Skeletal maturity at onset of the adolescent growth spurt and at peak velocity for growth in height: a threshold effect?

In 191 Polish boys of the Wroclaw Growth Study, the relationship between skeletal age and chronological age was examined at the onset of the adolescent growth spurt (take-off) and at peak velocity of height growth (PHV). It was found that, at PHV, skeletal age is markedly less variable than is chronological age, but at take-off no such reduction in variability is visible. The following interpretation of this finding is proposed. The onset of the spurt depends, ultimately, upon some maturational processes going on in the hypothalamus and shows little relationship with the advancement of the long bones at that time. Therefore, the spurt can begin at any level of skeletal maturity within the range normally observed at the chronological age at which it happens to begin in the individual. Peak height velocity, on the other hand, is reached when skeletal maturity is sufficiently advanced for testosterone to change its influence upon the bones from one which consists in stimulating cartilage growth to one which consists in stimulating epiphyseal fusion. Therefore, PHV is bound to occur within a range of skeletal maturity much more restricted than that within which take-off can occur.