Impact of age, sex, body constitution, and the COVID-19 pandemic on the physical fitness of 38,084 German primary school children

Physical fitness (PF) is a vital health indicator, but many children do not meet the WHO physical activity guidelines. Low PF in children raises the risk of non-communicable diseases (NCDs) and negatively impacts their quality of life, a situation amplified during the COVID-19 pandemic. This study uses data from 38,084 German third-graders (7.8 to 9.4 years) across seven cohorts (school years 2017/18-2023/24) who participated in a statewide fitness evaluation program in the Federal State of Thuringia. We aimed to examine age, sex, and pandemic effects on endurance (6-minute run), coordination (star run), speed (20-meter linear sprint), lower limb power (powerLOW; standing long jump), upper limb power (powerUP; ball-push test), and static balance (one-legged stance with eyes closed). Data on height and mass was incorporated to clarify the task-specific impact of the body mass index (BMI) on performance. Our results corroborated reported age and sex effects, demonstrating linear development across the age range for all tests, and in static balance only, girls scored higher than boys. The relationship between BMI and task-specific performance revealed an inverted U-shaped function in weight-bearing tests (first four). Functions were steeper for boys than girls, implying a stronger BMI impact on performance for boys. BMI and age interaction suggest that being overweight may limit age-related performance benefits. Negative pandemic declines in endurance, coordination, powerLOW, and powerUP were more pronounced in "fitter" schools. In conclusion, task-specific performances should be interpreted considering age, sex, and body constitution. Pandemic-related performance declines emphasize the role of access to physical activity resources for all children.