Comparison of 3D ankle kinematics between minimal inertial measurement units configuration and optical motion-capture system under diverse walking conditions

Accurate assessment of three-dimensional ankle kinematics is essential for advancing biomechanical analyses and supporting evidence-based clinical interventions. While optical motion-capture systems (OMCS) are considered the gold standard, their high cost and limited portability restrict their use outside the laboratory. Inertial measurement units (IMUs) offer more practical alternative; however, concerns remain regarding their validity and reliability in diverse walking conditions. This study evaluated the validity and repeatability of IMU-derived ankle kinematics relative to OMCS across three walking surfaces: level ground, inward wedge, and outward wedge. Ankle movements in the sagittal, frontal, and transverse planes were analyzed using a minimal sensor configuration. High agreement and repeatability were observed in the sagittal and transverse planes, particularly during level- and inward-inclined walking. Conversely, the frontal plane demonstrated limited agreement under wedged conditions, likely because of complex multisegmental foot dynamics and reference frame misalignment from static calibration. Despite these limitations, consistent repeatability across conditions supports the use of IMUs for tracking ankle kinematics outside the laboratory. These findings suggest that IMUs are applicable as practical tools for clinical and biomechanical studies, particularly where streamlined setups are warranted.