Relationship between body mass index and physical fitness of children and adolescents in Xinjiang, China: a cross-sectional study

BACKGROUND: Xinjiang is an economically underdeveloped area in China, but the obesity rate of children and adolescents is increasing year by year. Physical fitness and body mass index (BMI) are very important factors for healthy development, whereas few studies focus on the relationship between them in this region. This study aimed to explore the relationship between physical fitness and BMI of children and adolescents aged 7 to 18 in Xinjiang. METHOD: A total of 17,356 children and adolescents aged 7–18 years were involved. BMI was divided into five levels by percentiles, from very low to very high. Physical fitness was evaluated by five indicators: grip strength, standing long jump, sit-and-reach, 50 m dash, and endurance running. Single-factor analysis of variance was used to compare the Z-scores of the five physical fitness indicators among different BMI levels for the four age groups by gender. A nonlinear quadratic regression model was used to evaluate the relationship between BMI and each indicator in the four age groups. RESULT: There is a significant correlation between the five health-related indicators (grip strength, standing long jump, sit and reach, 50 m dash, endurance run) at two age groups (13-15 yrs., 16-18 yrs) of children and adolescents in Xinjiang, China. The range of the Pearson coefficient is 0.048 ~ 0.744. For the other two age groups (7-9 yrs., 10-12 yrs.,) significant correlations are found only in some indicators, and the Pearson coefficient ranges from 0.002 to 0.589. The relationship between BMI and physical fitness presents an U-shaped or inverted U-shaped curve in most age groups(R(2) ranges from − 0.001 to 0.182. Children and adolescents with normal BMI score higher on physical fitness tests, and boys (R(2) ranges from − 0.001 to 0.182) are more pronounced than girls (R(2) ranges from 0.001 to 0.031). CONCLUSION: Children and adolescents with a BMI above or below the normal ranges have lower physical fitness than those with normal BMI. BMI and physical fitness have an U-shaped or inverted U-shaped curve relationship, and the impact is more evident in boys than girls. Targeted actions such as improving the quality of physical education classes, advocating students to keep a balanced diet and physical exercise should be taken designedly.