Sheldon's trunk index and the growth of the thoracic and lumbar trunk.

The trunk index (TI), a ratio of the area of the thoracic trunk to that of the lumbar trunk, is measured on a somatotype photograph marked according to defined criteria. Photographs of 82 boys from the Harpenden Growth Study were measured at ages 5 to 18 years, in an order that obscured which photographs were of the same boy at different ages. Remeasurement two months later of 12 boys at each of ages 5, 11, and 18 years showed retest correlations of 0.97 or higher for thoracic and lumbar areas separately and of about 0.95 for their ratio. When 57 boys aged 17 to 20 years were measured by another worker, their TI values correlated 0.90 with those used in this study. Inter-age correlations among the unedited TI values were approximately 0.9 between ages a year apart and declined as age differences increased. Correlations with values at age 18 increased from about 0.7 at age 5 to 0.9 by age 16. Editing was done by remeasuring all values that deviated by more than 0.05 TI units from a regression line based on each subject's total array of values. In the edited data, correlations with TI values at age 18 increased, ranging from 0.8 at age 5 to 0.95 by age 16. Mean TI was quite stable, ranging only between 1.45 and 1.51 for the whole age span, with the lowest values appearing from 11 to 14 years. In 43 of the 59 boys whose series allowed determination of peak height velocity (PHV), a 'TI dip' appeared: one to three TI values fell more than 0.05 TI units below the boy's overall regression line shortly before PHV. Distance and velocity curves are given for growth of the thoracic and lumbar trunk areas. Peak velocity of growth of the lumbar area occurred on average a little earlier than that for the thoracic area; the TI dip was in part a result of this. Alterations of fat distribution as seen by skinfolds probably also contributed. Judging by their individual regression lines, about 80% of the boys showed no more than chance variation from a horizontal slope, their TI neither increasing nor decreasing overall. An additional 10% appeared to show significant slopes only because their series started or ended too near their TI dips. The remaining 10% of boys appeared to show real changes in TI as they grew. Examples of the most extreme changes are shown.