Physical Fitness Index and body mass index: A cross-sectional study based on 1.3 million college students

To explore differences in physical fitness levels among different grade groups and identify the association between body mass index (BMI) and physical fitness index (PFI).

This study collected data from 1307857 participants (600999 females & 706858 males) who were undergraduate students aged from 18 to 24 years old. PFI was calculated using the z-scores of 5 sex-specific physical fitness items, namely sit-and-reach, broad jump, pull-up/sit-up, 50-meter dash, and 800/1000-meter run. BMI was classified into 4 categories based on the Asian standards recommended by the WHO: (1) BMI < 18.5 kg/m2; (2) 18.5 kg/m2 ≤ BMI < 23 kg/m2; (3) 23 kg/m2 ≤ BMI < 25 kg/m2; (4) BMI ≥ 25 kg/m2. ANOVA was used to detect variations in PFI among BMI categories and differences in physical fitness levels within grade groups. Finally, quadratic models were constructed to explore the association between BMI and PFI.

(1) An inverted "J" shape association was identified between BMI and PFI. (2) Both boys and girls in higher grades had higher PFI than those in lower grades.

The association between BMI and PFI is non-linear. Physical development and physical activity engagement may assist in improving the physical fitness level of college students. Therefore, colleges should foster a more physical activity-friendly environment to reduce overweight and obesity rates, thereby enhancing fitness levels.