A novel, synergy-based kinematic model of the foot and ankle for gait analysis

Investigating foot biomechanics in musculoskeletal modeling is challenging. Multi-segment foot models, the most common approach in the literature, simplify the foot into a chain of a few rigid segments, neglecting many bone-to-bone motions and typically requiring several markers to track the various segments. This study aims to develop and characterize a synergy-based multibody kinematic optimization (SB-MKO) model that allow to reconstruct the kinematics of all the bones using a minimal number of degrees of freedom and markers. A novel kinematic model was developed by exploiting experimentally derived kinematic coupling among bones, or motion synergies, identified from weight-bearing CT scans through principal component analysis. The model describes the foot–ankle complex using five degrees of freedom (one at the ankle, three in the foot and one for toe motion). The intrinsic baseline accuracy of this modeling strategy was quantified against in-vitro CT-based bone kinematics, and robustness was evaluated by reducing the number of skin markers. The model was also preliminarily tested on clinical gait data from healthy, flat, and diabetic feet and compared with a standard multi-segment foot model. In the in-vitro test, the SB-MKO achieved average errors below 7° and 4 mm for rotations and translations, matching or improving the accuracy of existing models. Similar results were obtained using only four foot markers. In clinical gait analysis, the model showed superior discrimination between populations and enabled assessment of transverse arch deformation, which is not accessible with conventional models. The proposed SB-MKO provides a sound, low-complexity framework for reconstructing bone-level foot and ankle kinematics, substantially simplifying gait analysis and showing strong potential for clinical applications.