Fig. 1.

a) Demonstration of dynamic gait stability defined based on the Feasible Stability Region (FSR) theory. Unlike static stability, which only involves the relative position of the body’s center of mass (COM) to the base of support (BOS), the FSR theory considers both position and velocity of the COM with respect to its BOS. When the COM’s motion state (i.e., the combination of its position and velocity relative to the BOS) is below the lower limit of FSR (like point A), COM lacks adequate forward momentum to bring itself above the BOS. Dynamic stability is thus negative indicating an unstable state against backward falling. When a person’s COM motion state is within the FSR, the person assumes a stable upright body posture without a need to adjust the existing BOS. When the COM motion state is above the FSR, the COM has excessive forward momentum that likely leads to a forward fall. The value of dynamic stability (s) is quantified as the shortest distance from the COM motion state to FSR’s lower limit (the solid line between the motion state A and the FSR). Also shown are the COM motion state trajectories during the stance phase in Tai-Chi (TC) and regular gaits for a representative participant. Comparisons between TC and regular gaits of b) COM position, c) COM velocity, and d) dynamic gait stability at touchdown (TD) and liftoff (LO). The COM position and velocity were relative to the rear edge of the BOS and normalized by foot length (l_BOS) and $\sqrt{g\times bh}$, respectively (where g represents the acceleration due to gravity and bh the body height). Both the COM position and velocity relative to the BOS are unitless. Therefore, dynamic gait stability is a dimensionless measurement. e) Demonstration of the center of pressure (COP) path on the transverse plane during both gaits for the participant in a). f) Comparison of the COP mediolateral (ML) and anteroposterior (AP) excursion between regular and TC gaits. COP excursion was determined as the maximal displacement of the COP path in the respective direction. Also shown is the effect size of each comparison expressed as Cohen’s d. ***: p < 0.001 vs. regular gait; **: p < 0.01 vs. regular gait.