Introduction: A sedentary lifestyle is becoming a major societal issue. Against a backdrop of increasing inactivity, physical fitness is now a key determinant of health as well as a priority for national and European public policies, particularly those targeting young people. The objective of our study was to analyze the relationship between this health determinant and body mass index (BMI) among adolescents. Method: Physical fitness tests were conducted between 2007 and 2014 in six regions of France among 49,631 middle and high school students aged 11 to 18. The distributions of sprint speeds and 3-minute shuttle run times were mapped by age, sex, and body mass index (BMI) using analysis of variance and density functions. Results: Average speeds in the shuttle run and sprint tests did not vary over the study period but increased with age. They were higher among boys. Children with a normal BMI had higher running speeds than those who were overweight or obese (p<0.05). Density functions based on speed gradients allow for the identification of an optimal BMI range for physical fitness. Discussion-Conclusion: The physical fitness of children and adolescents, as assessed by speeds in the shuttle run and sprint tests, varies by sex and age. Like survival or athletic performance functions, it responds asymmetrically to changes in BMI, depending on a lack of active muscle mass or excess adipose mass. A healthy weight within the normal BMI range is associated with optimal physical fitness. Due to the simplicity of its measurement, physical fitness (assessed by movement speed) can be used as an indicator of the level of physical activity practiced to evaluate its health impacts on large populations.