The Trunk Index is the ratio of the thoracic trunk over the abdominal trunk. In other words, the trunk is divided at the waist into two sections and the area of each is measured by means of a planimeter on the standard somatotype photographs. Its discovery paved the way for solving three of the objections to somatotyping. It distinguished between endomorphy and mesomorphy; it remained constant from about the third year of life to old age, despite variations in fatness or leanness, and it opened the way for the complete objectification of somatotyping.
Sheldon's trunk index and the growth of the thoracic and lumbar trunk
Ann Hum Biol. 1979 Jul-Aug;6(4):315-36.
- 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.
A longitudinal study of the growth of trunk surface area measured by planimeter on standard somatotype photographs
Ann Hum Biol. 1976 Mar;3(2):181-6. Singh R.
- Thoracic and abdominal surface areas have been measured by planimeter on 340 photogrammetric somatotype photographs of 15 boys of the Harpenden Growth Study followed at regular intervals from 3 to 20 years of age. Mean distance and velocity curves are given, estimated by Patterson's method for extracting maximum information from mixed longitudinal data. Both areas showed a marked adolescent growth spurt with the age at peak coinciding with the age of peak height velocity of the same children. The correlation coefficients between thorax and abdominal surface areas rose from an average of about 0-3 at ages 5-8 to 0-5 at ages 9-12 and 0-7 at ages 13-19. Trunk index (thorax/abdomen area) fell gradually with age from 5 to 9 and thereafter remained constant.