section_path: Biological development/Growth spurt
section_name: Growth spurt
paragraph_text: Growth spurt
  
  The physical development of girls during their teenage years can be broken down into three distinct stages. At the start, which generally coincides with the beginning of rapid growth, there is the development of breast buds and pubic hair. The peak period of physical growth occurs approximately one year later in concert with stage two of sexual maturity. Approximately 1 to 1.6 years after the onset of secondary sex characteristics, girls enter into the third stage which typically includes menarche. By this time, they will have finished their growth spurt and experience a notable broadening of the hips as well as an adult fat distribution. Additionally, breast development is complete and hair in both the pubic region and armpits (axillary hair) will be darker and more widespread. In comparison to girls, it can be tricky to define when exactly sexual development in boys begins.
  
  For boys, puberty typically takes around 5 years to finish, as opposed to just 3 + 1 ⁄ 2 years for girls (menarche). By this point in time, they have already experienced their growth spurt and there are evident changes in their body shape – wider hips and fat distribution is more adult-like. Breast development will also be completed by this stage. In boys, four stages in development can be correlated with the curve of general body growth at adolescence. The initial sign of sexual maturation in boys usually is the "fat spurt". The maturing boy gains weight and becomes almost chubby, with a somewhat feminine fat distribution. This probably occurs because estrogen production by the Leydig cells in the testes is stimulated before the more abundant Sertoli cells begin to produce significant amounts of testosterone. During this stage, boys may appear obese and somewhat awkward physically. Approximately 1 year after the scrotum begins to increase in size, stage II can be seen. During this time, there is a redistribution of subcutaneous fat and the start of pubic hair growth. Following 8 to 12 months of the peak velocity in height gain, stage III ensues. This period is marked by an obvious widenening of hips with a more adult-like fat distribution and full development of the breasts. All together, these three stages culminate in a complete growth spurt for most individuals. At this time, axillary hair appears and facial hair appears on the upper lip only. A spurt in muscle growth also occurs, along with a continued decrease in subcutaneous fat and an obviously harder and more angular body form. Pubic hair distribution appears more adult but has not yet spread to the medial area of the thighs. The penis and scrotum are near adult size. Stage IV for boys, which occurs anywhere from 15 to 24 months after stage III, is difficult to pinpoint. At this time, the spurt of growth in height ends. There is facial hair on the chin and the upper lip, adult distribution and color of pubic and axillary hair, and a further increase in muscular strength.
  
  The adolescent growth spurt is a rapid increase in the individual's height and weight during puberty resulting from the simultaneous release of growth hormones, thyroid hormones, and androgens. Males experience their growth spurt about two years later, on average, than females. During their peak height velocity (the time of most rapid growth), adolescents grow at a growth rate nearly identical to that of a toddler—about 10.3 cm (4 inches) per year for males and 9 cm (3.5 inches) per year for females. In addition to changes in height, adolescents also experience a significant increase in weight (Marshall, 1978). The weight gained during adolescence constitutes nearly half of one's adult body weight. Teenage and early adult males may continue to gain natural muscle growth even after puberty.
  
  The accelerated growth in different body parts happens at different times, but for all adolescents, it has a fairly regular sequence. The first places to grow are the extremities—the head, hands and feet—followed by the arms and legs, then the torso and shoulders. This non-uniform growth is one reason why an adolescent body may seem out of proportion.
  
  During puberty, bones become harder and more brittle. At the conclusion of puberty, the ends of the long bones close during the process called epiphysis. There can be ethnic differences in these skeletal changes. For example, in the United States, bone density increases significantly more among black than white adolescents, which might account for decreased likelihood of black women developing osteoporosis and having fewer bone fractures there.
  
  Another set of significant physical changes during puberty happen in bodily distribution of fat and muscle. This process is different for females and males. Before puberty, there are nearly no sex differences in fat and muscle distribution; during puberty, boys grow muscle much faster than girls, although both sexes experience rapid muscle development. In contrast, though both sexes experience an increase in body fat, the increase is much more significant for girls. Frequently, the increase in fat for girls happens in their years just before puberty. The ratio between muscle and fat among post-pubertal boys is around three to one, while for girls it is about five to four. This may help explain sex differences in athletic performance.
  
  Pubertal development also affects circulatory and respiratory systems as an adolescents' heart and lungs increase in both size and capacity. These changes lead to increased strength and tolerance for exercise. Sex differences are apparent as males tend to develop "larger hearts and lungs, higher systolic blood pressure, a lower resting heart rate, a greater capacity for carrying oxygen to the blood, a greater power for neutralizing the chemical products of muscular exercise, higher blood hemoglobin and more red blood cells".
  
  Despite some genetic sex differences, environmental factors play a large role in biological changes during adolescence. For example, girls tend to reduce their physical activity in preadolescence and may receive inadequate nutrition from diets that often lack important nutrients, such as iron. These environmental influences, in turn, affect female physical development.