School bag carriage in schoolchildren: relative load, carrying duration, perceived heaviness, and back pain

Carrying school bags represents a routine mechanical exposure for school-aged children and may influence posture, gait, and musculoskeletal health. Current recommendations commonly express school bag weight as a percentage of body weight; however, relationships among relative load, anthropometric characteristics, perceived heaviness, carrying duration, and back pain remain unclear. This cross-sectional study examined these relationships in 221 schoolchildren aged 6–15 years. Anthropometric characteristics, school bag weight, carrying duration, perceived heaviness, and self-reported back pain were assessed. Mean relative school bag load was 14.49 ± 3.79% of body weight. Relative load was inversely associated with BMI, body weight, and body height. Carrying duration showed a weak positive association with perceived heaviness. Exploratory cluster analysis identified three descriptive profiles characterized by higher BMI/lower relative load, lower BMI/higher relative load, and longer carrying duration. Still, the cluster membership was not significantly associated with back pain. These findings indicate that relative load, carrying duration, perceived heaviness, and reported back pain capture partly distinct aspects of school bag carriage. Preventive recommendations should therefore remain proportionate to the evidence and should interpret the effect of a single school bag parameter on musculoskeletal pain cautiously.