Analysis of the growth spurt at age seven (mid-growth spurt).

A statistical analysis of the mid-growth spurt from the data of the First Zurich Longitudinal Study is presented. A small but significant mid-growth spurt was found in most height and length measurements and in some girth and width measurements, such as chest circumference, bihumeral and biiliac width. The mid-growth spurt seemed to be slightly more pronounced in boys than in girls. The peak of the mid-growth spurt occurred between 6.5 and 8.5 years. The height of the peak varied from 0.3 to 0.7 cm/year for the different measurements (estimated from the smoothed median velocity curves). In a peak-centered analysis a mid-growth spurt of standing height was observed in two thirds of the children. It occurred about one year earlier in girls than in boys, and averaged 1.4 cm/year. The data indicate that the mid-growth spurt is due to a transient increased growth rate of the bones, particularly the long bones, and possibly of fat and muscle tissue. In contrast to the growth spurt of the extremities, the spurt of the rump height is not a true growth phenomenon, but the result of a postural change occurring at about 6.5 years of age.