Comparison of hydrostatic weighing at residual volume and total lung capacity.

Hydrostatic weighing (HW) was performed at both residual volume (RV) and total lung capacity (TLC) (both measured on land) to determine if underwater weighting at extreme lung volumes affected the measurement of body density. Subjects were 72 middle-aged males (mean age = 43.4 yr) and 51 middle-aged females (mean age = 40.2 yr). Subjects were first assessed for underwater weight at RV for at least 10 trials. Subjects were than instructed to inspire maximally and hold their breath underwater for as long as they could. Three trials at TLC were used for assessment of underwater weight. Forced vital capacity and residual volume (oxygen dilution) were determined separately on land. Small but statistically significant differences in body density (Db) were observed with the use of RV (1.0354 g/ml for men and 1.0196 g/ml for women) vs TLC (1.0367 g/ml for men and 1.0221 g/ml for women) (p less than 0.05). Percent fat values for the RV and TLD data differed by only 0.5% for men and 0.9% for women. Results indicated that the difference between percent fat determination by HW at RV and TLC, was negligible. It was concluded that HW at TLC may be the method of choice for subjects who are uncomfortable with performing the technique of underwater weighing at RV.