A standardised protocol for three-dimensional ankle kinematics: From motion capture to functional workspace analysis

Understanding the complex biomechanics of the ankle is essential for advancing rehabilitation protocols and improving the design of assistive devices. This work proposes a standardised and reproducible protocol for the three-dimensional assessment of ankle kinematics using a marker-based gold-standard motion capture system. Ankle movements of 34 healthy adult women and men were investigated using a VICON motion capture system equipped with 12 infrared cameras. A total of 32 reflective markers were attached to the lower legs, feet and shoes of each participant to capture joint motion and foot–shoe interactions. Participants performed controlled plantarflexion–dorsiflexion, inversion–eversion and abduction–adduction movements, which were analysed using combined local coordinate frames to resolve ankle motion in all three anatomical planes. The ankle joint exhibited the greatest mobility in the frontal plane, with a maximum pitch angle of 106.5; dorsiflexion and plantarflexion were dominated by pitch motion, with an average range of 59.1° in pitch; inversion and eversion showed a more evenly distributed motion pattern, with average angular displacements of 37.4° in roll; abduction and adduction were characterised by yaw motion, with 28.3°. Results demonstrated consistent intra-subject repeatability across trials, with noticeable inter-subject variability, confirming the effectiveness of the proposed protocol in capturing natural variations in human motion. A secondary analysis revealed relative displacement between the foot and shoe, with an average slip of approximately 1 mm and peak values exceeding 10 mm in extreme cases, highlighting the importance of footwear-foot coupling in kinematic studies. The proposed methodology provides a robust foundation for the quantitative characterisation of ankle mobility, enabling reproducibility across laboratories and supporting future developments in rehabilitation robotics, ergonomic footwear design and motion analysis research.