Maximum Force of Peak Two under Progressive Schoolbag Loads and Walking Durations in Adolescent Girls Involved in Sports

Maximum Force of Peak Two (MFP2) represents the propulsive push-off force generated during the terminal stance phase of gait and serves as an important indicator of lower-limb loading during walking. In adolescent girls involved in sports, schoolbag carriage and prolonged walking may alter MFP2, reflecting biomechanical adaptations in gait mechanics and increased mechanical demands on the musculoskeletal system. The study examined the effects of progressive schoolbag loads and walking durations on MFP2 in adolescent girls involved in sports, with particular emphasis on bilateral gait adaptations. A repeated-measures design was employed involving 69 adolescent girls actively participating in sports. Participants completed walking trials under six schoolbag load conditions (0%, 8%, 12%, 16%, 20%, and 25% of body weight) and seven walking durations (0, 5, 10, 15, 20, 25, and 30 minutes). MFP2 was recorded bilaterally using a Zebris Force Distribution Measurement (FDM) platform. Data were analyzed using paired-sample t-tests and repeated-measures multivariate analysis of variance. MFP2 increased significantly with progressive schoolbag loads in both feet (p < 0.001). Paired-sample analyses revealed significant left–right differences across most load and duration conditions, indicating bilateral asymmetry in propulsive force generation. Walking duration exerted a significant effect on the right foot (p < 0.001) but not on the left foot (p = 0.107). Significant load × time interactions were observed for both feet (p < 0.05), demonstrating that the influence of schoolbag load on MFP2 varied according to walking duration. Progressive schoolbag loading substantially increased MFP2 in both feet and contributed to bilateral asymmetry in propulsive force generation during walking. The findings suggest that heavier schoolbag loads, particularly when combined with prolonged walking durations, increase the mechanical demands placed on the lower limbs of adolescent girls involved in sports. These results support existing recommendations that schoolbag loads should be limited to approximately 10–15% of body weight and highlight the importance of considering both load magnitude and carriage duration when developing strategies to promote musculoskeletal health.