In this study the relationship between morphological data and active drag, as measured on the MAD system (system to measure active drag), and the effect of a 2.5-year period of growth was examined in a group of children (mean age at the start of the study, 12.9 years). During this period the children showed a mean increase in height from 1.52 to 1.69 m, and in weight from 40.0 to 54.7 kg. Also the body cross-sectional area (Ap), previously reported to relate strongly to drag in a group of adult swimmers, showed an increase in size of 16%. However, the drag did not change; the mean drag force for all subjects swimming at 1.25 m•s−1 was 30.1 N (±2.37) in 1985 and 30.8 N (±4.50) in 1988. The increase in height resulted in a decrease in the Froude Number (Fr) and hence in a decrease in wave-making resistance. Furthermore, form indices derived from ship-building technology demonstrated changes that indicated a more streamlined body form. Therefore it was concluded that during growth a complex process takes place in which different factors determining drag, such as height, body shape (Cd), and Ap, change in directions, having opposite effects on drag.
The authors are with the Department of Exercise Physiology and Health, Faculty of Human Movement Sciences, Vrije Universiteit and Universiteit van Amsterdam, Amsterdam, The Netherlands.