<b>BMI and Relative Backpack Load as Risk Factors for Flatfoot Among Adolescents Aged 10–15 Years: A Cross-Sectional Study</b>

Background: Flatfoot is a prevalent musculoskeletal condition during adolescence, adversely affecting plantar pressure distribution, postural alignment, and functional mobility through compromise of the medial longitudinal arch. Both body mass index (BMI) and backpack loading have been independently implicated in arch deformation during the developmental years; however, their combined influence on flatfoot prevalence and severity in school-going adolescents in Pakistan remains poorly characterised. Objective: To evaluate the influence of BMI category and relative backpack weight (RBW) as risk factors for flatfoot prevalence and severity among adolescents aged 10–15 years attending private schools in Karachi, Pakistan. Methods: A cross-sectional observational study was conducted among 399 school-going adolescents (250 males, 149 females; mean age 12.50 ± 1.55 years) recruited by convenience sampling from eight private schools. BMI was calculated from measured height and weight, RBW was expressed as a percentage of body weight with a threshold of ≤/>10%, and foot posture was classified using Clarke's angle derived from static inkpad footprints into high arch, normal, mild, moderate, and severe flatfoot. Associations were tested using chi-square analysis with effect sizes reported as Cramér's V. Results: Overall flatfoot prevalence was 34.6% (right foot), 39.4% (left foot), and 27.8% bilaterally, with severe flatfoot present in 23.3% and 25.0% of right and left assessments respectively. Significant associations were identified between BMI and Clarke's angle classification for both feet and bilaterally (p ≤ 0.005; Cramér's V = 0.148–0.190), and between RBW and Clarke's angle classification across all laterality outcomes (p ≤ 0.004; Cramér's V = 0.193–0.249), with bilateral flatfoot demonstrating the strongest association with RBW (Cramér's V = 0.249; p < 0.001). Conclusion: Underweight BMI status and heavy relative backpack load are significantly associated with flatfoot prevalence and bilateral arch compromise in school-going adolescents, identifying a specific and potentially modifiable risk profile warranting targeted school-based intervention and longitudinal investigation.