Abstract: Passive Lower-Limb Exoskeletons (PLLEs) have emerged as a frontier in the fields of assistive locomotion and rehabilitation robotics due to their significant advantages in biomechanical compatibility, energy efficiency, and system lightweight design. This paper systematically classifies and reviews the research progress on PLLEs over recent decades, focusing on the mechanical power transmission mechanisms and system architectures for locomotion-load assistance. By utilizing a bio-inspired evaluation framework based on human lower-limb load regulation mechanisms, the study addresses the limitations of exoskeleton assistance performance. It highlights the significant impact of human–machine locomotion coupling effects on stress distribution in lower limb joints and the collaboration between humans and exoskeletons, which hinders their application in rehabilitation and prevention. The study proposes an integrated continuous metamorphic mechanism in a biomimetic design to enhance human–machine cooperation of PLLEs by achieving passive compliant dynamic responses aligned with lower limb biomechanics. Two potential research directions are then suggested: I. Bio-kinematic dimensional synthesis of PLLEs considering wearable uncertainties for workspace dexterity and accommodation pre-metamorphosis; II. Bio-dimensional synthesis based on PLLEs' rigid-flexible coupled dynamics for axial impact buffering control during metamorphic process of lower limbs.

Key words: Passive lower-limb exoskeleton')">Passive lower-limb exoskeleton, Load regulation, Human–machine cooperation, Metamorphic reconfiguration, Bio-inspired evaluation, Passive compliance