Body mass index and individual physical fitness tests in Taiwanese youth aged 9–18 years

To evaluate the relationship between body mass index (BMI) and four components of physical fitness in a national cross-sectional sample of Taiwanese youth.A subsample of 102 765 youth (50 940 girls, 51 825 boys) 9-18 years was selected from a nationwide sample. Height, weight and four fitness items were measured: sit and reach (flexibility), standing long jump (power), sit-ups (abdominal muscular strength/endurance), and 800/1 600 m run/walk (cardiorespiratory endurance). BMI was calculated. Sex-specific regressions using a non-linear quadratic model of each fitness item on BMI were done in four age groups: 9-10, 11-12, 13-15, 16-18 years.The relationship between BMI and fitness varied among tests. Lower fitness in three items (sit-ups, jump, distance run) was evident in boys and girls with higher BMIs in each age group. Slopes of decline with increasing BMI varied by age group and sex. Relationships became parabolic for the three fitness items with age and peaks of the parabola were sharper in adolescent boys than girls. Relationships for the sit and reach contrasted with the other tests and differed among age groups and between sexes.BMI significantly and differentially influenced individual fitness tests, but effects varied with age and sex. Higher BMIs were generally associated with lower fitness.