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

The mean WWI of Chinese adolescents aged 13-18 years was 9.34 ± 1.14 cm/√kg and the mean PFI was -0.02.

• Sample consisted of 9204 adolescents aged 13-18 years from different regions of China.
• Participants were selected using stratified whole-cluster random sampling.
• WWI is described as 'a new type of index for assessing adolescents' body composition.'
• PFI was derived from assessments of grip strength, standing long jump, sit-up, sit and reach, 1000/800 m run, and 50 m dash.

Differences in PFI between different WWI groups were statistically significant in both the 13-15 and 16-18 age groups.

• Kruskal-Wallis H test was used to assess differences between WWI groups.
• H-value for the 13-15 age group was 19.673 (P < 0.001).
• H-value for the 16-18 age group was 29.177 (P < 0.001).
• Statistical analyses included t-test, Mann-Whitney U test, Kruskal-Wallis H test, Pearson correlation analysis, and curvilinear regression analysis.

The association between WWI and PFI followed an inverted 'U' curve, with the highest PFI of 0.021 occurring when WWI was 8.2.

• Both lower and higher WWI values were associated with negatively affected physical fitness.
• The peak PFI value of 0.021 was identified at a WWI of 8.2 cm/√kg.
• Curvilinear regression analysis was used to characterize the non-linear relationship.
• The inverted 'U' shape indicates an optimal WWI range for physical fitness in adolescents.

The effect of WWI on PFI was more pronounced in boys than in girls.

• Sex-stratified analyses revealed differential associations between WWI and PFI.
• Both sexes showed the inverted 'U' curve pattern, but the magnitude of the effect differed.
• The paper recommends that sex differences be considered in physical fitness promotion and intervention strategies.

Chinese adolescents' physical fitness levels show a decreasing trend year by year, motivating investigation of body composition indices like WWI.

• The declining physical fitness trend is described as hurting 'adolescents' physical health and future achievements.'
• WWI has 'received extensive attention in recent years' as an assessment tool for body composition.
• Little prior research had examined the association between WWI and PFI among Chinese adolescents at a nationwide level.
• The study assessed adolescents from different regions of China to provide nationally representative data.