A Passive Backpack Exoskeleton Based on a Four-Bar-Sinusoidal Coupled Mechanism for Centroid Regulation and Load Reduction

Abstract Excessive oscillation of the load centroid in the sagittal plane imposes impact and oscillatory force on the shoulders and back, contributing to upper-body muscle fatigue. To address this issue, this paper investigates a hip-driven backpack exoskeleton with a passive adaptive centroid adjustment mechanism, integrating a four-bar linkage with a sinusoidal mechanism to dynamically regulate the load centroid. Kinematics models of the human-load and human-exoskeleton-load systems are developed based on an extended six-bar representation, enabling comparative analysis of centroid trajectories. Dynamics models for the upper torso, single-leg, and double-leg support phases are constructed to evaluate pressure and joint torques variations. opensim-based musculoskeletal simulations further analyze the torque distribution and metabolic activity of key gait-related muscles. Results demonstrate that the exoskeleton effectively reduces load centroid fluctuation and redistributes joint torques. Experimentally, it achieves an 85% reduction in peak load centroid displacement, a 49% decrease in total shoulder pressure, and a 50% reduction in lumbar swing amplitude during the single support phase. Simulations show lumbar and hip torque reductions of 52.68% and 23.64%, respectively, and a 32.85% decrease in erector spinae metabolic activity. This work addresses a gap in passive exoskeletons by introducing a rigid mechanism that adaptively shifts the load centroid without active control, verified through integrated modeling and experiment. It provides an effective strategy to reduce upper-body strain and improve gait efficiency during load-bearing locomotion.