The purpose of this study was to examine the age- and athletic event-related differences in trunk muscularity among junior and senior athletes. The cross-sectional areas (CSAs) of the rectus abdominis, lateral abdominal muscle group, psoas major and erector spinae were determined on magnetic resonance imaging for 188 junior athletes aged 12-18 years and 164 senior athletes aged ≥ 19 years in six different athletic events (male: archery, table tennis, Nordic combined, basketball, and wrestling; female: archery, table tennis, and gymnastics). To examine the age-related differences for each athletic event and sex, the subjects were classified into the following age groups: 12-15 years, 16-18 years and senior (≥ 19 years). The main results were as follows. 1) The magnitude relationships of trunk muscle CSA/height² differed among the age groups by athletic event and muscle. 2) The values of CSA/height² of the lateral abdominal muscle group and erector spinae were significantly larger in female gymnasts aged 12-15 years than in female athletes from the other athletic events. These results respectively indicated that 1) each trunk muscle has the potential to specifically develop according to the athletic events and 2) selective hypertrophy of the trunk muscles can be induced by training reflecting athletic event-specific characteristics in female junior high school athletes.

The objectives of this study were 1) to quantify the differences in body densities and percent body fat using various methods for evaluating body composition (e.g., underwater weighing (UWW), air displacement plethysmography (ADP), skinfold caliper (SKF) measurement, ultrasound (US), bioelectrical impedance analysis (BIA), and dual-energy x-ray absorptiometry (DXA)), and 2) to examine the relationship between trends of the differences in body density and percent body fat obtained by these methods and characteristics of morphology and body composition. To this end, the body compositions of 73 healthy male adults were measured using UWW, ADP, SKF, US, and BIA. Twenty-seven of these 73 subjects underwent further measurement using DXA. Differences in body densities determined with ADP, SKF, and US were compared with those measured using UWW as a reference, and the differences in percent body fat estimated with UWW, ADP, SKF, US and BIA were compared with those measured by DXA as a reference. The results of this study indicate that 1) ADP is useful as a method for evaluating body density, as the results differed insignificantly from the reference method and showed no systematic errors due to differences in morphological characteristics and body composition, and 2) UWW measurements exhibited the smallest difference in percent body fat from the reference method, however, more than in any other method, there were systematic errors due to differences in morphological characteristics and body composition, specifically, trunk composition.