Association between body roundness index and strength fitness in senior middle school students

Adolescent obesity poses a major challenge to declining strength fitness. Body Roundness Index (BRI) provides superior quantification of central adiposity over traditional metrics. However, evidence regarding the BRI-strength fitness correlation remains underexplored in youth populations.Therefore, this study aims to explore the association between BRI levels and muscular fitness in senior middle school students.

A cross-sectional study was conducted in 2024 among 3,822 senior senior middle school students (grades 10-12) from Jiangxi, Zhejiang, and Fujian provinces in China using stratified random cluster sampling. Data were collected through sociodemographic questionnaires, anthropometric measurements (height/waist circumference), and strength fitness tests (grip strength/sit-ups/standing long jump). Multivariable logistic regression and restricted cubic spline (RCS) models were used to analyze associations.

A total of 3,822 senior middle school students were included. Significant gender differences were found in grip strength, sit-ups, standing long jump, and BRI (all P < 0.01). The rates of substandard performance were 13.8% for grip strength, 19.7% for sit-ups, 17.4% for standing long jump, and 74.9% for overall strength fitness (P ₇₅ cutoff). Strength fitness deficits varied significantly across physical activity levels, grade, parental education, household income, and other sociodemographic factors (P < 0.05). Higher BRI levels (Q4) were associated with increased risk of failing sit-ups and standing long jump, especially in boys. BRI showed a non-linear dose-response relationship with grip strength, standing long jump, and overall strength fitness, and a linear negative association with sit-ups.

An appropriate BRI level may contribute to the enhancement of strength-related fitness, while an excessively high BRI could potentially inhibit strength performance. Therefore, more attention should be payed on abdominal fat management and healthy dietary habits promotion and physical activity. Meanwhile, gender differences require personalized intervention strategies.