Abstract: Accurate knowledge of the kinematics of the in vivo Ankle Joint Complex (AJC) is critical for understanding the biomechanical function of the foot and assessing postoperative rehabilitation of ankle disorders, as well as an essential guide to the design of ankle–foot assistant devices. However, detailed analysis of the continuous 3D motion of the tibiotalar and subtalar joints during normal walking throughout the stance phase is still considered to be lacking. In this study, dynamic radiographs of the hindfoot were acquired from eight subjects during normal walking. Natural motions with six Degrees of Freedom (DOF) and the coupled patterns of the two joints were analyzed. It was found that the movements of the two joints were mostly in opposite directions (including rotation and translation), mainly in the early and late stages. There were significant differences in the Range of Motion (ROM) in Dorsiflexion/Plantarflexion (D/P), Inversion/Eversion (In/Ev), and Anterior–Posterior (AP) and Medial–Lateral (ML) translation of the tibiotalar and subtalar joints (p?<?0.05). Plantarflexion of the tibiotalar joint was coupled with eversion and posterior translation of the subtalar joint during the impact phase (R2?=?0.87 and 0.86, respectively), and plantarflexion of the tibiotalar joint was coupled with inversion and anterior translation of the subtalar joint during the push-off phase (R2?=?0.93 and 0.75, respectively). This coordinated coupled motion of the two joints may be a manifestation of the AJC to move flexibly while bearing weight and still have stability.

Key words: Dynamic biplane radiography , · Tibiotalar joint , · Subtalar joint , · In vivo kinematics , · Coupled motion , · Bionic design