The association between weight-adjusted waist index and physical fitness index among Chinese adolescents.

The physical fitness level of Chinese adolescents shows a decreasing trend year by year, which hurts adolescents' physical health and future achievements. The weight-adjusted waist index (WWI), as a new type of index for assessing adolescents' body composition, has received extensive attention in recent years. However, little research has been conducted on the association between WWI and the physical fitness index (PFI) among Chinese adolescents nationwide. In this study, 9204 adolescents aged 13-18 years from different regions of China were assessed for weight, waist circumference, grip strength, standing long jump, sit-up, sit and reach, and 1000/800 m run using stratified whole-cluster random sampling, and a 50 m dash was assessed. WWI and PFI of the participants were derived sequentially. The t-test, Mann-Whitney U test, Kruskal-Wallis H test, Pearson correlation analysis, and curvilinear regression analysis were used to analyze the associations existing between WWI and PFI. The WWI of Chinese adolescents aged 13-18 years was (9.34 ± 1.14) cm/kg; the PFI was - 0.02. Overall, the results of the analysis showed that the differences in PFI between the different WWI groups were statistically significant in the 13-15 and 16-18 age groups (H-value 19.673, 29.177, P-value < 0.001). The highest level of PFI in adolescents was 0.021 when the WWI was 8.2. The association between WWI and PFI among Chinese adolescents aged 13-18 years was characterized by an inverted "U" curve, with both lower and higher WWI negatively affecting physical fitness. The effect of WWI on PFI was more pronounced in boys than in girls. In the future, changes in WWI should be considered in the promotion and intervention of physical fitness, to keep WWI in the normal range and promote the improvement of physical fitness.