Relationship between body composition and physical fitness among 13-year-old adolescents

This study aimed to examine sex differences in physical fitness and body composition, and to explore the relationship between body composition and physical fitness among 13-year-old adolescents. A cross-sectional study was conducted on 80 adolescents (42 girls, 38 boys; mean age 13.47 ± 0.28 years) from an urban primary school in Niš, Serbia. Physical fitness was assessed using the Eurofit battery: Sit-and-Reach, Standing Long Jump, Handgrip Strength, 30-Second Sit-Ups, 10×5 m Shuttle Run, and the 20-Meter Shuttle Run. Body composition was measured via bioelectrical impedance (body fat percentage - BF%) and skinfold caliper (sum of five skinfolds - S5SF). Independent t-tests, Pearson correlations, and multiple regressions were performed separately by sex. Boys significantly outperformed girls in muscular strength, explosive power, speed, and aerobic capacity (p < .001), while girls had higher flexibility (p = 0.035) and BF% (p < .001). S5SF and BF% were negatively correlated with most fitness tests, especially shuttle run, long jump, and sit-ups. BMI showed weaker and inconsistent associations but was positively linked to handgrip strength, particularly in boys. Regression analyses identified S5SF and BF% as stronger predictors of physical fitness outcomes than BMI, with sex-specific patterns. Excess adiposity adversely affects physical fitness in adolescents, with stronger predictive value for BF% and skinfolds than for BMI. These findings support the use of direct body composition measures and emphasize the need for sex-sensitive approaches in adolescent fitness monitoring and physical education.