A novel fluid-bladder backpack interface mitigates biomechanical loading during load carriage: A musculoskeletal modeling study

Traditional backpacks often cause altered gait, increased joint loading, and discomfort due to poor biomechanical load transfer. This study designed and evaluated a novel backpack featuring a soft polymer fluid-filled bladder as an ergonomic interface, assessing its impact on lower extremity biomechanics across incremental loads. Ten healthy males walked under four conditions: unloaded, and with the backpack loaded at 3 kg, 6 kg, and 9 kg. Motion capture and force plate data informed a subject-specific OpenSim model. Analyses estimated peak joint angles, muscle forces, and joint reaction loads. A repeated-measures ANOVA with Greenhouse-Geisser correction was used to assess the effect of load. Increasing load significantly raised hip, knee, and ankle joint reaction forces along primary weight-bearing axes and major antigravity muscle forces (e.g., quadriceps). Kinematic adaptations included increased hip and knee flexion. Critically, the load transmission pattern suggested the fluid-bladder may provide a dynamic buffering effect, showing a non-uniform force increase between mid and high loads rather than a purely linear rise. While load carriage elevates biomechanical demand, the fluid-bladder interface demonstrates potential as an ergonomic solution by potentially improving force distribution. Future work directly comparing this design to a traditional backpack is required to confirm its ergonomic advantages. This design merges functional hydration with biomechanically-informed load carriage to enhance comfort.