The adaptive role of the knee joint in maintaining the orbital stability during slope walking.

Maintaining gait stability on sloped surfaces is a biomechanically demanding task. Previous studies revealed the effect of slopes on overall gait stability, but the joint-level mechanisms underlying the stability still remain unclear. Given that most active exoskeletons for gait assistance focus on providing additional torque to specific joints without considering the contribution of each joint to the stability, understanding the role of each joint in stabilization can enable the design of a safer intervention. In this study, we aimed to identify joint-specific contributions to gait stability by analysing maximum Floquet multipliers (max FMs) and their corresponding eigenvector across multiple gait phases and slope conditions. Results were obtained from the data of 13 participants walking on a treadmill at five slope gradients (-12°, -6°, 0°, 6° and 12°). The max FMs remained mostly invariant across slopes and phases, whereas the eigenvector components of the bilateral knees showed significant phase- and slope-dependent changes. These variations exhibited alternating patterns between limbs and were moderately correlated with joint angle variability, highlighting the knee's adaptive role in maintaining stability. Our findings provide new insights into joint-level stabilization strategies, which could potentially serve as valuable biomechanical insight for future exoskeleton design. Moreover, our findings suggest that interference at the knee during slope walking could largely affect gait orbital stability relative to other lower limb joints.