The purpose of this study was to evaluate the linear and curvilinear relationship
between body mass index (BMI) and physical fitness in children and
adolescents.
Participants were 4567 (2505 girls) children aged 6–16 years. Weight and
height were measured and BMI was calculated and adjusted for age and sex using
WHO z-scores. Physical fitness was evaluated with the following tests: Yo-Yo test,
standing long jump, seated 2-kg medicine ball throw, and 20-m dash. Participants
were grouped into two groups according to their maturity status, estimated as years
from peak height velocity. Associations were determined with linear and nonlinear
quadratic regression models.
The nonlinear quadratic regression coefficient was significant for the 20-m
dash among girls of both maturity status levels, and in maturity status level 1 boys;
for the standing long jump among boys of both maturity status levels, and in maturity
status level 1 girls. The Yo-Yo test was only significant for maturity status level 1
boys. For the medicine ball throw, the linear regression coefficient was significant for
both maturity status levels in both sexes. Almost all physical fitness items were
observed to have meaningfully large nonlinear relationships with BMIz, but they
were not all significant due to the small sample size, especially in maturity status
level 2.
Conclusion: The association between BMI and physical fitness is nonlinear in the
majority of its components, and those with lower and higher BMI had poorer fitness.
