides with the spurt in skeletal growth, for both are caused by the same hormones. Boys’ muscle widths reach a peak velocity of growth that is greater than that reached by girls. But, since girls have their spurt earlier, there is actually a period, from about 121/2 to 131/2, when girls on average have larger muscles than boys of the same age, as well as being taller. Simultaneously with the spurt there is a loss of fat, as described above.

The marked increase in muscle size in boys at adolescence leads to an increase in strength. Before adolescence, boys and girls are similar in strength for a given body size and shape; after, boys have much greater strength, probably due to development of more force per gram of muscle as well as to absolutely larger muscles. They also develop larger hearts and lungs relative to their size, a higher systolic blood pressure (the pressure resulting from a heart contraction), a lower resting heart rate, a greater capacity for carrying oxygen in the blood with more hemoglobin, and a greater power for neutralizing the chemical products of muscular exercise such as lactic acid. In short, the male becomes at adolescence more adapted for the tasks of hunting, fighting, and manipulating all sorts of heavy objects, as is necessary in some forms of food gathering.

It is as a direct result of these anatomical and physiological changes that athletic ability increases so much in boys at adolescence. The popular notion of a boy’s “outgrowing his strength” at this time has little scientific support. It is true that the peak velocity of strength is reached a year or so later than that of height, so that a short period may exist when the adolescent, having completed his skeletal and probably also his muscular growth, still does not have the strength of a young adult of the same body size and shape. But this is a temporary phase; considered absolutely, power, athletic skill, and physical endurance all increase progressively and rapidly throughout adolescence.