The foot contributes to propulsion and postural stability function by changing its morphology during walking through the truss/windlass mechanisms. We quantified the truss mechanism regarding foot stiffness and the windlass mechanism regarding the movement coordination between the foot arch and metatarsophalangeal (MTP) joint. We aimed to clarify the relationship of these mechanisms with propulsive force and static foot alignment. Forty-eight healthy young adults participated and walked at a comfortable speed. The ground reaction force (GRF), ankle power, and sagittal plane motion of the foot arch and MTP joint were recorded using a three-dimensional motion analysis system. The vertical GRF and foot arch motion were used to quantify foot stiffness as the truss coefficient, and the foot arch and first MTP joint were used to quantify movement coordination as the windlass coefficient. The Foot Posture Index (FPI) and arch height index (AHI) were used to assess static foot alignment. A canonical correlation analysis was performed using the foot- and gait-related index group, and then a single-correlation analysis was performed. The canonical correlation analysis showed that a composite variable consisting of FPI, AHI, and the truss coefficient was related to the anterior GRF (A-GRF) and ankle power. The truss coefficient was positively correlated with A-GRF and ankle power, whereas the windlass coefficient was positively correlated with ankle power. Therefore, the truss and windlass coefficients can be used as indices to evaluate foot function, suggesting that maintaining foot stiffness and foot arch coordination with the first MTP joint is important for propulsion.
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