The biologic marker of early adolescence is puberty, defined as the developmental phase of physical maturation to full reproductive capacity. It is a time of rapid conspicuous changes in body size and appearance, which are associated with the maturation of internal reproductive structures and based on major alterations of endocrine function. The most visible somatic changes include the pubertal growth spurt and the development of secondary sex characteristics.
Most early adolescents experience a transient increase of growth velocity or growth rate of the body, which results in the attainment of adult size (Tanner). From infancy on, the annual rate of growth falls steadily throughout childhood, picks up again rather suddenly at the time of puberty (this is the “growth spurt”) and declines to zero in later adolescence, when skeletal maturation is completed and the growth potential exhausted. There are wide interindividual variations in the slope of the growth curve, the age at onset of the pubertal spurt, the shape of this spurt, and its peak growth velocity. Some of the variability is due to socioeconomic differences: generally, children and adolescents from higher socioeconomic levels provided with better nutrition, tend to grow more rapidly (Tanner; Takahashi). There are also marked sex differences. Whereas both boys and girls are of comparable stature until the end of childhood, their growth curves diverge in puberty. Boys start their growth spurt about two years later than girls do. Thus, at the onset of their own growth spurt, boys are already taller (on the average, by 10 cm) than girls usually are when they start growing faster. In addition, boys have, on the average, a higher peak growth velocity than girls do. Girls who are in the midst of their pubertal growth spurt tend to be taller than boys of the same age, but when the boys catch up in pubertal development, the girls fall behind and end up with a shorter adult stature than the boys.
The growth spurt is not restricted to body size. Practically all muscular and skeletal components of the body are included. Bone and muscle diameters increase, with boys showing a considerably greater increase in number and size of muscle cells than girls do. Consequently, physical strength increases rapidly in both sexes with boys significantly surpassing girls. Subcutaneous fat decreases in most boys and in some girls. All these changes combined result in the characteristic adult sex differences in body build: most conspicuously, the relatively greater height and broader shoulders of men, and the relatively wider hips of women. There are many internal sex differences (for instance, in blood chemistry) that develop in puberty, but they will not be discussed here.
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