Association between Body Mass Index and Upper Body Postural Alignment in Saudi School-Aged Males: A Cross-Sectional Comparative Study

Background: Childhood obesity has been associated with musculoskeletal disorders and low back pain; however, evidence regarding the impact of body mass index (BMI) on upper-body postural alignment across different school-age groups in Saudi children remains limited. Objective: This study aimed to investigate the association between BMI and upper-body postural alignment in Saudi school-aged boys and determine if these relationships differ according to school level. Methods: This cross-sectional comparative study recruited 432 healthy Saudi boys aged 6–18 years from governmental and private schools in the Western Region of Saudi Arabia. Participants were categorized into three BMI groups (normal weight, overweight, and obese; n = 144 per group) and three school levels (primary, middle, and secondary; n = 144 per level). The upper-body postural alignment included forward head, shoulder protraction, shoulder slope, shoulder downward rotation, dorsal kyphosis, and trunk inclination angles was assessed using standardized digital photogrammetry. Results: Factorial MANOVA demonstrated a significant multivariate main effect of BMI category (p < 0.001), whereas neither school level (p = 0.893) nor the BMI × school level interaction (p = 0.132) was significant. Univariate analyses indicated that BMI significantly affected all postural variables (p < 0.001). Bonferroni-adjusted comparisons demonstrated that forward head and shoulder downward rotation angles decreased with increasing BMI, whereas shoulder protraction, shoulder slope, dorsal kyphosis, and trunk inclination angles increased (p < 0.001). These patterns were generally consistent across school levels, with Cohen's d values ranging from moderate to very large. Conclusion: Increasing BMI was consistently associated with progressive deterioration in upper-body postural alignment among Saudi school-aged boys, regardless of school level. Excess body weight exerted a substantially greater influence on postural alignment than school level, highlighting the importance of early postural screening combining weight management and postural rehabilitation to promote optimal musculoskeletal development.