Respiratory muscle force and ventilatory function in adolescents.

In 94 girls and 90 boys, aged 12.5-20.3 yr, the relationship of respiratory pressures or forces with lung volumes and ventilatory flows was studied. There was great variability in respiratory muscle performance, which helps to explain differences in lung volumes between individuals. Respiratory muscle force increases almost proportionally with thoracic dimensions, so that inspiratory and expiratory pressures generated at the level of residual volume (RV), functional residual capacity (FRC) and total lung capacity (TLC) are approximately constant with age. In the oldest boys there is evidence that the continued increase in lung volumes when they stop growing is due to a 'muscularity effect'. Boys generate larger pressures than girls at all lung volumes. Thus boys attain a larger TLC, and in spite of narrower airways, the same peak expiratory flow and a larger FIV1/FVC ratio than girls. Effort independent flows (FEV1 and MMEF), however, are larger in girls.