Effect of backpack loads on shoulder strap tension in male adolescents: a biomechanical study

Purpose: The widespread use of heavy backpacks among adolescents raises concerns about the impact on musculoskeletal health. This study aims to investigate the effects of different backpack loads on shoulder strap tension during walking and running, using a springloaded shock-absorbing backpack. Methods: Fifteen healthy male adolescents participated in the study. Each carried a backpack with loads of 3.5 kg, 7 kg and 10.5 kg while walking at 2 m/s and running at 4 m/s. Shoulder strap tension was measured using custom-made tension sensors, and kinematic data were collected with a 3D motion capture system. Statistical Parametric Mapping (SPM) was applied to analyze tension variations across different gait phases. Results: Shoulder strap tension increased significantly with heavier loads during both walking and running (p < 0.001). The tension varied throughout the gait cycle, with distinct patterns observed between walking and running. During walking, tension peaked at mid-stance, while in running, tension decreased during the absorption phase and increased sharply during propulsion. Significant differences between 7 kg and 10.5 kg loads were noted at specific gait intervals ( p < 0.05). Conclusions: Backpack load significantly influences shoulder strap tension, with variations closely linked to gait phases. These findings highlight the importance of ergonomic backpack design, particularly for reducing shoulder strain in adolescents.