Analysis of the adolescent growth spurt using smoothing spline functions.

Height growth velocity curves between 4.5 and 17.75 years were estimated, using smoothing spline functions, for 112 boys and 110 girls from the Zurich Longitudinal Study (1955--1976). Parameters characterizing the growth process, such as peak height velocity and age at peak height velocity, were calculated directly from the estimated curves. The variability of parameters describing the adolescent growth spurt is large, both between and within sexes. Peak height, defined as increase of height velocity during the growth spurt, and age at peak height velocity both characterize the sex difference in growth in a highly significant manner. Peak height of at least 4 cm/year is found in 70% of the boys, but in only 11% of the girls. The age at peak height velocity averages 12.2 years in girls and 13.9 years in boys and has a wide range of 5.7 years and 3.8 years respectively. The sex difference in adult height of 12.6 cm is composed of the following 4 factors: +1.6 cm caused by more prepubertal growth in boys, +6.4 cm by the boys' delay in spurt, +6.0 cm by the more extensive spurt in boys and -1.4 cm by more post-spurt growth in girls. Correlations between parameters indicate that the adult height depends neither on the duration of growth, nor on the duration and height of the peak. Minimal pre-spurt height velocity and peak height velocity, but not peak height, are age- and height-dependent. Partial correlations given adult height reveal two compensating mechanisms between growth in the prepubertal and in the pubertal period. Small prepubertal height and low height velocity with respect to adult height are followed by a late adolescent spurt and vice versa. Small height at the onset of the spurt with respect to adult height is followed by a longer lasting, but not higher spurt and vice versa.