Passive ankle and hindfoot kinematics within a robot-driven tibial movement envelope

Accurate description of individual bone kinematics is essential for understanding individual foot and ankle joint function and interactions. While invasive and noninvasive techniques, including robotic simulators, have advanced the direct measurement of individual joint kinematics during gait, efforts to quantify the passive adaptability of the foot and ankle remain limited. This study aimed to systematically describe the passive kinematics and ranges of motion of the ankle and hindfoot joints during prescribed planar tibial motions. Five fresh-frozen lower limb cadaveric specimens were attached to a 6-axis industrial robot, loaded to 25 % body weight, and prescribed tibial dorsi/plantarflexion, external/internal rotation, and varus/valgus alignment motions with various underfoot perturbations. Tibiotalar, talofibular, tibiofibular, subtalar, talonavicular, and calcaneocuboid joint kinematics were calculated using joint-specific anatomical coordinate systems. One-way repeated measures ANOVA with post hoc Bonferroni corrections (α = 0.05) compared joint rotations and range of motions (ROM) across various underfoot perturbations. Significant passive adaptive kinematic changes occurred in the hindfoot joints during prescribed dorsi/plantarflexion and external/internal rotation, indicated by functional shifts in hindfoot kinematics with different perturbations, while ROM remained consistent. In contrast, minimal passive adaptive motions were observed in the hindfoot joints during varus/valgus alignment, indicating a reduced role in foot mobility during coronal plane tibial motion. These findings emphasize the importance of accurately measuring individual bone kinematics, particularly the significant contributions of hindfoot joints to the passive mobility and stability of the foot and ankle.