Background Very low body mass, extreme mass changes, and extremely low per cent body fat are becoming increasingly common in many sports, but sufficiently reliable and accurate field methods for body composition assessment in athletes are missing.

Methods Nineteen female athletes were investigated (mean (SD) age: 19.5 (±3.3) years; body mass: 59.6 (±7.6) kg; height: 1.674 (±0.056) m; BMI: 21.3 (±2.3) kg/m²). Three observers applied diagnostic B-mode-ultrasound (US) combined with the evaluation software for subcutaneous adipose tissue measurements at eight ISAK sites (International Society for the Advancement of Kinanthrometry). Regression and reliability analyses are presented.

Results US measurements and evaluation of subcutaneous adipose tissue (SAT) thicknesses (including fibrous structures: D_included; n=378) resulted in an SE of estimate SEE=0.60 mm, R²=0.98 (p<0.001), limit of agreement LOA=1.18, ICC=0.968 (0.957–0.977). Similar values were found for D_excluded: SEE=0.68 mm, R²=0.97 (p<0.001). D_included at individual ISAK sites: at biceps, R²=0.87 and intraclass-correlation coefficient ICC=0.811 were lowest and SEE=0.79 mm was highest. Values at all other sites ranged from R²: 0.94–0.99, SEE: 0.42–0.65 mm, and ICC: 0.917–0.985. Interobserver coefficients ranged from 0.92 to 0.99, except for biceps (0.74, 0.83 and 0.87). Evaluations of 20 randomly selected US images by three observers (D_included) resulted in: SEE=0.15 mm, R²=0.998(p<0.001), ICC=0.997 (0.993, 0999).

Conclusions Subject to optimal choice of sites and certain standardisations, US can offer a highly reliable field method for measurement of uncompressed thickness of the SAT. High accuracy and high reliability of measurement, as obtained with this US approach, are essential for protection of the athlete’s health and also for optimising performance.